Phylum Brachiopoda Lecture Notes PDF

Phylum Brachiopoda  Phylum Brachiopoda Brachiopods were the dominant shelly marine invertebrates of the Palaeozoic. Brachiopods originated in the early Cambrian and diversified in the Ordovician. Although they survived t...

Phylum Brachiopoda  Phylum Brachiopoda Brachiopods were the dominant shelly marine invertebrates of the Palaeozoic. Brachiopods originated in the early Cambrian and diversified in the Ordovician. Although they survived the end-Permian mass extinction they declined through the Mesozoic. They are classified into three subphyla: Linguliformea and Craniiformea, and Rhynchonelliformea. They are exclusively marine filter feeders. The brachiopod shell shape is sometimes indicative of substrate type. Communities of brachiopods can be used to study paleoenvironments. Some groups like the lingulids have changed very little in shape over the last 500 million years. This is why some brachiopods are referred to as living fossils. If they have a common name, it is “lamp shells,” since the shells of one living group, the terebratulides, bear some resemblance to the oil lamps of Biblical times. Most modern brachiopods live in marginal marine environments attached to the substrate and species tend to be morphologically similar. The “lamp shells” got their nickname from However, Palaeozoic brachiopods exploited a wider range of marine their resemblance to a Biblical oil lamp. environments and their morphology was extremely diverse, ranging from erect coral-like forms to flattened saucer shapes.  Phylum Brachiopoda Morphology The typical brachiopod shell has one valve that is larger than the other. It is called the ventral (or pedicle) valve, since it usually has an opening for a fleshy stalk called the pedicle, with which the brachiopod attaches itself to the substrate. The opposite, smaller valve is known as the dorsal (or brachial) valve, because the lophophore (brachium) attaches to it. Lophophore (filters particles and detritus out of the water) occupies most of the shell’s internal volume in the mantle cavity. Crammed into the back of the shell is the coelomic cavity, which contains most of the internal organs. There is also a kidney-like nephridium used for excretion of metabolic wastes. The brachiopod has no eyes to sense light, but a set of bristles around the margin called setae help sense the environment. The adductors run perpendicularly from the dorsal to the ventral valves, and pull the two valves together, closing the shell. The diductors insert on the middle of the ventral valve, and also on the cardinal process of the dorsal valve, so they pull the dorsal valve around its hinge line and cause it to open.  Phylum Brachiopoda Morphology The opening for the pedicle is called the pedicle foramen, and it usually perforates the beak, or the pointed part of the ventral valve. The pedicle foramen may be enclosed on the anterior end by a single plate, called a deltidium, or by a pair of deltidial plates. Pedicle foramen In some groups, there is no round pedicle foramen, but instead a notch called a delthyrium. If the delthyrium is shallow, then a similar notch on the dorsal valve called the notothyrium may enlarge the pedicle opening. If a plate encloses the notothyrium, it is called the chilidium, or there may be a pair of chilidial plates. In pentameride brachiopods, there is a large, spoon- Delthyrium shaped platform in the ventral valve for the hinge muscles called the spondylium. This group is also characterized by a median septum, and a corresponding spoon-shaped feature in the dorsal valve called the cruralium. The septum, cruralia, and spondylia of the pentamerides subdivides the internal volume of the shell into five chambers that give the group its name. Morphology Phylum Brachiopoda The calcareous support of the lophophore, or brachium, is called the brachidium, and it is attached to the hinge area by the crus (plural, crura). It may have an elaborate loop to support the main part of the lophophore. The shape of the brachidia can be highly variable. In many spirifers they are arranged in a conical spiral pointing laterally. The hinge line can be straight (strophic) or curved (astrophic). It can have a large flat or curved surface between the beak and the posterior margin of the other valve, which is known as an interarea. The convex posterior portion extremity of the shell is known as the umbo. Some hinges come to wing-like lateral points called the cardinal extremities. The edge of the shell along its line of closure is called the commissure. It may be straight, or have corrugated edges (plication), and it may also have a deep trough on one valve (sulcus) that is matched by a large elevated area (fold) on the opposite valve. The surface of the shell may show concentric growth lines, indicating the enlargement of the shell from its embryonic origin around the hinge area, as well as fine radial ribs that run from the beak to the commissure known as costae.  Phylum Brachiopoda Morphology Astrophic Strophic  Phylum Brachiopoda Classification Traditional classification: inarticulates vs. articulates Articulate 1. Inarticulates: shells lack defined hinges and are made of calcium Inarticulate phosphate (phosphatic); they do not have teeth or sockets so rely on muscles to hold and move them. Example: Order Lingulida. 2. Articlulates: shells with articulated hinges (with teeth and sockets) and made of calcium carbonate. Examples: all brachiopods other than Lingulida. Modern classification: Subphyla Linguliformea, Rhynchonelliformea, and Craniiformea Linguliformea Craniiformea Rhynchonelliformea  Phylum Brachiopoda Subphylum Linguliformea Order Lingulida (Cambrian to Recent) The lingulides are best known from the common living genus, Lingula, which lives buried in mudflats using its long pedicle to dig a burrow. Their extraordinary longevity and conservatism is probably due to the fact that they are very successful at living deep in the mud of brackish lagoons, and have tracked that common habitat through 600 million years with little pressure to change in response to predators or competitors. Lingula Subphylum Craniiformea Order Craniida (Ordovician to Recent) Craniid brachiopods are circular to subcircular in shape. Their dorsal valve is calcified and they tend to encrust onto hard surfaces including other brachiopod shells. Isocrania costata Subphylum Rhynconelliformea Phylum Brachiopoda Order Athyridida (Ordovician to Jurassic) They have a complex spiral brachidium and retain their pedicle after maturation. Order Atrypida (Ordovician to Devonian) Juvenile atrypids possess a pedicle, which is lost during maturation. Composita subtilita They were among the first brachiopods to evolve a spiral- shaped lophophore brachidium. Known to be a tropical group, they were common at the beginning of the Paleozoic, but were wiped out by the Late Devonian Frasnian-Famennian extinction. Order Orthida (Cambrian-Permian) They have a long, straight (strophic) hinge, with a wide open triangular delthyrium and notothyrium, surrounded by a distinct Atrypa spinosa but narrow interarea on both valves. The majority of the taxa also have very fine costae radiating from the cardinal area, a gently biconvex shell that is circular or elliptical in outline, and fold and sulcus that are shallow or absent. Internally, they have no brachidium, but a short brachiophore instead. Hebertella  Phylum Brachiopoda Order Pentamerida (Cambrian-Devonian) Most pentamerides had deeply biconvex shells with highly curved (astrophic) hinges, and a small uncovered delthyrium-notothyrium. Their most distinctive feature is the large, scoop-shaped spondylium in the ventral valve, and a large septum and cruralium complex, dividing the inside of the shell into five chambers (hence the name pentamerides). Although pentamerides were never as diverse as other suborders, their Pentamerus oblongus great numerical abundance in the Silurian makes them a useful index of that period. Order Productida (Ordovician to Permian) Productids are best known for their spine-covered shells. Spines functioned as protection and acted as anchors when living on soft substrates, although some also cemented themselves to harder substrates. Order Rhynchonellida (Ordovician-Recent) Pulchratia symmetrica It has remained little changed since the Ordovician. Never abundant compared to other brachiopods, they managed to persist nevertheless, and today there are a few surviving genera. They have a short, bent hinge line and a pointed pedicle beak giving their posterior profile a distinctive pointed “V” shape. Most have a pedicle, and the delthyrium is partially closed. The majority of the genera have coarse plicated costae or ribs, giving them a highly crenulated, zig-zag commissure, and a deep fold and Camarotoechia sulcus.  Phylum Brachiopoda Order Spiriferida (Ordovician-Jurassic) This highly diverse and distinctive group got its name from the spiral arrangement of its lophophore. Spiriferides tend to have highly biconvex shells, with well- developed radial costae, and a large interarea on the ventral valve. The most typical group, the suborder Spiriferidina, are characterized by a very long, strophic hinge line, giving them a wing-like profile. This group typically has a very wide pedicle interarea with a large triangular delthyrium, and a deep fold and sulcus. Mediospirifer audaculus Order Strophomenida (Ordovician to Triassic) Strophomenides are the largest and most variable of brachiopod orders, with a number of unusual forms. They are typically concavo-convex to plano-convex, with a long, straight (strophic) hinge line, and a completely closed pedicle foramen. This implies that their pedicle was reduced or absent, and not important in adults, so it required them to live freely on the substrate once they matured enough to lose their pedicle. The earliest and most primitive group is the suborder Strophomenidina, which have a distinctive shell that is deeply Strophodonta demissa concavo-convex and has a long strophic hinge, giving them a “D”-shaped outline.  