Reassessing a cryptic history of early trilobite evolution

Abstract

Trilobites are an iconic Paleozoic group of biomineralizing marine euarthropods that appear abruptly in the fossil record (c. 521 million years ago) during the Cambrian ‘explosion’. This sudden appearance has proven controversial ever since Darwin puzzled over the lack of pre-trilobitic fossils in the Origin of Species, and it has generally been assumed that trilobites must have an unobserved cryptic evolutionary history reaching back into the Precambrian. Here we review the assumptions behind this model, and suggest that a cryptic history creates significant difficulties, including the invocation of rampant convergent evolution of biomineralized structures and the abandonment of the synapomorphies uniting the clade. We show that a vicariance explanation for early Cambrian trilobite palaeobiogeographic patterns is inconsistent with factors controlling extant marine invertebrate distributions, including the increasingly-recognized importance of long-distance dispersal. We suggest that survivorship bias may explain the initial rapid diversification of trilobites, and conclude that the group’s appearance at c. 521 Ma closely reflects their evolutionary origins.

Contrasting patterns of disparity suggest differing constraints on the evolution of trilobite cephalic structures during the Cambrian ‘explosion’

“…In contrast, the slower‐than‐random expansion of glabellar and eye ridge disparity from an initially small area of morphospace is more suggestive of a real diversification event. This hypothesis is supported by the proposal of a close relationship between the earliest (bigotinid and fallotaspidoid) trilobites based on similarities in these structures (Geyer 2019 b ), and is consistent with the recent suggestion by Holmes and Budd (2022) that trilobites probably arose close to when they appear in the fossil record.…”

“…1996; Lieberman 2003). In fact, the earliest faunas contain a low diversity of probably closely related trilobites (bigotinids and fallotaspidoids), which exhibit only limited provincialism (Holmes & Budd 2022). Regardless, it is important to consider how the presence or absence of a cryptic history might affect the observed development of disparity in the earliest trilobites.…”

Quantitative ichnology: a novel framework to determine the producers of locomotory trace fossils with the ichnogenus Gordia as a case study

“…Stratigraphic chart of different Gordia ichnospecies and their putative trace makers. Purella cristata (Parkhaev 2017), trilobites (Holmes & Budd 2022), Phragmochaeta canicularis (Conway Morris & Peel 2008) and Palaeonema phyticus (Poinar 2001) are considered to be the earliest appearances in the body fossil record of uncontroversial molluscs, arthropods, polychaetes and nematodes, respectively. Gordia with abrupt turns resembling Gordia indianaensi , Gordia marina from Río Huso and Río Alagón sections of the Cíjara Formation (Jensen & Palacios 2016), and Gordia nodosa from the Fjord Formation are considered as the earliest appearances of these ichnospecies.…”

Cambrian origin but no early burst in functional disparity for Class Bivalvia

“…Unlike trilobites [12] and several echinoderm classes [13], bivalve taxonomic diversity apparently remained low through the Cambrian, producing at most four genera across the first 36 Myr of the Class's evolutionary history [14,15] (electronic supplementary material, figure S1). Bivalves did participate in the ensuing Great Ordovician Biodiversification Event, establishing all six of their major clades [16,17] and generating 176 genera among 50 families (electronic supplementary material, figure S1), before suffering significant losses in the end-Ordovician mass extinction that we treat here as the end of this initial phase of the group's evolution.…”