A CAMBRIAN ORIGIN FOR CRINOIDS? PROBABILISTIC MODELING OF EVOLUTIONARY TREES SUPPORTS A LONG FUSE OF LATENT DIVERSIFICATION ACROSS THE CAMBRIAN-ORDOVICIAN BOUNDARY

Unraveling the origin of higher taxa is one of the oldest and most frequently addressed research topics in paleontological studies yet it is perhaps one of the most poorly constrained and least understood problems in macroevolution. For example, clade origins may be studied solely with respect to the historical patterns of apomorphy acquisition (requiring a tree and model of character change) or by investigating the timing of lineage splitting between clades recognized by distinct synapomorphies (requiring taxonomy and evolutionary models but may or may not explicitly use trees). Many current approaches combine temporal data from fossil occurrences with a phylogenetic hypothesis to estimate divergence times within a fully tree-based framework. Unfortunately, phylogenetic methods often require outgroup criteria to provide directionality to branching relationships, which can be problematic when sister group relationships are unknown and/or homologies are uncertain. Moreover, divergence time estimates are poorly estimated when nodes are simply dated as the FAD of their oldest descendants.

The origin of the Class Crinoidea (Echinodermata) has been a subject of major debate for the last 20 years, when Ausich & Babcock (1998) first argued that the oldest tentative crinoid, Echmatocrinus brachiatus Sprinkle 1973, was probably an octocoral. More recently, another debate has emerged as to whether the Crinoidea are sister to edrioasteroids (such as edrioblastoids) or to the diploporitan Eumorphocystis.

Using an empirically constrained crinoid suptertree and occurrence data of Ordovician species (N=597), including the oldest representatives (e.g., Guensburg & Sprinkle 2003), I apply multiple probabilistic approaches to estimate the age of the MRCA of total-group Crinoidea. Importantly, I do not condition on outgroup selection. Results indicate crinoids likely originated during the latest Cambrian. This does not falsify either sister group hypothesis, but it does suggest a substantial “latent” diversification may have taken place during the Cambrian-Ordovician transition. Although the latest Cambrian is characterized by a highly incomplete echinoderm record, additional studies searching for the oldest crinoid fossils are warranted, especially in geographically undersampled paleocontinents.