Phylum Brachiopoda Order Terebratulida (Devonian-Recent) The best known of the living brachiopods are the terebratulides, which have a distinctive shape often compared to a biblical oil lamp. Shell is strongly biconvex, with a large pedicle foramen (and therefore a large pedicle) and beak that overhangs the short, curved hinge with no interarea. Since there is a large pedicle foramen, the delthyrium is closed by delthyrial plates. Most taxa have a relatively smooth shell with little ornamentation and no fold or sulcus. Internally, they have a loop-shaped brachidium. Terebratulina septentrionalis  Phylum Brachiopoda Ecology and paleoecology Brachiopods are exclusively benthic marine animals. As filter feeders they do not actively search for food and most brachiopods live on, or partially enclosed by, the substrate. They are dependent on currents to bring food and oxygen and carry away waste products. Fossil forms exploited a range of benthic habitats.  Phylum Brachiopoda Discriminating between brachiopods and bivalves Brachiopods look superficially very similar to bivalves. This similarity is the consequence of sharing a similar lifestyle; most species of each group are sessile filter feeders living in the shallow marine environment. As such they represent an example of evolutionary convergence. Both have bilateral symmetry. Bivalves grow a left and right shell with the line of symmetry along the margins of the valves. Brachiopods grow a front and back (dorsal and ventral) shell, with the line of symmetry cutting each valve in half. Most brachiopods have a plane of symmetry that runs through both shells, so that one half of the shell is the mirror image of the other. By contrast, most clams are symmetrical between the valves, so the right valve is the mirror image of the left valve. Of course, there are many groups that break this rule, such as the scallops. Phylum Mollusca  Mollusks are an extremely diverse and abundant Phylum. Most mollusks are marine. Some groups live in fresh water and land. The phylum includes animals with an external shell, for example snails and oysters, as well as mainly soft-bodied forms, for example slugs and squids. Mollusks are first known from the early Cambrian. They utilize two/three part chitin, calcite and aragonite shell. Mollusks are divided into three main groups: 1. Gastropoda 2. Bivalvia 3. Cephalopoda Gastropoda Gastropods are the most diverse group of mollusks, living in terrestrial and all aquatic environments. The bivalve shell shape is constrained strongly by function so that mode of life can often be interpreted from the shell morphology. Cephalopods are the most morphologically complex mollusks. As active predators, occupying the same ecological niche as fish, cephalopods may be considered to be the most sophisticated invertebrate group. Cephalopoda Bivalvia  Basic morphology Most mollusks have an elongate, unsegmented body with a distinct head. The internal organs are held between a muscular foot and a calcareous shell secreted by an underlying tissue known as the mantle. The mantle tends to overhang the body forming a chamber at the posterior, the mantle cavity. This cavity contains the gills. The mouth opens anteriorly, at the other end of the mollusk. Ancestral mollusk internal morphology Sensory organs, such as eyes and tentacles, are concentrated in the head. Shell morphology is extremely diverse. Shells may be coiled or straight, chambered or undivided, singular or two-valved. Primarily, shells provide protection but they also may be used in burrowing or boring, or to enable buoyancy. The evolutionary development of some mollusks, such as octopus and squid, has tended to result in the loss of the shell.  Class Gastropoda Gastropods may have a calcareous shell or be entirely soft bodied. They have a well-developed head and sensory organs and an expanded muscular foot. In terrestrial gastropods the gills are lost and the mantle cavity is modified into an air-filled “lung”. Each complete coil is called a whorl. Dextral shell Gastropods are coiled and can be planispiral and therefore may have a superficial resemblance to ammonites. Posterior region Anterior region has muscular foot has head with tentacles  Torsion: Class Gastropoda Embryonic gastropods begin their development with their anus and gills at the rear of the body. As they develop, the entire visceral mass (a cluster of internal organs in mollusks that contains the digestive, excretory, reproductive, nervous, and respiratory systems), is forced to twist around into a U-bend. There is probably no simple, single answer to the puzzle of gastropod torsion. It probably brought mantle cavity in the front of the animal, so the snail can easily ventilate its gills, wash away wastes, and taste the oncoming water. Some scientists also suggest that torsion allowed the gastropod to retract into its mantle cavity for protection. In addition to these puzzles is the fact that some gastropods secondarily lose their torsion. Detorted gastropods are most common among the unshelled opisthobranchs.  Class Gastropoda Sublass Opisthobranchia Sublass Pulmonata Sublass Prosobranchia  Class Gastropoda Evolutionary history Cambrian gastropods were typically marine, herbivorous grazers with low, coiled shells. Cambrian gastropods By the Carboniferous, forms with a siphonal notch were common, indicating the presence of a siphon and therefore an infaunal mode of life. Palaeozoic gastropods generally occupied shallow water environments and were greatly affected by the end-Permian extinction. During the Mesozoic, prosobranchs diversified and deep burrowing, long-siphoned prosobranchs originated in the Cretaceous. These forms dominate the gastropod fauna today. Carnivorous gastropods were important predators in the Cenozoic. The Cenozoic also saw the radiation of gastropods into freshwater Early Paleozoic gastropods environments and the appearance of planktic opisthobranchs. Radiation of the air-breathing pulmonates into the terrestrial environment began in the Jurassic. Late Paleozoic gastropods Pleistocene gastropods Cretaceous gastropods  Class Gastropoda Planorbis (Jurassic–Recent) Belonging to the subclass Prosobranchiata, this freshwater gastropod has an almost planispiral shell (diameter approximately 1 cm). Although the morphology varies within the genera, most species have smooth shells. Living in a range of freshwater environments, Planorbis feeds on algae and plants. Planorbis Turritella (Cretaceous-Recent) It belongs to the subclass Prosobranchiata. The multiwhorled shell with a high pointed spire has a simple, circular aperture and spiral ribbing on the external surface. There is no siphonal canal. The height is approximately 5 cm. The mode of life is benthonic, infaunal, filter feeding. Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms suspended in water, typically by having the water pass over or through a specialized Turritella filtering organ that sieves out and/or traps solids.  Class Gastropoda Hygromia (Eocene-Recent) Buccinum (Pliocene-Recent) Hygromia is a terrestrial gastropod, the It has a moderately high-spired shell with an oval bristly snail, belonging to the subclass aperture and short siphonal canal. Pulmonata. Typically the shell has an external, ribbed sculpture. It has a modified, untwisted, mantle cavity The height is approximately 8 cm. that functions as an air-breathing lung. Living in sea water up to a depth of 200 m, Buccinum The shell is generally thin, smooth, and lives semi-infaunally with its siphon extended above conispiral. the sediment drawing clean water into the mantle Its height is approximately 5 mm. cavity. Buccinum is carnivorous. Hygromia is found in a range of terrestrial habitats and is most common in humid Hygromia environments with calcium-rich soils. Patella (Eocene-Recent) Patella is characterized by an uncoiled, conical, cap-like shell. Pronounced ribs that radiate from the Buccinum apex strengthen the shell. The height of the cone is approximately 3 cm. Patella lives in the intertidal zone and clings to rocks using its foot. Patella  Class Bivalvia Bivalves are laterally compressed mollusks enclosed within a pair of hinged shells or valves. The valves are closed by the adductor muscles. The shell is opened by relaxing these muscles and water currents are drawn into the cavity. In the majority of bivalves the gill is modified for filter feeding, although the earliest bivalves may have been deposit feeders (an organism that moves along the surface within soft sediments to ingest organic matter found in the sediment). Some bivalves retain this feeding strategy. Most forms are capable of limited movement and the foot can protrude into the sediment to enable them to burrow. Most bivalves are marine but there are some freshwater forms. As the shell forms of different species are adapted to the nature of the substrate, bivalves are very useful in paleoenvironmental reconstruction. Bivalve shells are multilayered. Two phases comprise the shell – an organic matrix and a crystalline calcareous component, in the form of aragonite or calcite (CaCO3). Internal morphology Class Bivalvia Beak: Tiny pointy origin of the shell. Umbo: “Humpiest” part of the shell of which beak is a part.  