Comment on "The early evolution of the tetrapod humerus".
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Animals↗
Biomedical subjects
P E Ahlberg
Publications and source records attributed to P E Ahlberg.
A uniquely specialized ear in a very early tetrapod.
The Late Devonian genus Ichthyostega was for many decades the earliest known tetrapod, and the sole representative of a transitional form between a fish and a land vertebrate. However, despite being known since 1932 (ref. 1) from a large collection of specimens, its morphology remained enigmatic and not what was expected of a very primitive tetrapod. Its apparent specializations led it to be considered as a "blind offshoot" or "sidebranch" off the tetrapod family tree, and recent cladistic analyses have disagreed about its exact phylogenetic position within the tetrapod stem group. In particular, its braincase and ear region defied interpretation, such that conventional anatomical terms seemed inapplicable. Using new material collected in 1998 (ref. 9), preparation of earlier-collected material, and high-resolution computed tomography scanning, here we identify and interpret these problematic anatomical structures. They can now be seen to form part of a highly specialized ear, probably a hearing device for use in water. This represents a structurally and functionally unique modification of the tetrapod otic region, unlike anything seen in subsequent tetrapod evolution. The presence of deeply grooved gill bars as in its contemporary Acanthostega suggest that Ichthyostega may have been more aquatically adapted than previously believed.
Adaptation, Physiological↗
A primitive sarcopterygian fish with an eyestalk.
The discovery of two Early Devonian osteichthyan (bony fish) fossils has challenged established ideas about the origin of osteichthyans and their divergence into actinopterygians (teleosts and their relatives) and sarcopterygians (tetrapods, coelacanths, lungfishes and related groups). Psarolepis from China and an unnamed braincase from Australia combine derived sarcopterygian and actinopterygian characters with primitive features previously restricted to non-osteichthyans, suggesting that early osteichthyan evolution may have involved substantial parallellism between sarcopterygians and actinopterygians. But interpretation of these fossils has been hampered by poor phylogenetic resolution. Here we describe a basal sarcopterygian fish, Achoania gen, et sp. nov., that fills the morphological gap between Psarolepis and higher sarcoptergyians. We also report the presence of eyestalk attachments in both Achoania and Psarolepis, showing that this supposedly non-osteichthyan feature occurs in basal sarcopterygians as well as the actinoptergyian-like Australian braincase.
Animals↗
Zebrafish in context: uses of a laboratory model in comparative studies.
With the recent interest in the reintegration of evolutionary and developmental biology has come a growing need for understanding the phylogenetic relations and degree of generality of the model organisms upon which we rely so heavily. In vertebrate biology the zebrafish Danio rerio has become a paradigmatic system for studies at levels of organization from molecular to interspecific. Studies of model systems in development are often techniques-driven rather than questions-based; however, informative hypotheses for developmental research can be derived from phylogenetic distributions of characters. With some understanding of how general the characters of interest are, a thoughtful comparison of the requirements of the questions with the lists of available embryos, reagents, and protocols can guide choices of new vertebrate models. We describe here the phylogenetic placement of zebrafish within the vertebrate world and discuss how generally observations on zebrafish can be taken to apply. We outline a practical protocol for investigating development in a comparative context, illustrated with an example from an ongoing study of teleost tail fin evolution. The principles and procedures presented here apply equally well to any comparative study with an interest in evolution, at any level of phylogeny from intraspecific studies to comparisons across phyla.
Animals↗