GEOL 433 Fishes and Basal Tetrapods 2024 PDF

FISHES AND BASAL TETRAPODS GEOL 433 PHYLOGENETIC REL ATIONSHIPS  VERTEBRATES The skeleton of vertebrates is made of bone and cartilage. Bone consists of a network of collagen fibers on which needle-like crystals of hydroxyapatite (a form of apatite, calcium phosphate, C...

FISHES AND BASAL TETRAPODS GEOL 433 PHYLOGENETIC REL ATIONSHIPS  VERTEBRATES The skeleton of vertebrates is made of bone and cartilage. Bone consists of a network of collagen fibers on which needle-like crystals of hydroxyapatite (a form of apatite, calcium phosphate, CaPO4) accumulate. Hence bone has a flexible component and a hard component, which explains why bones may undergo a great deal of strain before they break. Cartilage is a flexible, usually unmineralized tissue, containing collagen and elastic tissues. The first vertebrates probably had a cartilaginous skeleton. The internal skeleton of vertebrates allows them to grow very large because the skeleton can grow with the animal. The vertebrate skeleton is maintained and remodeled constantly within the body.  CONODONTS The commonest early vertebrates were the conodont animals. Conodonts are microfossils that are the jaw elements of a jawless basal fish which are common in certain marine facies from the Cambrian to the end of the Triassic. Typically just the separate conodont elements are found, but groups or assemblages of conodonts occur which represent all of the elements of one conodont animal. About ten complete conodont animals with soft-tissue preservation and all the conodont elements have been found. They were eel-like animals about 5 cm long and had a short lobed head with large eyes. Conodonts are commonly used in biostratigraphy. M AIN T Y P E S OF C ON OD ON T ELEMENTS CONODONT ELEMENTS CONODONT ASSEMBLAGES AND THE CONODONT ANIMAL CONODONT ANIMAL CONODONTS AND STRATIGRAPHY  CLASSIFICATION OF FISHES “Class Agnatha”: A paraphyletic group of jawless fishes, including armored and unarmored Paleozoic ostracoderms, and modern lampreys and hagfishes; Early Cambrian to Recent. Class Placodermi: Heavily armored fishes with jaws and a hinged head shield; Mid-Silurian to Late Devonian. Class Chondrichthyes: Cartilaginous fishes, including modern sharks and rays; Late Ordovician to Recent. Includes the acanthodians which are small fishes with many spines and large eyes that range from Late Ordovician to Early Permian Class Osteichthyes: Bony fishes, with ray fins (Subclass Actinopterygii) or lobe fins (Subclass Sarcopterygii), the later including ancestors of the tetrapods; Late Silurian to Recent.  JAWLESS FISHES The vertebrate head is unique in providing an organized structure that contains the brain, the major sense organs and the mouth. The first vertebrates had no jaws. The jawless fishes are sometimes referred to as ostracoderms. Ostracoderms were jawless, they were generally armored, although some were not, and they had their heyday in the Devonian. The oldest examples are from the Early Cambrian Chengjiang biota in China. Sacabambaspis from the Mid-Ordovician is the oldest well-preserved fish. Ostreostracans like Hemicyclaspis have a semicircular head shield bearing openings on top for the eyes and nostrils, as well as porous regions on the sides that may have served for the passage of electrical sense organs. Heterostracans like Pteraspis are more streamlined in shape, and were perhaps more active swimmers. Both of these forms have their mouths underneath the head shield, and they probably fed by sieving organic matter from the sediment. These armored jawless fishes died out at the end of the Devonian, and their place was taken by fishes with jaws. Jawless fish still exist today, with about 50 species of lampreys and hagfishes. MODERN LAMPREY BASAL VERTEBRATE MYLLOKUNMINGIA, A JAWLESS FISH FROM EARLY CAMBRIAN OF CHINA  EARLY JAWLESS FISHES Mid-Ordovician Sacabambaspis Devonian Hemicyclaspis Devonian Pteraspis  FOSSIL SACABAMBASPIS LIFE RECONSTRUCTION OF SACABAMBASPIS  FOSSIL HEMICYCLASPIS LIFE RECONSTRUCTION OF HEMICYCLASPIS FOSSIL PTERASPIS LIFE RECONSTRUCTION OF PTERASPIS  JAWED FISHES Jaws probably evolved during the Ordovician. Study of the anatomy of modern vertebrates suggests that jaws may have evolved from the strengthening bars of cartilage or bone between the gill slits. The transition cannot be followed in fossils because the gill skeleton of jawless fishes was not mineralized. Some of the oldest jaw-bearing fishes were the placoderms, such as Coccosteus, which had an armor of large bony plates over the head and shoulder region. The osteichthyans, or bony fishes, also appeared in the Devonian. There are two groups: (1) those with ray-like fins, the actinopterygians, ancestors of most fishes today from carp to salmon, and seahorses to tuna; and (2) the lobefins, the sarcopterygians, that had thick, muscular, limb like fins. Today the lobefins are rare, being represented by only three species of lungfishes, and the rare coelacanth.  