Class Bivalvia Heterodont dentition Pachydont dentition Types of dentition: No hinge teeth Schizodont dentition Dysodont dentition Desmodont dentition Taxodont dentition Isodont dentition  Class Bivalvia Bivalve ecology As shell shape is constrained by function, the mode of life of bivalves can be interpreted from the shell morphology. The major life habits of bivalves are: (i) burrowing in soft substrates; (ii) boring and cavity dwelling; (iii) attached (cemented or by byssus threads); (iv) unattached recumbant; and (v) intermittent swimming. Infaunal bivalves Bivalves that burrow in soft substrates tend to be equivalved and have a distinct pallial sinus. Burrowing is achieved by the foot, which penetrates the sediment and swells. The muscles in the foot then contract, drawing the shell down through the sediment.  Class Bivalvia Boring bivalves Bivalves that bore into hard substrates typically have elongate, thin shells that are resistant to abrasion. The shell edges are used to penetrate the material aided by acid secretions from the mantle. Epifaunal and swimming bivalves Epifaunal bivalves exploit three living strategies: (i) attachment to the substrate by byssus threads (a group of thread-like structures made of the protein collagen); (ii) cementation to hard surfaces; and (iii) recumbent, free lying on the sediment surface stabilized by the shell morphology. Byssally attached bivalves secrete threads of collagen that adhere to the substrate. Cemented bivalves produce a calcareous fluid at the mantle margin that crystallizes, fixing the bivalve firmly to the substrate. Recumbent, unattached bivalves have extremely asymmetric valves. Bivalves that are able to swim do so only intermittently. The shells of swimming valves tend to be thin, to reduce weight, although some forms have pronounced radial ribs.  Class Bivalvia Evolutionary history The earliest bivalves are known from Lower Cambrian rocks. These primitive forms are extremely small and may be descended from an unusual group of primitive mollusks called rostroconchs. Strangely, no bivalves are recorded from the Middle and Upper Cambrian. A major period of diversification occurred in the early Ordovician. Groups arose with taxodont, dysodont, and heterodont hinges and a range of feeding strategies. Deposit feeders, byssally attached bivalves, and burrowers colonized marginal and nearshore environments. After this rapid radiation the group stabilized and bivalves were not a particularly diverse or Cambrian bivalve abundant group during the Palaeozoic. Non-marine bivalves appeared in the Devonian and were abundant in the Carboniferous, particularly in deltaic environments. Ordovician bivalves Devonian freshwater bivalve  Class Bivalvia Evolutionary history During the early Mesozoic, bivalves underwent a second radiation. The presence of a muscular foot and the development of siphons allowed them to successfully exploit an infaunal mode of life. By colonizing new infaunal environments, bivalves expanded into the intertidal zone, burrowed deeper into the sediment, and developed mechanisms for boring into hard substrates. Diversification of epifaunal bivalves also occurred in the Mesozoic. The most important of these groups were the rudist bivalves. They colonized carbonate shelves, adopting conical forms similar to corals or encrusting or lying on hard substrates. This group was short lived, originating in late Jurassic times and becoming extinct at the end of the Cretaceous. Most bivalves survived the end-Cretaceous mass extinction. Bivalves have been ubiquitous in most shallow marine environments throughout the Cenozoic and they continue to be abundant in most present-day marine environments. Rudist bivalves Triassic bivalves  Class Bivalvia Radiolites (Cretaceous) Known as rudist bivalves, such Teredo (Eocene–Recent) highly modified forms of Teredo is a highly specialized bivalve able to bore into wood. cementing bivalves were The cylindrical shell is extremely reduced. common in the Cretaceous. Sharp sculpture on the external surface is used for tunneling Radiolites has two strikingly into the substrate. different valves. The lower valve The animal is essentially worm-like and lives encased in the is conical with thick walls and burrow, growing to fill the excavated space. Teredo the upper valve is reduced to a small, flat lid. Ensis (Eocene-Recent) Such coral-like rudists tended to Characterized by an extremely elongated, thin, featureless shell grow in groups, feeding in calm, (approximately 12 cm in length) with both posterior and clear waters above the sea bed. anterior gapes. Ensis lives infaunally in muds and sands in the intertidal zone. During feeding, the anterior part of the shell is close to the sediment–water interface. During low tide, the animal burrows actively down into deeper sediments using the muscular foot. Ensis Mya (Oligocene–Recent) This bivalve has an elongate, smooth shell with a posterior gape. The shell is approximately 3 cm from umbo to shell edge. Dentition is absent and there is a deep pallial sinus. Radiolites Modern species are infaunal, living in soft sediment in burrows 30 cm deep. Mya  Class Cephalopoda All cephalopods are marine. The body of cephalopods is elongated so that the mantle cavity is anterior and the visceral mass is at the posterior end of the animal. Living cephalopods swim using jet propulsion. Water is drawn into and expelled from the mantle cavity through the hyponome, a modified part of the foot. Cephalopods are divided into three subclasses: Nautiloidea: Nautiloids have an external, chambered shell with simple sutures between its chambers. Ammonoidea: Ammonoids also have an external shell, always coiled, with variable and more complicated sutures. Coleoidea: Coleoids have an internal and reduced shell. In some coleoids the shell is absent. Nautiloids (external shell) Coleoidea (internal shell) Ammonoids (external shell)  Class Cephalopoda Nautiloids (Subclass Nautiloidea) The animal occupies the final shell chamber, the body chamber. The head, sensory functions, and hyponome are situated near the chamber opening and the visceral mass is at the rear. The animal is connected to the rest of the shell by the siphuncle, a tube extending from the body chamber to the protoconch (the first part of the shell to be formed; new chambers are added onto this as the animal grows). Septal necks are where the septa are pierced to allow the tube of soft tissue (siphuncle) through. The septum is the wall of the chamber. A suture is a line along which the septum of the shell fuse. The umbilicus is the diameter of the depression between the inside margins of the last coil. The most recent line of growth, which traces the edge of the aperture opening, is called the peristome. All of the chambered sections of the shell are collectively called the phragmocone. Support structures for the siphuncle are called septal foramen.  Class Cephalopoda Buoyancy in Nautilus Nautilus is the only living cephalopod that retains an external, coiled Nautilus has an adjustable buoyancy mechanism that gives it shell. Living in cool waters in the southwest Pacific, at depths neutral buoyancy at different depths of the water column. between 150 and 300 m, Nautilus relies on buoyancy control to The shell chambers contain gas and sea water, the adjust its position in the water column. proportions of which can be changed. Nautilus is an opportunistic feeder (sustains itself from a number of Initially the chambers contain sea water. The siphuncle different food sources), grasping mainly crustaceans and small fish removes ions from solution in the sea water, drawing water with its tentacles. from the chambers into the mantle cavity. Gas bubbles then Nautilus is a poor swimmer. Sea water is drawn into the mantle diffuse into the space, making the animal more buoyant and cavity and expelled through the hyponome. As water is ejected a able to float higher in the water column. By pumping ions into force is exerted on the shell, causing it to lurch forwards. When the the chambers, Nautilus brings water back into the chambers, mantle cavity is emptied, the shell swings backwards generating a making the animal less buoyant so that it sinks to lower see-sawing motion. Consequently, Nautilus swims only for short depths. distances and relies on buoyancy control to maintain its position in the water column during feeding. It rests during the day on the sea floor. Nautiloids include not only the living genus Nautilus, but a wide variety of primitive forms going back as far as the Cambrian. The primitive forms tend to be straight-shelled (orthoconic) or slightly curved (cyrtoconic). Othoceras sp.  Class Cephalopoda Ammonoids (Subclass Ammonoidea) Most ammonoids had a chambered, planispirally coiled shell (the shell coils in a single horizontal plane with the diameter increasing away from the axis of coiling/centre) with complex sutures. The shell can be divided into three parts: (i) the body chamber, where the animal lived (ii) the phragmocone, the chambered part of the shell (each chamber represents part of a previous body chamber) (iii) the protoconch, the first chamber to form As with nautiloids, the chambers were connected by a tube called the siphuncle, although in ammonoids the siphuncle usually ran along the outer, ventral margin of the shell rather than through the center of the chambers. The buoyancy mechanism of ammonoids was therefore similar to that of nautiloids.  