THE ROLE OF FOSSILS IN DECIPHERING TRENDS IN METAZOAN DISPARITY

There are multiple hypotheses regarding the patterns of morphological diversity during the Early Paleozoic, but few empirical tests at the kingdom-level. Results based on fossil organisms at lower taxonomic levels indicate the majority of clades exhibit an initial peak in disparity. However, it is unclear if this pattern scales to the level of metazoans or if the processes associated with fossilization exaggerate or diminish this trend. A newly constructed metazoan morphospace containing 212 extant organisms from 34 phyla coded for 1790 discrete characters was used to test how the loss of morphological information during fossilization changes our perception of disparity, particularly in reference to the Cambrian Explosion.

The effect of fossilization on metazoan morphospace was determined using two methods, 1) by coding eight well-preserved Cambrian taxa and culling the datasets to the characters that could be coded for those animals, and 2) excluding characters unlikely to be observable in fossil organisms based on the morphologic characteristics of the features. In both cases, fossilization reduced the number of characters that could be coded by 50% and the resulting morphospace emphasizes the relative disparity within skeletonized phyla (e.g. vertebrates and arthropods), while shrinking the relative disparity of all other phyla. This exaggerates the importance of skeletal and gross anatomical characters that are preserved in instances of exceptional preservation. In addition, overall fossilizable metazoan disparity is increased by 40% with respect to the original dataset indicating our perception of metazoan disparity is amplified during the initial metazoan diversification in association with the origin of fossilizable characters. These results support the idea that the ‘Cambrian Explosion’ may more so represent an explosion in fossilizable characters and fossils rather than a dramatic increase in phenotypic disparity.

It is likely that since the anatomical features that underlie the deep connections within metazoans are not preserved, trends in kingdom-level metazoan disparity, unlike those at the class or phylum-level, cannot be addressed with fossil taxa or skeletal features alone.