JAWED FISHES OF THE DEVONIAN Placoderm Coccossteus Acanthodian Climatius Actinopterygian Cheirolepsis Lungfish Dipterus Lobefin Osteolepis  EVOLUTION OF RAY-FINNED BONY FISHES AND JAWS Carboniferous palaeonisciform Cheirodus Triassic neopterygian Semionotus Cretaceous teleost Mcconichthus Evolution of actinopterygian jaws  EVOLUTION OF SHARKS AND RAYS During the Carboniferous, numerous extraordinary shark- like fishes arose, and these were important marine predators. A second shark radiation took place in the Triassic and Jurassic. Hybodus was a fast-swimming fish, capable of accurate steering using its large pectoral fins. It is rare to find whole shark fossils because the bulk of the skeleton is cartilaginous and is not preserved. The apatite teeth and scales are more commonly found isolated, and these and other fish teeth and scales, sometimes called ichthyoliths, are useful in biostratigraphy. Modern sharks, called collectively neoselachians, are faster swimmers and more ferocious flesh eaters than their precursors. Neoselachians radiated dramatically during the Jurassic and Cretaceous to reach their modern diversity of 1100 species. These sharks can open their mouths wider than their precursors, and range in size from 1 to 16 m long. The skates and rays, unusual neoselachians, are specialized for life on the seafloor. ANCIENT AND MODERN SHARKS AND RAYS FOSSIL FISH TEETH AND SCALES  THE ORIGIN OF TETRAPODS: FINS TO LIMBS When a fish evolved into a land animal, it already had lungs as well as gills. So, the main problem for the first tetrapods, the four-legged land vertebrates, was support – in water, an animal “weighs” virtually nothing, but on land the body has to be held up from the ground, and the internal organs have to be supported. In addition, reproductive, osmotic (water balance) and sensory systems had to adapt, but this did not happen all at once. Tetrapods arose from lobefin fishes, which have complex bony and muscular pectoral (front) and pelvic (back) paired fins. Close study of lobefins such as Osteolepis and Eusthenopteron shows that they share many characters of the limbs and skull with the earliest tetrapods. The Late Devonian lobe-finned fish Tiktaalik is representative of the transition between fish and early tetrapods such as Acanthostega and Ichthyostega which ranged up to 1 m long. Their heads are still very fish-like in shape and sculpturing and their limbs and tail are still adapted for swimming. LATE DEVONIAN TIKTAALIK TIKTAALIK AND EVOLUTION OF TETRAPODS  SKULL OF THE LATE DEVONIAN AMPHIBIAN ACANTHOSTEGA FINS AND LEGS OF THE FIRST TETRAPODS  AMPHIBIANS Amphibians today include 7200 living species of mainly small animals that live in or close to water. They show many adaptations to life on land, but they still rely on water for breeding and water balance. Frogs and toads (anurans), known since the Triassic, have specialized in jumping. Other modern amphibians include the urodeles – salamanders and newts, which date from the Jurassic, and the caecilians, which look like earthworms and also date from the Jurassic. All living amphibians appear to be closely related, forming a clade, the Lissamphibia. These are part of a larger clade, the Subclass Batrachomorpha, which includes an important paraphyletic group, the temnospondyls. The second amphibian lineage, the Subclass Reptiliomorpha, included important groups in the Carboniferous and Permian, as well as the ancestors of reptiles, birds and mammals. Some Permian reptilomorphs, such as Seymouria, were adapted to a fully terrestrial life.  FOSSIL AMPHIBIANS Early Permian temnospondyl Eryops Early Triassic temnospondyl Benthosuchus Early Permian reptiliomorph Seymouria  EVOLUTION OF REPTILES Amphibians only made it halfway on to land, and they still produce swimming tadpoles. Reptiles and their descendants made a clean break from the water by evolving a kind of egg that did not have to be laid in water. The cleidoic (“closed”) egg, sometimes called the amniotic egg, is enclosed within a tough semipermeable shell, hence its name. This egg type is seen in all members of the clade Amniota (reptiles, birds, mammals): it is the familiar hen’s egg that we eat for breakfast. Primitive mammals, such as the platypus, still lay eggs, but most mammals have suppressed the egg and it “hatches” inside the mother’s womb. The amniote eggshell is usually hard and made from calcite, but some lizards and snakes have leathery eggshells. The shell retains water, preventing evaporation, but allows the passage of gases, oxygen in and carbon dioxide out. Inside the shell is a set of membranes and food is in the form of yolk. THE CLEIDOIC EGG OF AMNIOTES  EARLY EVOLUTION OF AMNIOTES Amniotes radiated during the Late Carboniferous, giving rise to three main clades These are distinguished by the pattern of openings in the side of the skull, especially the temporal openings behind the eye socket. The primitive state is termed the anapsid (“no arch”) skull pattern, since there are no temporal openings. The two other skull patterns seen in amniotes are the synapsid (“same arch”), where there is a lower temporal opening, and the diapsid (“two arch”) pattern, where there are two temporal openings. Tha Anapsida include various early forms such as Hylonomus (the oldest-know amniote), as well as some Permian and Triassic reptiles, and possibly the turtles. The Synapside include the “mammal-like reptiles”, and the mammals, and the Diapsida includes a number of early groups, as well as the lizards and snakes and the crocodiles, pterosaurs, dinosaurs and birds. All modern amniotes produce the cleidoic egg, and the structure is so similar that we can be sure this egg arose at the base of clade Amniota.  HYLONOMOUS AND AMNIOTE SKULL PATTERNS PHYLOGENY OF THE MAJOR GROUPS OF FISHES AND TETRAPODS  FOSSIL AND RECENT ANAPSID REPTILES Procolophon Proganochelys Fossil Snapping Turtle  SYNAPSIDS OF THE PERMIAN Dimetrodon Lycaenops Dicynodon  TRANSITION TO THE MAMMALS Early Triassic cynodont Thrinaxodon Early Jurassic mammal Megazostrodon Skull of an early synapsid Skull of a mammal MARINE VERTEBRATES AND EVOLUTIONARY FAUNAS