Class Cephalopoda Coiling There are 3 forms of coiling mode: Involute coiling: The inner coils are almost completely hidden by more recent coils. This type of coiling is demonstrated by Nautilus. Evolute coiling: Describes a coiling growth opposite to involute, in which the inner coils are easily seen, giving a wider umbilicus. This type of coiling is common in Ammonoids. Convolute coiling: Newly formed whorls partially overgrow prior whorls in ammonoid species. Involute coiling Convolute coiling Evolute coiling  Class Cephalopoda Ammonoid suture Sutures are the lines marking the junction between the chamber wall and the ammonoid shell. The suture pattern is an important feature used in ammonoid classification. Sutures are described in terms of saddles and lobes. Saddles are curved sections of line that “point” towards the body chamber. Lobes are the converse – curves that are directed away from the body chamber. Palaeozoic ammonoids generally had simple, straight suture lines, whereas most Mesozoic ammonoids are characterized by complex sutures.  Class Cephalopoda Ammonitic suture Ceratitic suture Goniatitic suture All ammonites are ammonoids, but not all ammonoids are ammonites. Ammonites are a specific group within the larger classification of ammonoids.  Class Cephalopoda Sexual dimorphism Mature ammonite shells collected from the same horizon can often be divided, on the basis of size, into two distinct morphological groups. The smaller ammonites are referred to as microconchs and the larger type as macroconchs. Microconchs may also have a modified aperture with lateral extensions, the lappets. The function of the lappets is unknown but it may be linked to sexual reproduction. Although microconchs and macroconchs may simply be closely related species, new characters appear in both groups simultaneously suggesting that they are males and females of the same species, though which is which is unknown.  Class Cephalopoda Heteromorphs Some ammonite groups developed peculiar, “heteromorph” shell forms, particularly in the late Cretaceous. Originally these forms were considered non-functional, evolutionary dead-ends. However, physical modeling has shown that they were stable and well adapted for floating within the water column. Class Cephalopoda  Class Cephalopoda Difference between ammonoids and nautiloids If the internal shell/fossil can be observed then this method is useful. The shape of the septa curvature differs between Nautiloids and Ammonoids. A Nautiloid shell will usually be smooth on the exterior with no ornament or ribbing. In contrast, an Ammonoids shell can have ornament and ribbing on its exterior. The siphuncle of a Nautiloid will always connect the body chamber to the original chamber through the centre of all previous chambers. In Ammonoids, the siphuncle also interconnects previous chambers but it has the tendency to run closer to the outer (ventral) margin rather than the centre. Coleoids (Subclass Coleoidea) Class Cephalopoda The subclass Coleoidea includes all living cephalopods except Nautilus. The shell in coleoids is internal and reduced or even absent. Cuttlefish have an internal shell with a buoyancy function. Squids are streamlined swimmers with an internal cartilaginous rod. Octopuses are benthic, shell-less coleoids with “webbed” arms that allow the animal to drift with the currents. Belemnites make up the majority of the fossil coleoids. Characterized by an internal skeleton with a robust, bullet-shaped, calcite counterweight, belemnites are abundant in Jurassic and Cretaceous rocks. Belemnites Belemnites had an internal skeleton unlike that of any living coleoid. It can be divided into three parts: (i) the robust anterior counterweight – the rostrum or guard; (ii) the buoyancy mechanism – the phragmocone, a chambered conical section with a siphuncle; and (iii) the pro-ostracum – the support for an open body chamber.  Class Cephalopoda Evolutionary history The first cephalopods were straight-shelled nautiloids. They appeared in late Cambrian times and underwent a rapid diversification in the Ordovician when they gave rise to the coiled forms that existed throughout the Palaeozoic and Mesozoic. Although never as abundant or diverse as Straight-shelled nautiloids ammonoids, they survive through to the present day. Ammonoids evolved from straight-shelled ancestors in the early Devonian. Their evolutionary history has been marked by a sequence of radiations followed by extinction. Peaking in diversity during the Jurassic, they declined through the remainder of the Mesozoic, becoming extinct at the end of the Cretaceous. The history of coleoids is less well known due to their reduced, internal shell. The first true coleoids are recorded from Carboniferous rocks. Belemnites became abundant in the Jurassic and Cretaceous. Squid and cuttlefish are known from the Jurassic, diversifying in the Cenozoic after the end- Cretaceous extinction event.  Class Cephalopoda Kosmoceras (Middle Jurassic) Dactylioceras (Lower Jurassic) This ammonite has a compressed shell This ammonite is typically serpenticone and the external with pronounced ribs that bifurcate surface is ribbed (the shell diameter is approximately 6.5 towards the venter. cm). The suture pattern is ammonitic. Sexual dimorphism is known. In the microconch, the smaller dimorph, the aperture is compressed and the lappets Dactylioceras are developed. The shell diameter of the macroconch is approximately 5 cm. Amaltheus (Lower Jurassic) Amaltheus has a compressed, oxycone shell sculpted with curved, sinuous ribs. A “pie crust” keel is developed along the venter. The suture pattern is ammonitic. The shell diameter is approximately 8 cm. Kosmoceras Amaltheus  Class Cephalopoda Scaphites (Cretaceous) Scaphites is a heteromorphic ammonite typical of the Cretaceous period. This partially uncoiled ammonite has a body chamber in the form of a hook with a slightly constricted aperture that faces upwards. The external surface of the shell is ribbed. Scaphites Neohibolites (Lower Cretaceous) This belemnite has a small, spindle-shaped guard (approximately 4 cm in length) with a long ventral groove in the area around the alveolus. The soft-body morphology of belemnites is known from exceptionally preserved individuals associated with fossil lagerstätten. Such specimens have long hooked tentacles and ink sacs. Neohibolites Subphylum Trilobitomorpha  Phylum Arthropoda Arthropods include all insects, millipedes, centipedes, scorpions, and crustaceans, such as crabs and lobsters. Arthropods have 1. Exoskeleton (external skeleton) (provides protection, attachment framework for musculature, barrier against desiccation in terrestrial organisms, and sensing) 2. Segmented body and bilateral symmetry 3. Jointed limbs (or appendages; this is what gives the group its name). The skeleton is external and is shed and replaced as the animal grows. This process, known as molting or ecdysis, is a major controlling factor in the success of the group. On the positive side it allows a single individual to assume multiple body plans through life; for example it allows caterpillars to turn into butterflies. On the negative side, it uses up costly resources and makes the animal helpless and prone to predation each time it occurs.  Trilobites Between the Cambrian and the Permian, Trilobites were amongst the most important elements of marine communities. Trilobites were mainly bottom dwellers, and tended to develop marked provincialism in their faunas. This makes them useful palaeogeographic indicators. Trilobites went extinct in the end-Permian mass extinction event, but had been in decline throughout the Upper Palaeozoic. They were so rapidly changing and abundant that they are the biostratigraphic standard for the Cambrian and Ordovician.  Trilobite morphology Trilobites have a calcified exoskeleton, which is responsible for their favorable preservation compared to most other arthropods. Divided across the body into: cephalon (or head) thorax pygidium (or tail) The calcite carapace is divided into three longitudinal lobes, one on each side (pleural lobes) and one down the center (axial lobe). DORSAL VIEW: Upper side of the animal. VENTRAL VIEW: Underside or belly portion ANTERIOR SIDE: Cephalon region POSTERIOR SIDE: Pygidium region  Trilobite morphology Cephalon The raised axial (central) area of cephalon is called glabella, whereas the portion of cephalon that surrounds glabella comprises cheeks. The section of the head outside the facial sutures is known as the free cheek. The part inside, adjacent to the glabella, is called the fixed cheek. The eyes of the trilobite could attach to either the free or fixed parts of the cephalon. Thorax It is made up of a series of nearly identical segments, which are usually two to forty two in number and not fused together. Pygidium It is composed of a variable number of segments, which are fused together, immovable and may be one to thirty in number. Pygidium may be larger or smaller than the cephalon. Like the segments of thorax, it is also divided into an axial lobe and two pleural lobes. In some forms, the posterior end of pygidium has an extension of an axial spine, which is termed as telson. The spines occurring in the pygidium region are known as pygidial spines. Based on the size of pygidium, trilobites may be defined as: a. Micropygous: When pygidium is small. b. Macropygous: When pygidium is larger than cephalon. c. Isopygous: Pygidium and cephalon are equal in size.  