PARAREPTILE COMPLETENESS


higher CCMb than either of the other two metrics (Fig. 6). For the first three of these clades, and to a lesser extent for procolopho- noids, very little postcranial material is known or at least incorporated in the current phylo- genetic data sets. Thus, the phylogenetic matrices focus on cranial characters by neces- sity, which tends to artificially inflate the CCMb, since a poorly preserved postcranium will not have a substantial negative effect. In nycteroleteroids and pareiasaurs, the two


CCM metrics have similar values, and the SCM is lower, this difference being statistically significant in pareiasaurs. In this case, the difference between the SCM and the CCMa is likely due to the fact that a specimen can receive a complete CCMa score even if the entire skeleton is not preserved. For example, only one complete humerus is required to score characters relating to the humerus, so a fossil preserving a complete humerus will receive the complete CCMa score for the humerus. In contrast, the SCM score will reflect the fact that only one of the two humeri is actually preserved. Pareiasaurs and nycteroleteroids are known from incomplete skeletons and not only from skulls (Supplementary Table 2). Several elements of the postcranial skeleton, including limbs, girdles, and the vertebral column, are known, but not entirely, and often the paired elements are not both preserved. Mesosaurids are the only group to have a


lower CCMa than SCM. Two of three genera, Mesosaurus and Stereosternum, are known from virtually complete skeletons. Their CCMa values are slightly lower than theirSCMvalues but still very high (97.1% and 96.4% for the CCMa vs. 99.7% and 99.6% for the SCM, respectively). In contrast, the gap between the SCM and the CCMa in the third genus, Brazilosaurus, is large enough to explain the discrepancy: this genus has a CCMa of 54.3% while its SCM is 74.9%, a difference of more than 20%. Brazilosaurus is known only from a skull, some limb fragments, and several almost complete vertebral columns (including the tail) with ribs. In the CCMa, the vertebrae and ribs together attribute 5.0% of the completeness to the genus, whereas in the SCM, the presacral ribs alone are 20.1% of the volume of the skeleton, and the presacral vertebrae 17.8%.


691


In contrast, the completeness of the skull is significantlymore important in the CCMa than in the SCM (Table 4). Mesosaurs are also the parareptile clade


with the highest completeness in all metrics. The explanation is probably both ecological and taphonomical. Mesosaurids are aquatic animals; as previously mentioned they are one of the earliest groups of amniotes to readopt an aquatic lifestyle. Living in an aquatic environ- ment with a high sedimentation rate, they were more likely to be preserved and in better condition than terrestrial parareptiles. This may also explain why their fossils are much more abundant; 30 specimens of Mesosaurus alone have been described according to the Paleobiology Database (Uhen and Sessa 2013), but the totalnumber of unpublished specimens in collections is much higher, in the hundreds. Nevertheless, this link between an aquatic


lifestyle and a high completeness does not seem to affect another parareptile clade comprising aquatic animals, the lanthanosuchoids. Indeed, their completeness results are some of the lowest among parareptiles (Fig. 6). Several hypotheses can be formulated to explain this phenomenon. First, not all lanthanosuchoids are regarded as aquatic; Feeserpeton, Acleistorhinus,and Colobomycter (together representing half of the lanthanosuchoid genera included in this study) are only known from skulls and were probably terrestrial. This can partly explain why the overall completeness of lanthanosuchoids is so low. But although the three other genera are generally considered to be aquatic, they too are only known from skulls (except Lanthaniscus, which also includes some vertebrae). Actually, no paleoecological study has ever been carried out on lanthanosuchoids. Those assumed to be


aquatic are designated as such because their flat and broad skulls resemble those of stego- cephalians (Ivakhnenko and Kurochkin 2008). Their way of life is not known well enough to confidently state whether their environment should have had an impact on their complete- ness or not. In addition, body size does not seem to


have an effect on the completeness of parareptile fossils. For example, pareiasaurs, some of the heaviest and largest tetrapods of the late Permian, have completeness values similar to


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