Abstract
The last common ancestor of all living arthropods had biramous postantennal appendages, with an endopodite and exopodite branching off the limb base. Morphological evidence for homology of these rami between crustaceans and chelicerates has, however, been challenged by data from clonal composition and from knockout of leg patterning genes. Cambrian arthropod fossils have been cited as providing support for competing hypotheses about biramy but have shed little light on additional lateral outgrowths, known as exites. Here we draw on microtomographic imaging of the Cambrian great-appendage arthropod Leanchoilia to reveal a previously undetected exite at the base of most appendages, composed of overlapping lamellae. A morphologically similar, and we infer homologous, exite is documented in the same position in members of the trilobite-allied Artiopoda. This early Cambrian exite morphology supplements an emerging picture from gene expression that exites may have a deeper origin in arthropod phylogeny than has been appreciated.
Highlights
The last common ancestor of all living arthropods had biramous postantennal appendages, with an endopodite and exopodite branching off the limb base
This suggested that the biramous limb in crustaceans differs from the condition seen in many Cambrian arthropods, which was reinterpreted as a uniramous limb and an exite rather than an endopodite and exopodite, respectively[5]
We present evidence for a morphologically distinctive exite in Cambrian arthropods exposed by computed microtomography of fossils from the Cambrian (Series 2, Stage 3) Chengjiang Biota of Yunnan, China
Summary
The last common ancestor of all living arthropods had biramous postantennal appendages, with an endopodite and exopodite branching off the limb base. Correspondences between biramous appendages of crustaceans and Palaeozoic arthropods such as trilobites underpin the presumed homology of the endopodite, exopodite and protopodite and their origin at or deeper than the root of the arthropod crown group[1,2].
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