Historical perspectives on the evolution of tetrapodomorph movement (original) (raw)
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Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 2019
The hypothesis that tetrapods evolved from elpistostegids during the Frasnian, in a predominantly aquatic context, has been challenged by the discovery of Middle Devonian tetrapod trackways predating the earliest body fossils of both elpistostegids and tetrapods. Here I present a new hypothesis based on an overview of the trace fossil and body fossil evidence. The trace fossils demonstrate that tetrapods were capable of performing subaerial lateral sequence walks before the end of the Middle Devonian. The derived morphological characters of elpistostegids and Devonian tetrapods are related to substrate locomotion, weight support and aerial vision, and thus to terrestrial competence, but the retention of lateral-line canals, gills and fin rays shows that they remained closely tied to the water. Elpistostegids and tetrapods both evolved no later than the beginning of the Middle Devonian. The earliest tetrapod records come from inland river basins, sabkha plains and ephemeral coastal lakes that preserve few, if any, body fossils; contemporary elpistostegids occur in deltas and the lower reaches of permanent rivers where body fossils are preserved. During the Frasnian, elpistostegids disappear and these riverine-deltaic environments are colonised by tetrapods. This replacement has, in the past, been misinterpreted as the origin of tetrapods.
Follow the footprints and mind the gaps: a new look at the origin of tetrapods
Earth and Environmental Science Transactions of the Royal Society of Edinburgh
ABSTRACTThe hypothesis that tetrapods evolved from elpistostegids during the Frasnian, in a predominantly aquatic context, has been challenged by the discovery of Middle Devonian tetrapod trackways predating the earliest body fossils of both elpistostegids and tetrapods. Here I present a new hypothesis based on an overview of the trace fossil and body fossil evidence. The trace fossils demonstrate that tetrapods were capable of performing subaerial lateral sequence walks before the end of the Middle Devonian. The derived morphological characters of elpistostegids and Devonian tetrapods are related to substrate locomotion, weight support and aerial vision, and thus to terrestrial competence, but the retention of lateral-line canals, gills and fin rays shows that they remained closely tied to the water. Elpistostegids and tetrapods both evolved no later than the beginning of the Middle Devonian. The earliest tetrapod records come from inland river basins, sabkha plains and ephemeral c...
Ever since Owen: changing perspectives on the early evolution of tetrapods
2008
The traditional notion of a gap between fishes and amphibians has been closed by a wealth of fish-like fossil tetrapods, many discovered since the mid 1980s. This review summarizes these discoveries and explores their significance relative to changing ideas about early tetrapod phylogeny, biogeography, and ecology.
Early tetrapod relationships revisited
Biological Reviews, 2003
In an attempt to investigate differences between the most widely discussed hypotheses of early tetrapod relationships, we assembled a new data matrix including 90 taxa coded for 319 cranial and postcranial characters. We have incorporated, where possible, original observations of numerous taxa spread throughout the major tetrapod clades. A stem-based ( total-group ) definition of Tetrapoda is preferred over apomorphy-and node-based ( crown-group ) definitions. This definition is operational, since it is based on a formal character analysis. A PAUP* search using a recently implemented version of the parsimony ratchet method yields 64 shortest trees. Differences between these trees concern : ( 1) the internal relationships of aïstopods, the three selected species of which form a trichotomy ; ( 2) the internal relationships of embolomeres, with Archeria crassidisca and Pholiderpeton scutigerum collapsed in a trichotomy with a clade formed by Anthracosaurus russelli and Pholiderpeton attheyi ; ( 3 ) the internal relationships of derived dissorophoids, with four amphibamid species forming an unresolved node with a clade consisting of micromelerpetontids and branchiosaurids and a clade consisting of albanerpetontids plus basal crown-group lissamphibians ; ( 4 ) the position of albenerpetontids and Eocaecilia micropoda, which form an unresolved node with a trichotomy subtending Karaurus sharovi, Valdotriton gracilis and Triadobatrachus massinoti ; (5 ) the branching pattern of derived diplocaulid nectrideans, with Batrachiderpeton reticulatum and Diceratosaurus brevirostris collapsed in a trichotomy with a clade formed by Diplocaulus magnicornis and Diploceraspis burkei. The results of the original parsimony run -as well as those retrieved from several other treatments of the data set (e.g. exclusion of postcranial and lower jaw data ; character reweighting ; reverse weighting ) -indicate a deep split of early tetrapods between lissamphibian-and amniote-related taxa. Colosteids, Crassigyrinus, Whatcheeria and baphetids are progressively more crownward stem-tetrapods. Caerorhachis, embolomeres, gephyrostegids, Solenodonsaurus and seymouriamorphs are progressively more crownward stem-amniotes. Eucritta is basal to temnospondyls, with crown-lissamphibians nested within dissorophoids. Westlothiana is basal to Lepospondyli, but evidence for the monophyletic status of the latter is weak. Westlothiana and Lepospondyli form the sister group to diadectomorphs and crown-group amniotes. Tuditanomorph and microbrachomorph microsaurs are successively more closely related to a clade including proximodistally : ( 1 ) lysorophids ; ( 2 ) Acherontiscus as sister taxon to adelospondyls ; ( 3 ) scincosaurids plus diplocaulids ; ( 4) urocordylids plus aïstopods. A data set employing cranial characters only places microsaurs on the amniote stem, but forces remaining lepospondyls to appear as sister group to colosteids on the tetrapod stem in several trees. This arrangement is not significantly worse than the tree topology obtained from the analysis of the complete data set. The pattern of sister group relationships in the crownward part of the temnospondyl-lissamphibian tree re-emphasizes the important role of dissorophoids in the lissamphibian origin debate. However, no specific dissorophoid can be identified as the immediate sister taxon to crown-group lissamphibians. The branching sequence of various stem-group amniotes reveals a coherent set of internested character-state changes related to the acquisition of progressively more terrestrial habits in several Permo-Carboniferous forms.
