Assessing the completeness of the fossil record: comparison of different methods applied to parareptilian tetrapods (Vertebrata: Sauropsida)

Abstract

As paleontological studies are generally distorted by gaps and biases in the fossil record, it is important to assess its completeness. Here we address the fossil record of Parareptilia, a Permian–Triassic amniote clade, applying two measures of specimen completeness: the skeletal completeness metric (SCM) and the character completeness metric (CCM). The SCM quantifies how much of the skeletal material of a taxon is preserved, whereas the CCM measures the amount of phylogenetic information available. The latter was implemented using two different approaches. In this study, we compare three completeness metrics. Two CCM implementations show a strong correlation with each other, but only the second implementation of the CCM correlates significantly with the SCM, possibly due to character selection in phylogenetic data sets. There is no correlation between diversity of parareptiles and their completeness, implying that the observed fluctuations in diversity are not driven by the completeness of the fossils. The mean completeness of parareptiles through time is consistently high compared with previously studied tetrapod clades, suggesting that most parareptile taxa are based on reasonably complete specimens. Clade-specific differences reveal no link between body size and completeness. However, the analyses confirm the impact of ecology, with aquatic mesosaurids being better preserved than terrestrial taxa.

Skeletal completeness of the non‐avian theropod dinosaur fossil record

“…); synapsid‐grade pelycosaurs (Brocklehurst & Fröbisch ); parareptiles (Verrière et al . ); and bats (Brown et al ). Silhouettes used include work by S. Hartman, and D. Bogdanov (see http://phylopic.org for full licensing information).…”

“…Verrière et al . () examined only genus‐level taxa of parareptiles, and this may potentially explain the higher completeness of parareptiles in relation to all other terrestrial groups.…”

“…This was further developed by Verrière et al . (), who modelled bone volumes using more precise natural shapes and mapping two‐dimensional outlines, representing each cranial bone, onto the external surface area of the skull (truncated pyramid) to obtain percentage volumes for each.…”

“…), parareptiles (Verrière et al . ), pelycosaurs (Brocklehurst & Fröbisch ) and sauropodomorphs (Mannion & Upchurch ) and so they were also compared to the distribution of theropod SCM2.…”

Does exceptional preservation distort our view of disparity in the fossil record?

“…Tutin & Butler [48] found a significant relationship between skeletal completeness and character completeness metrics for plesiosaurs, and between plesiosaur and ichthyosaur completeness, suggesting common controls on skeletal preservation in both groups; this could feasibly include a lagerstätten bias. Lagerstätte biases in completeness may not be present for all groups; however, Verriere et al [49] found no evidence for lagerstätte effects on parareptile diversity and in turn no correlation between their diversity and skeletal or character completeness metrics.…”