Trilobite morphology Facial sutures are lines of weakness present on the cheeks. When a trilobite molted, its cephalon broke apart along this curved seam that runs from the front of the cephalon, through the eyes, and then posteriorly. On the basis of the position of facial suture with respect to the genal angle, the trilobites can be divided into four types: a. Marginal Facial Suture: This runs along the margin of the cephalon with the result that there is no free cheek on the dorsal side. b. Proparian Facial Suture: It cuts the margin on the lateral side in front of the genal angle. c. Gonatoparian Facial Suture: This suture cuts the margin at the genal angle. d. Opisthoparian Facial Suture: It cuts at the posterior side of the cheek.  Trilobite morphology The thorax of a typical trilobite was made up of a series of nearly identical segments, usually between two and 20 in number. In Ordovician and later trilobites these segments were usually jointed in such a way that the trilobite could roll up for defense. Bottom (ventral side) Hypostome-the hard mouthpart in trilobites found on the ventral side of the cephalon.  Trilobite eyes Trilobites were one of the first organisms on earth to have eyes, and therefore among the earliest organisms for which sight was an important sense (most marine invertebrates rely on touch and chemical clues). The earliest trilobites had compound eyes, much like those found in other arthropods. With their hundreds of tiny, closely packed calcite lenses (a holochroal eye), they were able to get a composite image of the world around them, although they could not resolve tiny details. Many trilobite eyes were arranged in a broad arc for excellent Holochroal eye composed of closely packed lenses peripheral vision, presumably to detect predators. Some had their eyes mounted on long stalks for a view above the seafloor, while others had eyes that were shaped like a large protruding half-dome, giving them excellent, almost 360° vision. The most sophisticated eyes are known as schizochroal, because each lens is surrounded by an area of interstitial opaque exoskeleton. On the other hand, many different groups of trilobites lost their eyes altogether, presumably because they lived in dark or muddy waters where vision no longer mattered. Schizochroal eye with lenses separated by solid cuticle  Evolutionary directions One involved them becoming extremely spiny. A second involved secondary loss of the eyes, and sometimes the addition of highly pitted fringes around the front of the head. The third involved a great reduction in size, and in the number of thoracic segments to only one or two. The three main adaptive strategies of trilobites away from a highly conserved body plan: (i) Trinucleus, a blind trilobite with a large frontal pitted region, which probably had a sensory function (ii) Agnostus, a tiny trilobite with a much reduced thorax (iii) Selenopeltis, a representative of the extremely spiny adaptation of trilobites.  Trilobite orders Order Agnostida Order Redlichiida Button-shaped pygidium equal in These are so primitive that the size to the cephalon (isopygous). segments of the pygidium are not Most were eyeless and blind even fused together. Their tiny size and widespread Have a large, semicircular distribution suggests that they cephalon, strong genal spines, floated in the open ocean. This is marginal facial suture; some have supported by the fact that they are a long tail spike called a telson. extremely widespread geographically making them excellent for biostratigraphic correlation around the globe. Order Corynexochida Order Asaphida Characterized by a box- Their large smooth isopygous like glabella, and an cephala and pygidia, are typical. opisthoparian facial Asaphids have only 6 to 9 thoracic region. segments, and their pleural lobes are rounded at the ends.  Trilobite orders Order Ptychopariida Order Illaenida They have a simple glabella that Frequently they are isopygous tapers anteriorly and a large area or have a pygidium larger than in front of the glabella. the cephalon. They also tend to have many They are typified by shield- thoracic segments and a small shaped forms. pygidium. Order Trinucleida Order Harpida They had extremely broad, They have a reduced pygidium and ornamented cephalic brim short thorax, so that their cephalon giving them a large, is more than half of the body. horseshoe-shaped head They are also typified by their shield. broad ornamented cephalic brim They had small eyes on and long genal spines. tubercles, a glabella that Most taxa were blind and lacked tapered anteriorly, and many eyes thoracic segments.  Trilobite orders Order Phacopida Order Lichida They are known by many thoracic Glabella extends to the anterior border segments, highly lobed, furrowed of the cephalon, subdivided into glabella with eyes on stalks, and elongate glabellar furrows, and an their ability to roll up and lock opisthoparian suture. their pygidium into the cephalon. Their most distinctive feature is the large pygidium Proetids were Order Proetida Order Odontopleurida opisthoparian, with a large vaulted glabella, and many Extreme development of taxa are isopygous. spines on their cephalic They had large brim, occipital ring, genal holochroal eyes, a angle, all along the furrowed pygidium thorax, and all over the without spines, small pygidium. and some also have genal spines.  Mode of living All trilobites were mobile and most seem to have crawled on the sea bed. A small number of species became burrowers or active swimmers. Most extremely, some trilobites became pelagic, with a permanently swimming, active lifestyle above the sea bed. These forms had eyes with up to 360° vision and a streamlined body. The most bizarre lifestyle adopted by trilobites is seen in a group of genera exemplified by Olenus. These trilobites had flattened bodies and many thoracic segments. They often occur in black shales, indicative of low oxygen availability above the sea bed, and had a great abundance. Each thoracic segment would have had a gill, allowing maximum extraction of oxygen, and it may be that they farmed sulfate-reducing bacteria, as do modern animals living in similar conditions near black smoker vents.  Trilobite evolution Trilobites were a major part of the marine benthos for over 250 million years. During all of that time their basic body plan remained the same, and the changes that did occur through evolution tended to be in details rather than in serious shifts of shape. This lack of innovation in a successful group is known as evolutionary conservatism. However, despite this, trilobites inhabited a wide range of niches and explored a wide range of marine environments from their evolutionary origins in the Cambrian. Cambrian Cambrian trilobites showed high diversity and included tiny, blind forms – the agnostids – and a range of more “familiar” body shapes. Ordovician Some Cambrian trilobites show a secondary loss of eyes. These This is the period of greatest diversity of trilobite body blind forms probably lived in deep water. forms, although more species have been recognized from A common factor amongst Cambrian trilobites is that they seem to Cambrian rocks. lack any adaptations for defense against predators. Trilobites were badly affected by the sudden glaciation that A major extinction in late Cambrian times may have been linked to caused the end-Ordovician mass extinction. the appearance of common, large predators, especially mollusks. Post-Ordovician Species that radiated after this extinction show a diverse array of Recovery from the late Ordovician mass extinction was defense strategies, including the ability to burrow, or extreme limited. spinosity. The last surviving trilobites became extinct in the end- Permian mass extinction. Vertebrates  Chordates The deuterostomes is a clade of animals that include both invertebrates and vertebrates. Vertebrates are members of the phylum Chordata, the chordates. Chordates are bilaterian (bilaterally symmetrical) animals, and within Bilateria. The cephalochordates and the urochordates are two groups of invertebrate deuterostomes that are more closely related to vertebrates than to other invertebrates. Along with the hagfishes and the vertebrates, they make up the chordates.  Chordates Derived Characters of Chordates All chordates share a set of derived characters: Notochord It provides skeletal support throughout most of the length of a chordate. Dorsal, hollow nerve cord The nerve cord of a chordate embryo develops into the central nervous system: the brain and spinal cord. Pharyngeal slits They allow water entering the mouth to exit the body without passing through the entire digestive tract. Muscular, post-anal tail It contains skeletal elements and muscles, and it helps propel many aquatic species in the water.  Chordates Lancelets Tunicates The most basal (earliest diverging) group Recent molecular studies suggest that the of living chordates are animals called tunicates (Urochordata) are more closely lancelets (Cephalochordata), which get related to other chordates than are their name from their bladelike shape. lancelets. As larvae, lancelets develop a notochord, The chordate characters of tunicates are a dorsal, hollow nerve cord, numerous most apparent during their larval stage, pharyngeal slits, and a post-anal tail. which may be as brief as a few minutes. The ancestral chordate may have looked Once a tunicate has settled on a substrate, something like a lancelet. it undergoes a radical metamorphosis in which many of its chordate characters disappear.  Chordates Pikaia Pikaia is considered to represent a primitive chordate. It’s documented from Middle Cambrian Burgess Shale (Canada). The rod running along its back resembles a backbone-like structure. The markings on the sides of its body form V-shapes which are the typical shape of chordate muscle bundles.  