The Greatest Step in Vertebrate History: A Paleobiological Review of the Fish‐Tetrapod Transition*
Physiological and biochemical zoology, 2004
Recent discoveries of previously unknown fossil forms have dramatically transformed understanding of many aspects of the fish-tetrapod transition. Newer paleobiological approaches have also contributed to changed views of which animals were involved and when, where, and how the transition occurred. This review summarizes major advances made and reevaluates alternative interpretations of important parts of the evidence.We begin with general issues and concepts, including limitations of the Paleozoic fossil record.We summarize important features of paleoclimates, paleo- environments, paleobiogeography, and taphonomy. We then review the history of Devonian tetrapods and their closest stem group ancestors within the sarcopterygian fishes. It is now widely accepted that the first tetrapods arose from advanced tetrapodomorph stock (the elpistostegalids) in the Late Devonian, probably in Euramerica. However, truly terrestrial forms did not emerge until much later, in geographically far-flung regions, in the Lower Carboniferous. The complete transition occurred over about 25 million years; definitive emergences onto land took place during the most recent 5 million years. The sequence of character acquisition during the transition can be seen as a five-step process involving: (1) higher osteichthyan (tetrapodomorph) diversification in the Middle Devonian (beginning about 380 million years ago [mya]), (2) the emergence of “prototetrapods” (e.g., Elginerpeton) in the Frasnian stage (about 372 mya), (3) the appearance of aquatic tetrapods (e.g., Acanthostega) sometime in the early to mid- Famennian (about 360 mya), (4) the appearance of “eutetrapods” (e.g., Tulerpeton) at the very end of the Devonian period (about 358 mya), and (5) the first truly terrestrial tetrapods (e.g., Pederpes) in the Lower Carboniferous (about 340 mya). We discuss each of these steps with respect to inferred functional utility of acquired character sets. Dissociated heterochrony is seen as the most likely process for the evolutionarily rapid morphological transformations required. Developmental biological processes, including paedomorphosis, played important roles. We conclude with a discussion of phylogenetic interpretations of the evidence.
Three-dimensional limb joint mobility in the early tetrapod Ichthyostega
Nature, 2012
The origin of tetrapods and the transition from swimming to walking was a pivotal step in the evolution and diversification of terrestrial vertebrates. During this time, modifications of the limbs-particularly the specialization of joints and the structures that guide their motions-fundamentally changed the ways in which early tetrapods could move 1-4 . Nonetheless, little is known about the functional consequences of limb anatomy in early tetrapods and how that anatomy influenced locomotion capabilities at this very critical stage in vertebrate evolution. Here we present a three-dimensional reconstruction of the iconic Devonian tetrapod Ichthyostega and a quantitative and comparative analysis of limb mobility in this early tetrapod. We show that Ichthyostega could not have employed typical tetrapod locomotory behaviours, such as lateral sequence walking. In particular, it lacked the necessary rotary motions in its limbs to push the body off the ground and move the limbs in an alternating sequence. Given that long-axis rotation was present in the fins of tetrapodomorph fishes 5-7 , it seems that either early tetrapods evolved through an initial stage of restricted shoulder 8,9 and hip joint mobility or that Ichthyostega was unique in this respect. We conclude that early tetrapods with the skeletal morphology and limb mobility of Ichthyostega were unlikely to have made some of the recently described Middle Devonian trackways 10 .