Chordate Origins and Phylogeny PDF

Chordate Origins and Phylogeny § Comparative vertebrate anatomy - the study of structure, of the function of structure, & of the range of variation in structure & function among vertebrates: § Kingdom: Animal § Phylum: Chordata § Subphylum: Vertebrata § Vertebrate characteristics: § 1 - notocho...

Chordate Origins and Phylogeny § Comparative vertebrate anatomy - the study of structure, of the function of structure, & of the range of variation in structure & function among vertebrates: § Kingdom: Animal § Phylum: Chordata § Subphylum: Vertebrata § Vertebrate characteristics: § 1 - notochord (at least in the embryo) § 2 - pharynx with pouches or slits in wall (at least in the embryo) § 3 - dorsal, hollow nervous system § 4 - vertebral column § Notochord = rod of living cells ventral to central nervous system & dorsal to alimentary canal § Fate of notochord during development: § Head region - incorporated into floor of skull § Trunk & tail - surrounded by cartilaginous or bony vertebrate (except in Agnathans) § Adults: § Fishes & amphibians - notochord persists the length of the trunk & tail but is constricted within the centrum of each vertebra § Reptiles, birds, & mammals - notochord almost disappears during development (e.g., remains as a pulpy nucleus in the vertebrae of mammals) § Protochordates - notochord remains as the chief axial skeleton § Agnathans - lateral neural cartilages are located on notochord lateral to the spinal cord § § Pharynx - region of alimentary canal exhibiting pharyngeal pouches in embryo; pouches may open to the exterior as slits: § permanent slits - adults that live in water & breathe via gills § temporary slits - adults live on land § Dorsal, hollow central nervous system - consists of brain & spinal cord & contains a central cavity (called the neurocoel) §Vertebrate beginnings § Among the oldest & best known = ostracoderms § fishes that occurred in the late Cambrian period (see The Cambrian Explosion) through the Devonian (about 400 - 525 million years before present) § had bony plates and scales (&, therefore, were easily fossilized) § jawless vertebrates called 'armored fishes' §Before ostracoderms? Myllokunmingia fengjiaoa (pictured below) & Haikouichthys ercaicunensis - primitive fish that have many similarities to living hagfishes and are the oldest vertebrates (530 mybf) ever found. § Before Vertebrates? § Cathaymyrus diadexus (literally the 'Chinese eel of good fortune') is not the fossil of an eel. At just 5 cm long, but 535 m.y. old, it is the earliest known chordate (fossil shown below; for a 'reconstruction' § Researchers think that Cathaymyrus is a fossil relative of modern lancelets (amphioxus). Non-vertebrate chordates still alive today include tunicates (or sea squirts; urochordates) & amphioxus (or branchiostoma). (cephalochordates) § Phylum Chordata - established in 1874 & included organisms with: § 1 - notochord 2 - pharyngeal pouches or slits § 3 - dorsal, hollow nervous system § 4 - cells that produce the hormone thyroxine § Subphylum Urochordata = tunicates Chordate 'ancestor' of vertebrates: § sessile (like adult tunicates) § tail evolved as adaptation in larvae to increase mobility § 'higher forms' - came about by retention of tail (neoteny) § Tunicate larva - also called 'sea squirt' § notochord is confined to the tail § notochord is lost during metamorphosis into sessile adult § possess pharyngeal slits § A 530 million-year-old (although perhaps as old as 560 million years) creature, Cheungkongella ancestralis, probably a tunicate, found in the Chengjiang fauna in China's southwest Yunnan Province, might be the earliest known fossil evidence of primitive chordates (Shu, D.-G., L. Chen, J. Han, X.-L. Zhang. 2001. An early Cambrian tunicate from China. Nature 411:472 - 473.) § Subphylum Cephalochordata= Amphioxus (or Branchiostoma) Vertebrate features: § notochord § dorsal, hollow nervous system § pharyngeal gill slits § 'circulatory' system - vertebrate pattern with 'pumping vessels' (but no heart) §Hemichordates = acorn worms § Bateson added acorn worms to the phylum Chordata in 1884 because they have: § 1 - a dorsal, hollow nervous system 2 - gill slits § 3 - a short diverticulum of the gut called the stomochord § Present consensus = the stomochord is not homologous with the notochord and Hemichordates are placed in a separate phylum Possible invertebrate ancestors: § 1 - annelid worms Evidence for: § bilateral symmetry § segmented § central nervous system with brain & longitudinal nerve cord Evidence against: nerve cord is solid nerve cord is ventral § 2 - echinodermata - chordate characteristics include: § radial cleavage - blastomeres in adjacent tiers lie directly above one another (as opposed to spiral cleavage) § anus forms near or at blastopore (deuterostomous) § mesoderm arises as outpocketing of the gut wall § indeterminate cleavage (i.e., fate of blastomeres isn't predetermined) § Phylum: Chordata Subphylum: Vertebrata Superclass: Pisces § Class Agnatha Class Placodermii Class Chondricthyes Class Acanthodii Class Osteichthyes § Superclass: Tetrapoda § Class Amphibia Class Reptilia Class Aves Class Mammalia Agnathans vs. Gnathostomes: § semicircular canals § agnathans have 1 or 2 § gnathostomes have 3 § jointed, paired lateral appendages § agnathans have none § gnathostomes do § jaws § agnathans have none § gnathostomes do Class Agnatha § Orders: § 1 - Osteostraci § 2 - Anaspida § 3 - Thelodonti § 4 - Galeaspida § 5 - Pituriaspida § 6 - Petromyzontia (lampreys) § 7 - Myxinoidea (hagfishes) Ostracoderms (Osteostraci, Anaspida, Heterostraci, & Coelolepid :) § 1 - extinct Paleozoic (Cambrian to Devonian) jawless fish with an external skeleton of bone ('bony armor') 2 - oldest known vertebrates 3 - many had flattened appearance (some may have been bottom- dwellers) Cyclostomes (Petromyzontia & Myxinoidea): Lampreys - parasitic with horny, rasping teeth (see drawing at right) Hagfishes - primarily scavengers  Gnathostomes Acanthodians: § 1 - earliest known gnathostomes (Silurian; about 440 mybp) § 2 - probably related to modern bony fishes § 3 - small (less than 20 cm long) with large eyes § 4 - Acanthodians most likely died out because of the rapidly increasing number of ray-finned fishes and sharks during the Permian Class Placodermii: § 1 - Silurian (about 420 million years before present) § 2 - probably off the main line of vertebrate evolution § 3 - many had bony dermal shields § 4 - some were probably predators (with large, sharp 'tooth plates') §Placoderms armored jawed fishes first appeared about 420 million years ago (MYA) during the Silurian Period. diversified dramatically by the beginning of the Devonian and came to dominate most marine and freshwater ecosystems before becoming extinct at the end of that period (355 MYA). About 200 genera have been discovered Placoderms (= plated skin) were named for their heavy armor of dermal bone, which formed large shields on the head and thorax. The rest of their bodies was covered with small bony scales or was without dental armor. The head and trunk shields of most placoderms were articulated by bony joints. This joint apparently allowed the forward part of the skull to tilt up, increasing the gape. Placoderms lacked teeth, but biting or grinding structures are often be found in the dermal bones lining their mouths. Placoderms evolved into a variety of body forms in a relatively short time. Many were torpedo-shaped, but there were notable expections, including the flatten Phyllolepida and the bottom feeding Antiarchi. Most placoderms were less than 30 cm (2 feet) in length, but some members of the dinichthyids (= terrible fish) reached or exceeded 6 m (20 ft), making them the first giants of the vertebrate lineage. Class Chondrichthyes - cartilaginous fishes § 1 - ancestors had bony skeletons so cartilaginous skeleton is specialized § 2 - pelvic fins of males are modified as claspers § 3 - placoid scales § 4 - numerous today but more abundant in the past Subclass Elasmobranchii - most common cartilaginous fishes § O. Cladoselachii - primitive sharks (300-400 mybp) § O. Selachii - 'modern' sharks § O. Batoidea - rays & skates Elasmobranchs: 1 - 1st pharyngeal slit modified as a spiracle 2 - naked gill slits (no operculum) 3 - mouth located ventrally Subclass Holocephali § O. Chimaeriformes (photo & drawing) § marine § gill slits have a fleshy operculum & the spiracle is closed § few scales § common ancestor with sharks but an independent line Class Osteichthyes - bony fishes § 1 - skeleton is partly or chiefly bone 2 - gill slits are covered by a bony operculum § 3 - skin has scales with, typically, little bone § 4 - most have a swim bladder § 5 - ray-finned or lobe-finned Subclass Actinopterygii - ray-fins Superorder Chondrostei § most primitive ray-fins § chiefly Paleozoic (300-400 mybp) § include present-day Sturgeons & Paddlefish (below) Superorder Neopterygii § Order Semionotiformes § dominant Mesozoic fishes § possess ganoid scales § two extant genera: 1 = operculum, 2 = dorsal fin, § Lepidosteus - predatory; includes present-day 3 = caudal peduncle (The gars narrow section of a fish's § Amia - includes present-day bowfins (or dogfish) body directly anterior to the insertion of the tail but § Division Teleostei - modern ray-finned fishes before the mid-body.), § recent bony fishes 