These legs were made for walking: 150 years of trilobite appendage morphology and evolution

Trilobites are an iconic part of the Paleozoic macroscopic shelly fossil record, with their biodiversity estimated at over 22,000 known species and a lengthy evolutionary history spanning 270 million years. Although direct evidence of the non-biomineralized ventral appendages of polymerid trilobites is only known from 41 species ranging from the early Cambrian to Middle Devonian, sites with exceptional preservation worldwide reveal substantial morphological and functional variability in this clade. Here, we exhaustively document all known instances of preserved trilobite appendicular morphology to date and explore their major evolutionary trends. Trilobites bore a pair of uniramous antennae followed by a series of biramous ventral appendages, the latter composed of a protopodite (appendage base), endopodite (walking leg) and exopodite (gill). Only 14 trilobite species show evidence of well-preserved and nearly complete appendage series. Comparisons between the biramous appendages suggests modular evolution between the endopodite and exopodite. Whereas endopodite organization is largely stable between trilobite groups of different ages, exopodite morphology is highly variable in number and width of articles, number and size lamellae and size relative to endopodite, likely in response to selective pressures from their environment and function (respiration vs. swimming). We also note a shift in protopodite morphology over trilobite evolutionary history. Cambrian redlichiids were typified by transversely elongate protopodites with short stud-like spines well-suited to durophagy, whereas this functional morphology appears to be absent in post-Cambrian trilobites. Our overview of trilobite appendage morphology, preservation and spatiotemporal distribution highlights the usually overlooked anatomical and functional variability of this successful euarthropod clade, and the biases that influence our understanding of their intricate evolutionary history.