Craniates The next major transition in chordate evolution was the appearance of a head. Chordates with a head are known as craniates. The origin of a head— consisting of a brain, eyes and other sensory organs, and a skull— enabled chordates to coordinate more complex movement and feeding behaviors. Derived Characters of Craniates The pharyngeal slits evolved into gill slits which are associated with muscles and nerves that allow water to be pumped through the slits. This pumping can assist in sucking in food, and it facilitates gas exchange. Craniates have a higher metabolic rate and a much more extensive muscular system. Craniates also have a heart with at least two chambers, red blood cells with hemoglobin, and kidneys that remove waste products from the blood.  Craniates Haikouella The most primitive of the fossils are the 3 cm long Haikouella. It resembled a lancelet through its mouth structure. It also had some of the characters of craniates (well-formed brain, small eyes, and muscle segments along the body, respiratory gills). However, it did not have a skull or ear organs. Myllokunmingia Found from Cambrian rocks, Chengjiang fauna of China. About the same size as Haikouella, Myllokunmingia Myllokunmingia had ear capsules and eye capsules, parts of the skull that surround these organs. Based on these and other characters, Haikouella paleontologists have identified Myllokunmingia as a true craniate.  Craniates Primitive fish are known from Cambrian and Ordovician rocks, all of them lacking preserved jaws. Together, these jawless fish are known as agnathans. Today the group is represented by hagfish, which are scavengers, and lampreys, which live parasitically. Hagfishes The most basal group of craniates is Myxini, the hagfishes. Hagfishes have a skull made of cartilage. Hagfishes have a small brain, eyes, ears, and a nasal opening that connects with the pharynx. Their mouths contain tooth-like formations made of the protein keratin. When attacked by a predator, a hagfish can produce several liters of slime in less than a minute. The slime coats the gills of the attacking fish, sending it into retreat or even suffocating it. Several teams of biologists and engineers are investigating the properties of hagfish slime in hopes of producing an artificial slime that could act as a space-filling gel. Such a gel might be used, for instance, to curtail bleeding during surgery.  Vertebrates Derived Characters of Vertebrates A more extensive skull and a backbone composed of vertebrae. The vertebrae enclose the spinal cord and have taken over the mechanical roles of the notochord. Lampreys Lampreys (Petromyzontida) are the most basal lineage of living vertebrates. As larvae, lampreys live in freshwater streams. The larva is a suspension feeder that resembles a lancelet and spends much of its time partially buried in sediment.  Vertebrates Fossils of Early Vertebrates Conodonts These were slender, soft-bodied vertebrates with prominent eyes controlled by numerous muscles. Most conodonts were 3–10 cm in length, although some may have been as long as 30 cm. They probably hunted with the set of barbed hooks at the anterior end of their mouth. These hooks were made of dental tissues that were mineralized. Their fossilized dental elements are so plentiful that they have been used for decades by petroleum geologists as guides to the age of rock layers in which they search for oil. Their teeth were phosphatic. Fishes also evolved during the Cambrian, but we know of their existence only through the preservation of isolated bony external plates.  Vertebrates Vertebrates with additional innovations emerged during the Ordovician, Silurian, and Devonian periods. The earliest vertebrates were fish, and all of them were marine, with fish not migrating into fresh water until the Devonian. These vertebrates had paired fins, an inner ear with two semicircular canals that provided a sense of balance. They lacked jaws. There were many species of these jawless, armored swimming vertebrates, but they all became extinct by the end of the Devonian period. One of the most conspicuous new groups of fishes was the ostracoderms. They were small animals with paired eyes like those of higher vertebrates. Lacking jaws and covered by bony armor, their small mouths allowed them to consume only small items of food. They had heterocercal tails (unequal lobes). Origins of Bone and Teeth Mineralization was associated with a transition in feeding In addition, when the bony armor of later jawless mechanisms. Early chordates probably were suspension feeders but vertebrates was examined under the microscope, scientists over time they became larger and were able to ingest larger found that it was composed of small tooth-like structures. particles, including some small animals. These findings suggest that mineralization of the vertebrate The earliest known mineralized structures in vertebrates—conodont body may have begun in the mouth and later was dental elements—were an adaptation that may have allowed these incorporated into protective armor. animals to become scavengers and predators.  Gnathostomes Derived Characters of Gnathostomes Jaws are hinged structures that with the help of teeth enable gnathostomes to grip food items firmly and slice them. According to one hypothesis, gnathostome jaws evolved by modification of the skeletal rods that had previously supported the anterior pharyngeal (gill) slits. Fossil Gnathostomes Jaws evolved in the Silurian. Gnathostomes appeared in the fossil record in the late Ordovician period, about 450 million years ago, and steadily became more diverse. The earliest gnathostomes in the fossil record include extinct lineages of armored vertebrates known as placoderms. They had paired fins which gave greater control on movement. Other groups of jawed vertebrates, collectively called acanthodians, emerged at roughly the same time and radiated during the Silurian and Devonian periods. Placoderms had disappeared by 359 million years ago, and acanthodians became extinct about 70 million years later. By 420 million years ago, gnathostomes had diverged into the three lineages of jawed vertebrates that survive today: chondrichthyans, ray-finned fishes, and lobe-fins (the latter two are clubbed under osteichthyans).  Gnathostomes Chondrichthyans (Sharks, Rays, and Their Relatives) They have a skeleton composed predominantly of cartilage, though often impregnated with calcium. The largest and most diverse group consists of the sharks, rays, and skates. A second group is composed of a few dozen species of ratfishes, or chimaeras. Their teeth and scales are characteristically vertebrate as is their overall body plan. Their first clear appearance in the fossil record is Devonian. They have a heterocercal tail. Osteichthyans Nearly all living osteichthyans have an ossified (bony) endoskeleton with a hard matrix of calcium phosphate. Most fishes can control their buoyancy with an air sac known as a swim bladder. They have a homocercal tail. In nearly all fishes, the skin is covered by flattened, bony scales that differ in structure from the tooth-like scales of sharks. http://www.bartleby.com/images/A4images/A4homoce.jpg They appeared in the late Silurian. During the Devonian they diverged rapidly into two main groups, the ray-finned fish that dominate modern aquatic environments and the lobe-finned fishes. This latter group includes modern lungfish and the coelocanth, a famous “living fossil”. Although rare in the modern world, this is the group that gave rise to all terrestrial Osteichthyans Chondrichthyans vertebrates – including us.  Gnathostomes Ray-Finned Fishes (actinopterygians) Nearly all the aquatic osteichthyans familiar to us are among the over 27,000 species of ray-finned fishes (Actinopterygii). Named for the bony rays that support their fins, the ray-finned fishes originated during the Silurian period. Ray-finned fish have flexible fins supported by a lightly built fan of radiating bones, the rays. There have been three main radiations of ray-finned fish: in the late Palaeozoic, the late Triassic/Jurassic, and the late Jurassic/Cretaceous. The latest of these radiations saw the spread of teleost fish which are characterized by features that include symmetrical tails, overlapping scales, specialized fins, and short jaws that are often adapted to take particular kinds of food. The oldest mid-Devonian ray-finned fishes, such as Cheirolepis differed from modern representatives in having asymmetrical tails and diamond-shaped scales that did not overlap.  Gnathostomes Lobe-Fins (sarcopterygians) The lobe-fins (Sarcopterygii), also originated during the Silurian period. The key derived character of lobe-fins is the presence of rod-shaped bones surrounded by a thick layer of muscle in their pectoral and pelvic fins. The fins produce a power stroke to move the fish along. This strongly built fin is a useful pre- adaptation to life on land. Another useful adaptation is the ability to breathe air. This is relatively common in fish, especially those living in warm, shallow water, which is prone to become stagnant. Modern lung-fish can breathe air indefinitely. Sarcopterygians reached their maximum diversity during the Devonian, and also gave rise to amphibians in that period. They have formed a very minor component of fish faunas since that time. By the end of the Devonian period, lobe-fin diversity dwindled, and today only three lineages survive. One lineage, the coelacanths (Actinistia), was thought to have become extinct 75 million years ago. However, in 1938, fishermen caught a living coelacanth off the east coast of South Africa. The second lineage of living lobe-fins, the lungfishes (Dipnoi), is represented today by six species in three genera, all of which are found in the Southern Hemisphere. Lungfishes arose in the ocean but today are found only in fresh water, generally in stagnant ponds and swamps. They surface to gulp air into lungs connected to their pharynx. The third lineage of lobe-fins that survives today is far more diverse than the coelacanths or the lungfishes. During the mid-Devonian, these organisms adapted to life on land and gave rise to vertebrates with limbs and feet, called tetrapods—a lineage that includes humans.  