TESTING COPE?S RULE IN ASAPHIDS (TRILOBITA) BY USING COMPARATIVE METHODS

MEROI ARCERITO, FACUNDO RENÉ; HALPERN K.; SFERCO M.E.; WAISFELD, B.G.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

IANIGLA - CCT Mendoza

Asaphids are significant members of the trilobite Evolutionary Ibexian II Fauna. The family radiated during the early Tremadocian, and thrived worldwide during most of the Ordovician, until vanishing at the Late Ordovician mass extinction. Thus, the study of their paleobiology is paramount in understanding the diversification, extinction, and ecological replacement within Trilobita during this period. From Cambrian to Ordovician, Trilobita in general and Asaphidae in particular registered an important body size increment that has been interpreted by some authors as an example of Cope?s rule. We have carried out a preliminary analysis that showed an increasing body size tendency through time in probably related asaphid species from NW Argentina (Cordillera Oriental, Western Gondwana). Here we present a more comprehensive analysis by using phylogenetic comparative methods to test for Cope?s rule among asaphids. Being a cosmopolitan family, several taxa from Late Cambrian (Stage 10) to Early Ordovician (Tremadocian-Floian) records from around the world were included in our database, however, we have emphasized on asaphid taxa from NW Argentina (i.e. Asaphellus, Notopeltis and Kayseraspis), where the first stages of asaphid radiation can be traced throughout a few number of potentially closely related taxa. In order to assess if ancestor-descendant relationships bear a tendency in body size, we first conducted a phylogenetic analysis using parsimony in TNT that resulted in 6 most parsimonious trees of 197 steps; then, each tree was age calibrated considering LAD (Last Appearance Date) of each taxon. On the other hand, pygidium size was used as a proxy for estimating body size throughout an average geometric mean calculated for each species. Both data sets were contrasted by testing the individual fitting of three evolutionary models in R, namely Brownian motion, Brownian motion with a trend and White noise or stasis. Unlike previous reports, our results do not support any phylogenetic tendency in body size. At least at this taxonomic level, the observed body size increment does not follow Cope?s rule. Instead, we find that the White noise model has a better fit to our data (Akaike weight= 0,9994), implying that species attributes have no phylogenetic signal and can be thought as independent draws from a normal distribution with stable means and variance across the time interval.