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Lecture 9

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ZOO 2090
Fred Laberge

Lecture 9 October 7, 2013 ZOO*2090 Tetrapods Objectives - Discuss the different hypotheses about the origin of tetrapods - Describe the anatomical trends from lobe-finned fishes to early tetrapods - Explain how modifications to the appendicular skeleton enabled locomotion on land - Describe early modes of locomotion in tetrapods Sarcopterygian - the “classic” view – Romer - Rhipidistia and Actinistia together form Crossoterygii which is a sister group of tetrapods with lungfish as an outgroup - Left  lungfishes and coelacanths as a sister group of tetrapods - Right  osteolepiformes as sister group of tetrapods - Crossopterygii has no meaning here - included more recent discoveries, but they maintained Crossopterygii - however, molecular data suggests that lungfishes are more closely related to tetrapods than the coelacanth (if you take only the living form) - tetropodomorpha  tetrapods + some advanced sarcopterygians What is a tetrapod? - vertebrate that possess a chiridium (muscular limb with well- defined joints and digits) - some authors use the term stegocephalians, which comprises the tetrapods (amphibians + amniotes) and other digit-bearing vertebrates (i.e., the early “tetrapods”) Labyrinthodonts (paraphyletic grouping) - early tetrapods have characteristics teeth (shared with some rhipidistians) - cross section of tooth showing enamel folds Acanthostega (extinct) - late Devonian (Greenland) Lecture 9 October 7, 2013 ZOO*2090 - shares with rhipidistians structure of vertebrae around notochord - but limbs have digits (8 fingers, 8 toes) - girdles are designed to bear weight - tail with fin rays - internal gills suggest it was aquatic Ichtyostega (extinct) - late Devonian (Greenland) - similar to Acanthostega - more robust; but lack internal gills (may have used only lungs) Temnospondyls (extinct) - large group of extinct tetrapods that appeared in the Carboniferous – with some forms persisting into Cretaceous - they typically have robust bodies and flat skulls - occupied a wide range of ecological niches, many were capable of excursions or life on land - they are considered by some authors as giving rise to modern amphibians Anthracosaurs (extinct) - embolomers and seymouriamorphs - medium to large size reptile-like semi-aquatic or terrestrial predators of mid-Carboniferous to Triassic - seymouriamorphs had aquatic larvae with external gills and terrestrial adults - considered reptiliomorphs (amphibians) by some authors  branches closer to amniotes than batrachomorph amphibians Lepospondyls (extinct) - diverse small tetrapods - present from Carboniferous to Permian that occupied specialized ecological niches - derived eel/snake-like forms are considered highly paedomorphic and aquatic - some of the lizard/newt-like microsaurs appear more terrestrial - teeth lack labyrinthine enamel folds - considered by most authors as giving rise to modern amphibians Locomotion - in a large animal like Eogyrinus the limbs and girdles were too small and poorly ossified for walking on land Lecture 9 October 7, 2013 ZOO*2090 - the limbs were likely used as points of pivot to support lateral flexions of the body in shallow water, as proposed for lobe-finned fishes - in other tetrapods the limbs and girdles of adults are massive and extensively ossified , suggesting a terrestrial habitat - but how did they move? – possibilities: o trot  simultaneous contact with the ground of diagonally opposing limbs  movement forward is ba
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