Tetrapods Derived Characters of Tetrapods In place of pectoral and pelvic fins, tetrapods have limbs with digits. Limbs support a tetrapod’s weight on land, while feet with digits efficiently transmit muscle-generated forces to the ground when it walks. The head is separated from the body by a neck that originally had one vertebra on which the skull could move up and down. Later, with the origin of a second vertebra in the neck, the head could also swing from side to side. The bones of the pelvic girdle, to which the hind legs are attached, are fused to the backbone, permitting forces generated by the hind legs against the ground to be transferred to the rest of the body. Except for some fully aquatic species, the adults of living tetrapods do not have gills.  Tetrapods Origin of Tetrapods The Devonian coastal wetlands were home to a wide range of lobe-fins. Those that entered particularly shallow, oxygen-poor water could use their lungs to breathe air. Some species probably used their stout fins to help them move across logs or the muddy bottom. Thus, the tetrapod body plan did not evolve “out of nowhere” but was simply a modification of a pre-existing body plan. The recent discovery of a fossil called Tiktaalik (from Ellesmere Island, in the Canadian Arctic) has provided new details on how this process occurred. Like a fish, this species had fins, gills, and lungs, and its body was covered in scales. But unlike a fish, Tiktaalik had a full set of ribs that would have helped it breathe air and support its body. Also unlike a fish, Tiktaalik had a neck and shoulders, allowing it to move its head about. Finally, the bones of Tiktaalik’s front fin have the same basic pattern found in all limbed animals: one bone (the humerus), followed by two bones (the radius and ulna), followed by a group of small bones that comprise the wrist. Although it is unlikely that Tiktaalik could walk on land, its front fin skeleton suggests that it could prop itself up in water on its fins.  Tetrapods Surprisingly, however, new evidence proves that millions of years before the existence Tiktaalik, other vertebrates had evolved legs and feet with toes for walking. This evidence is in the form of numerous trackways of walking vertebrates with toes. These trackways were recently found in marginal marine tidal flat deposits of early Middle Devonian age in Poland. Perhaps these early walkers, which presumably were very early amphibians, spent a great deal of time partly immersed in the water along the seashore, keeping their skin moist at a time when land plants did not yet provide effective cover.  Amphibians The most likely ancestors of amphibians, and all other tetrapods, are a group of extinct lobefin fish known as rhipidistians. These fish have a similar skull morphology to the earliest amphibians and the pattern of limb bones common to all subsequent tetrapods, including humans. This is the pattern of one upper bone, two lower bones, and many peripheral bones in each limb. Amphibians typically lay their eggs in water or in moist environments on land; the eggs lack a shell and dehydrate quickly in dry air. With their strong fins and ability to breathe air, lobefin fishes were excellently pre-adapted to a life lived partly on land that could intermittently become problematic. Modern lungfish leave water to escape from such conditions and to find food. The two best-known early amphibians are called Ichthyostega and Acanthostega from late Devonian sediments from Greenland. They show a fascinating blend of fish-like and amphibian-like characteristics. Icthyostega had seven toes on its hindlimbs, and Acanthostega had eight digits on its forelimbs. It shows that the pentadactyl limb is a later development and is not one of the shared characteristics of the whole group. From this highly diverse assemblage two groups are important for the evolution of more modern tetrapods: temnospondyls, which gave rise to modern amphibians, and the reptilomorphs (included groups such as the anthracosaurs), which likely included the ancestor of reptiles, mammals, and birds. Ichthyostega  Amniotes The amniotes are a group of tetrapods whose extant members are the reptiles (including birds) and mammals. Reptiles evolved from amphibians during the Carboniferous. Derived Characters of Amniotes Amniotes are named for the major derived character of the clade, the amniotic egg, which contains four specialized membranes: the amnion, the chorion, the yolk sac, and the allantois. Called extraembryonic membranes because they are not part of the body of the embryo itself, these membranes develop from tissue layers that grow out from the embryo. The amniotic egg is named for the amnion, which encloses a compartment of fluid that bathes the embryo and acts as a hydraulic shock absorber. The other membranes in the egg function in gas exchange, the transfer of stored nutrients to the embryo, and waste storage. The amniotic egg was a key evolutionary innovation for terrestrial life: It allowed the embryo to develop on land in its own private “pond,” hence reducing the dependence of tetrapods on an aqueous environment for reproduction. In contrast to the shell-less eggs of amphibians, the amniotic eggs of most reptiles and some mammals have a shell. The shells of bird eggs are calcareous (made of calcium carbonate) and inflexible, while the eggshells of many other reptiles are leathery and flexible.  Amniotes Early Amniotes The most recent common ancestor of living amphibians and amniotes likely lived about 350 million years ago. No fossils of amniotic eggs have been found from that time, which is not surprising given how delicate they are. Based on where their fossils have been found, the earliest amniotes lived in warm, moist environments, as did the first tetrapods. Over time, however, early amniotes expanded into a wide range of new environments, including dry and high latitude regions. The earliest amniotes were small and had sharp teeth, a sign that they were predators. Later groups also included herbivores, as evidenced by their grinding teeth and other features. The earliest well-known reptile is called Hylonomus and found in the hollow tree stumps of a Carboniferous fossil forest in eastern Canada. Early reptiles were small in size, and this may be a reflection of their physiology. Modern reptiles are cold blooded. Such temperature regulation is much faster for a small animal.  Reptiles The reptile clade includes tuataras, lizards, snakes, turtles, crocodilians, and birds, along with a number of extinct groups, such as plesiosaurs and ichthyosaurs. Fossil evidence indicates that the earliest reptiles lived about 310 million years ago and resembled lizards. Reptiles have diverged greatly since that time, but as a group they share several derived characters that distinguish them from other tetrapods. For example, unlike amphibians, reptiles have scales that contain the protein keratin (as does a human nail). Scales help protect the animal’s skin from desiccation and abrasion. In addition, most reptiles lay their shelled eggs on land. Many reptiles that resembled the popular conception of sea monsters emerged in early Mesozoic seas. Among them were the placodonts, which, like many early Mesozoic fishes, were blunt-toothed shell crushers. Cousins of the placodonts were the nothosaurs which have been found in Early Triassic. Placodonts and nothosaurs did not survive the Triassic Period. The more fully aquatic plesiosaurs evolved from the nothosaurs in mid-Triassic time. The limbs of plesiosaurs were wing-like paddles that propelled them through the water. The most fishlike reptiles of Mesozoic seas were the ichthyosaurs. The last important group of early Mesozoic marine reptiles to evolve were early crocodiles. Reptiles  Reptiles The Origin and Evolutionary Radiation of Reptiles The oldest reptilian fossils, found in rocks from Nova Scotia, date from the late Carboniferous period. As reptiles diverged from their lizard-like ancestors, one of the first major groups to emerge were the parareptiles, which were mostly large, stocky, quadrupedal herbivores. Parareptiles died out by about 200 million years ago, at the end of the Triassic period. During the Carboniferous, reptiles radiated into the three main evolutionary lines: anapsids, synapsids, and diapsids. These are differentiated by their skull structure. The anapsid lineage is primitive and has no holes in the upper skull apart from the eye and nose apertures. It is represented today by modern turtles and tortoises. The synapsid lineage evolved next, and has a single hole in the upper skull behind the eye. This group evolved into mammal-like reptiles and eventually into mammals. The diapsids evolved later and are characterized by two openings in the skull behind the eye. The diapsids are composed of two main lineages. One lineage gave rise to the lepidosaurs, which include tuataras, lizards, and snakes. This lineage also produced a number of marine reptiles, including the giant mosasaurs. The other diapsid lineage, the archosaurs, produced the crocodilians, pterosaurs, and dinosaurs.  