4 = caudal fin or "tail", § 95% of all living fish 5 = anal fins, § about 40 living orders 6 = pelvic fins, & 7 = pectoral § well-ossified skeleton fins § cycloid & ctenoid scales (flexible & overlapping) § pelvic fins often located far forward § no spiracle Subclass Sarcopterygii - lobe-finned fishes Crossopterygii - chiefly Paleozoic except Latimeria § 1 - resemble early amphibians § 2 - skeleton of fin lobe corresponds closely to proximal skeletal elements of early tetrapod limbs § 3 - skull similar to that of early amphibians § 4 - had swim bladders that may have been used as lungs Dipnoi - lungfish (3 living genera; Africa, Australia, & South America) § § African & South American species have inefficient gills & will drown if held under water § Australian species (Neoceratodus spp.) relies on gills unless oxygen content of water is too low Class Amphibia Oldest known = subclass Labyrinthodontia Fish-like features: 1- small bony scales in the skin 2- fin-rays in the tail (for swimming) 3- a skull similar to that of some Crossopterygians 4- a sensory canal system (like the lateral line system) that indicates a primarily aquatic existence § Subclass Lepospondyli § ancestry uncertain due to lack of fossil evidence § probably on a 'side branch' of vertebrate evolution § Subclass Lissamphibia - modern amphibians Anura - frogs & toads Urodela - tailed amphibians Gymnophiona (apodans) - wormlike, burrowing amphibians Modern amphibian characteristics: § 1 - aquatic larval stage with external gills § 2 - middle ear cavity with ear ossicle (columella) § 3 - no bony scales (except apodans) Class Reptilia - the first amniotes: § 1 - scaly § 2 - clawed § 3 - large, yolk-laden, shell-covered eggs laid on land Stem reptiles = Cotylosaurs (about 300 mybp) Reptile Subclasses: § 1 - Anapsida Cotylosauria - stem reptiles Chelonia - turtles & tortoises § unchanged for about 175 million years § identified by bony dermal plates to which ribs & trunk vertebrae are fused § 2 - Lepidosauria Rhynchocephalia (Sphenodonta) - only living representative is the Tuatara Squamata - lizards, geckos, & snakes § 3 - Archosauria § Thecodontia - stem archosaurs Pterosauria Saurischia - 2 major groups: sauropods & theropods (check this short video) Ornithischia (like Iguanodon) Crocodilia 4 - Euryapsida - marine reptiles, includes the plesiosaurs & ichthyosaurs 5 - Synapsida Pelycosauria - first stage in evolution to mammals Therapsida § Reptile subclasses - classified in part according to presence or absence of temporal openings § Synapsid type = mammal-like reptiles § Anapsid type = stem reptiles & turtles § Diapsid type = rhynchocephalians, lizards, & snakes § Euryapsid type = extinct plesiosaurs. Temporal fenestration has long been used to classify amniotes. Taxa such as Anapsida, Diapsida, Euryapsida, and Synapsida were named after their type of temporal fenestration. Temporal fenestra are large holes in the side of the skull. The function of these holes has long been debated. Many believe that they allow muscles to expand and to lengthen. The resulting greater bulk of muscles results in a stronger jaw musculature, and the longer muscle fibers allow an increase in the gape. Class Aves - birds 1 - may have arisen from an archosaurian reptile, perhaps a small bipedal dinosaur 2 - lost several dinosaur characteristics (e.g., long tail & teeth) but retained others (e.g., claws, scales, diapsid skull, single occipital condyle &, perhaps, feathers) (see AMNH website & ABC News website) Subclass Archaeornithes Genera: Archaeopteryx & Archaeornis Characteristics: 1 - solid bones 2 - weakly developed keel &, probably, weakly developed flight muscles Subclass Neornithes Superorder Odontognathae § extinct § many features of modern birds (e.g., hollow bones & short tail) Superorder Paleognathae § ratites § small wings but powerful leg muscles § Superorder Neognathae - birds adapted for sustained flight Modifications to reduce weight include: § loss of some bones § pneumatic bones § reduced tail § loss of teeth § loss of urinary bladder Class Mammalia Characteristics: 1 - hair 2 - mammary glands 3 - 3 middle ear bones 4 - muscular diaphragm 5 - sweat glands 6 - marrow within bones 7 - 2 sets of teeth 8 - biconcave, enucleate red blood cells 9 - well-developed cerebral cortex Subclass Prototheria - egg-laying mammals Monotremata - platypus + 2 spiny anteaters § 1 - lay eggs 2 - testes within the abdominal cavity 3 - no pinna 4 - no corpus callosum 5 - less stable body temperature § Subclass Theria Infraclass Metatheria Marsupialia - pouched mammals; young born alive, but at a very immature stage Infraclass Eutheria - placental mammals The Vertebrate 'Family Tree':