Reassessing a cryptic history of early trilobite evolution

Area of Science:

• Paleontology
• Evolutionary Biology
• Marine Biology

Background:

• Trilobites are key Paleozoic marine arthropods, appearing suddenly in the Cambrian fossil record (~521 Ma).
• The abrupt appearance of trilobites has long been debated, with a prevailing hypothesis of an unobserved Precambrian evolutionary history.
• Previous models assumed a cryptic history, leading to issues like convergent evolution of biomineralization and challenges to clade synapomorphies.

Purpose of the Study:

• To critically review the assumptions behind the cryptic evolutionary history model for trilobites.
• To investigate alternative explanations for the early Cambrian appearance and diversification of trilobites.
• To re-evaluate palaeobiogeographic patterns in early trilobites in light of modern dispersal data.

Main Methods:

• Review of existing literature and fossil record data concerning trilobite origins and diversification.
• Analysis of early Cambrian trilobite palaeobiogeographic patterns.
• Comparison of vicariance and dispersal models for marine invertebrate distributions.

Main Results:

• The cryptic history model presents significant evolutionary challenges, including extensive convergent evolution and the need to abandon established synapomorphies.
• Vicariance-based explanations for early Cambrian trilobite distribution are inconsistent with modern marine invertebrate dispersal patterns.
• Survivorship bias is proposed as a potential factor explaining the rapid diversification observed in early trilobites.

Conclusions:

• The sudden appearance of trilobites at ~521 Ma likely reflects their actual evolutionary origins, not a hidden Precambrian past.
• Rethinking assumptions about trilobite origins and diversification is necessary, emphasizing survivorship bias over cryptic evolution.
• The study challenges long-held views and provides a more parsimonious explanation for trilobite's early Cambrian prominence.