Anapsids A 2008 study reported the discovery of the oldest known fossil of the turtle lineage, dating to 220 million years ago. This fossil has a complete lower shell but an incomplete upper shell, suggesting that turtles may have acquired full shells in stages. The marine sediments in which this fossil was found also suggest that turtles may have originated in shallow coastal waters. However, as other scientists have argued, it is also possible that turtles originated on land and that the incomplete upper shell of this fossil may have been a specialized adaptation for an aquatic lifestyle. Permian and Triassic anapsids fall into three families. The Permian millerettids are known from South Africa, which was at temperate southern latitudes during that time. They were small, active, insectivores of moderate size, with skulls typically around 5 cm long. Late Permian and Triassic procolophonids lived in moderate to high southern latitudes and were omnivores or herbivores. Late Permian pareiasaurs are found in the northern hemisphere and could reach 2–3 m in length. They were heavily built herbivores. The oldest fossil turtles are late Triassic in age and they appear to have evolved either from the procolophonids or pareiasaurs.  Synapsids Synapsid, mammal-like reptiles dominated the late Carboniferous and Permian land masses. The first radiation was of the group known as pelycosaurs. The best known pelycosaur is Dimetrodon, with a skeleton characterized by a huge sail supported by extensions to its backbone. This sail probably helped the animal to maintain its preferred body temperature. Descendents of the pelycosaurs, therapsids, radiated widely in the late Permian. They stood more upright than reptiles, and their teeth were more highly differentiated, but in neither their posture nor their tooth patterns were they as advanced as mammals. Their descendents, the cynodonts, were common in the Triassic, and include the species Thrinaxodon, which shows evidence of having had whiskers. Pelycosaurs Although mammal-like reptiles were advanced in many ways, they all shared a sprawling gait with their amphibian ancestors. Synapsids were outcompeted by the ancestors of dinosaurs during the Triassic, and remained a minor group until after the end-Cretaceous extinction event. Cynodont Therapsids  Diapsids Lepidosaurs Pterosaurs One surviving lineage of lepidosaurs is represented by two They ranged in size from a wingspan of a few centimeters species of lizard-like reptiles called tuataras. Fossil evidence to over 15 m. indicates that tuatara ancestors lived at least 220 million years They were active flyers, and had narrow, membranous wings ago. attached to a modified fourth finger and probably to their The other major living lineage of lepidosaurs consists of the thighs. lizards and snakes, or squamates. Snakes descended from lizards The aerodynamics of their wings suggests that they were with legs. Today, some species of snakes retain vestigial pelvic and predominantly gliders and soarers. limb bones, providing evidence of their ancestry. They were covered in hair, and their highly energetic lifestyle means that they must have been warm blooded. Alligators and Crocodiles Alligators and crocodiles (collectively called crocodilians) belong to a lineage that reaches back to the late Triassic. The earliest members of this lineage were small terrestrial quadrupeds with long, slender legs. Later species became larger and adapted to aquatic habitats, breathing air through their upturned nostrils. Dinosaurs What is a dinosaur? Dinosaurs are archosaurs, a clade that includes crocodilians, pterosaurs, and birds. The archosaur lineage originated approximately 245 million years ago, just a few million years after the devastating Permian- Triassic mass extinction. The most important feature is related to the bones in its hips and legs. They had straight back legs, perpendicular to their bodies. This allowed them to use less energy to move than other reptiles that had a sprawling stance like today's lizards and crocodiles. Their weight was also better supported. Though descendants of reptiles, they were actually quite different from reptiles. In fact, they were in many ways more similar to birds. Lizard, Crocodile Dinosaur Dinosaur origins The dinosaurs were members of the Dinosauromorpha which evolved early in the Triassic Period. The dinosaurs inherited their advanced locomotory ability from early dinosauromorphs. Early dinosauromorphs probably spent much of their time standing or walking on all fours, but some were adapted to rise up and run on two legs. The upper portion of the legs of many early dinosauromorphs extended straight downward beneath their bodies rather than sprawling slightly out to the side, as they did in therapsids. This feature, which facilitated running, was passed on to the dinosaurs and seems to have been a key to their success. The first dinosaurs resembled those early dinosauromorphs that sometimes traveled on their two hind legs, but their skulls were differently formed, and their teeth were more highly developed. Dinosaurs did not become gigantic until Jurassic time, but a few dinosaur taxa attained large body sizes during the Triassic. The crocodiles, like the dinosaurs, evolved from early dinosauromorphs late in the Triassic Period. The two groups shared terrestrial habitats until the end of the Triassic, after which the dinosaurs rose to much greater prominence. The first dinosaurs appeared in the Late Triassic, around 225Ma. They were small, agile, and bipedal at first. The oldest dinosaur fossils are known from the Triassic period and have been found primarily in the Ischigualasto and Santa Maria Formations of Argentina and Brazil, and the Pebbly Arkose Formation of Zimbabwe. Herrerasaurus and Eoraptor, the best-known of the earliest dinosaurs, were carnivores living in Argentina alongside a fauna dominated by rhynchosaurs (early reptiles), with synapsids present Herrerasaurus too. Eoraptor Dinosaur classification Dinosaurs have been divided into saurischians (“lizard-hipped”) and ornithischians (“bird-hipped”), based on the arrangement of their hip bones. In saurischians, the two lower bones of the hip point away from each other, as in lizards, and in ornithischians, the two lower hip bones both point backwards, as in birds. Ironically, birds by ancestry are saurischians rather than ornithischians—their evolution of true “bird hips” occurred after the saurischian–ornithischian split. The saurischians are divided into theropods and sauropodomorphs. Theropods are the classic bipedal carnivorous dinosaurs, and also include birds. Sauropodomorphs include the enormous quadrupedal, herbivorous long-necked, long- tailed sauropods and their cousins the “prosauropods”. Ornithischians include several very different groups. Ornithischian pelvis with a closed structure Saurischian pelvis with an open structure  Sauropodomorphs They constitute the second great radiation of dinosaurian herbivores (after Ornithischians). They are characterised by enlarged narial opening, an unusual position for the longest pedal claw—on the first digit rather than the middle toe, leaf-shaped teeth. They were herbivores (early forms maybe omnivores). Primitive forms were bipeds; later forms were so large they were obligate quadrupeds. The most primitive known sauropodomorph is Saturnalia of the Late Triassic of Brazil. By the Late Triassic, sauropodomorphs had split into two distinctive groups: prosauropods and sauropods. Narial opening Enlarged digit Saturnalia Prosauropods Prosauropods were the dominant large-bodied herbivores on land from the Late Triassic through the Early Jurassic. Early sauropods were traditionally called prosauropods, but that term has started to become unpopular. Some like to use the term sauropodomorphs. The best example of an early sauropodomorph is Plateosaurus (a so-called “prosauropod). Riojasaurus, a Late Triassic prosauropod from South America, is one of only a few basal prosauropods that retain a short neck. It was also unusually large at 10 m. Prosauropods were typically about 6 m long. Other prosauropods from the Early Jurassic, such as Massospondylus, have proportionately longer cervical vertebrae, as does the Plateosaurus. Opposing teeth did not contact one another, and all the grinding must have been done in a gizzard (masses of small stones have been found inside the skeletons of several prosauropods). Prosauropods have long, lightly built necks and heads. They were clearly adapted to browse high in vegetation. Only Riojasaurus, the largest, was always quadrupedal because of its weight, but it had a very long neck to compensate. Prosauropods were the first animals to browse on vegetation high above the ground, and they represent a completely new ecological group of herbivores exploiting an important new resource in the zone up to perhaps 4 m above ground. The same adaptation was re-evolved later in sauropods, and again in mammals such as the giraffe. Riojasaurus Massospondylus Plateosaurus Sauropods Sauropods were rare in the Early Jurassic, when ornithischians appear to have undergone their major radiation, but diversified rapidly during the Middle Jurassic after prosauropods had gone extinct. Sauropods are the largest land animals that ever evolved. Sauropods were all herbivores. They had small heads and very long necks that allowed them to browse on anything Shunosauru