Was Mesosaurus fully aquatic?

A new paper by Demarco, Meneghel, Laurin and Piñeiro 2018
asks, “Was Mesosaurus (Fig. 1) a fully aquatic reptile?“ The authors report, “Mesosaurs are widely thought to represent the earliest fully aquatic amniotes,” but conclude, “more mature individuals might hypothetically have spent time on land. In this study, we have found that the variation of the vertebral centrum length along the axial skeleton of Mesosaurus tenuidens fits better with a semi-aquatic morphometric pattern, as shown by comparisons with other extinct and extant taxa.”

The authors report methods
“We measured the centrum length for each available vertebra in the mesosaur skeletons. All measurements were taken on digital images.” They also looked at Claudiosaurus and Thadeosaurus (Fig. 1), but did not conduct a phylogenetic analysis that included these and other closest sisters to Mesosaurus in the large reptile tree (LRT, 1263 taxa). For comparison, the authors looked at the unrelated vertebral profiles of Cotylorhynchus, Casea, Varanus and Varanops. 

All of the ancestors to Mesosaurus in the LRT
kept four functioning legs, so terrestrial locomotion remained within their abilities. That seems pretty clear. At Anarosaurus (Fig. 1) the Sauropterygia split off with Pachypleurosaurus and Diandongosaurus at the base. At Brazilosaurus the Thalattosauria + Ichthyosaurus split off with Wumengosaurus (Middle Triassic)  and Serpianosaurus (Middle Triassic) at the base. That means taxa from Galephyrus to Wumengosaurus had their genesis prior to the Early Permian, in the Late Carboniferous. That gives time enough for basal ichthyosaurs, like Grippia, to appear in the Early Triassic. This is a prediction that can be tested and confirmed with new discoveries in the Late Carboniferous.

Note that basal marine younginiform diapsids
are basal to the clade Enaliosauria, which includes mesosaurs, sauropterygians, thalattosaurs and ichthyosaurs in the LRT. Mesosaurs were not basal anapsids (contra Demarco et al. 2018 and all prior authors dealing with mesosaurs).

The authors report,
“The evidence suggests thatMesosaurus may have been slightly amphibious rather than strictly aquatic, at least when it attained a large size and an advanced ontogenetic age, though it is impossible to determine how much time was spent on land and what kind of activity was performed there. Thus, it is impossible to know if mesosaurids came onto land only to bask, like seals or crocodiles, or if they were a bit more agile.”

Since mesosaurs still had limbs, hands and feet,
we can imagine/surmise that they were able to crawl about on land. Based on their proximity to thalattosaurs and ichthyosaurs and the derivation from basal sauropterygians, they were aquatic as well.

It is noteworthy
that sauropterygians and ichthyosaurs experienced live birth. So, it is not surprising that mesosaurs, nesting between them, were also viviparous (Piñeiro et al. 2012).

Interesting
that mesosaurs despite their derived nesting, predate their late-surviving phylogenetic ancestors. This demonstrates the incompleteness of the fossil record and the likelihood of finding phylogenetic ancestors in earlier strata, which happens all the time…

References
Demarco PN, Meneghel M,  Laurin M and Piñeiro G 2018. Was Mesosaurus a fully aquatic reptile? Frontiers in Ecology and Evolutiion 6:109. doi: 10.3389/fevo.2018.00109
Piñeiro G, Ferigolo J, Meneghel, M and  Laurin M 2012. The oldest known amniotic embryos suggest viviparity in mesosaurs. Historical Biology. 24 (6): 620–630. doi:10.1080/08912963.2012.662230