SVP abstracts – Origin of aquatic reptiles?

Sobral and Schoch 2019
bring us news on a taxon at the genesis of aquatic reptiles.

I presume that means
no sea turtles, no marine iguanas, no mosasaurs, no sea crocs, no penguins. If so, the LRT already provides a long list of diapsid taxa at the base of the Enaliosauria (Fig. 1; including mesosaurs, ichthyosaurs, thalattosaurs, placodonts, pachypleurosaurs and plesiosaurs) along with other basal aquatic marine younginiforms (Fig. 2), a monophyletic clade distinct from terrestrial younginiforms that gave rise to protorosaurs and archosauriforms.

Figure 3. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria)

Figure 1. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria)

From the abstract:
“The Middle Triassic was a time of major changes in terrestrial tetrapod faunas, but the fossil record of this interval is largely obscure.”

Why do paleontologists always paint themselves into a corner like this? To make their discoveries more newsworthy?

“This is unfortunate, since many modern groups originated or diversified during this time. However, recent excavations in the upper Middle Triassic of Germany have revealed several new taxa, most of which are much smaller than those found in other tetrapod-bearing basins of similar age.”

Here’s Galesphyris (Fig. 2) at the base of the aquatic younginiforms in the LRT.

Figure 3. Spinoaequalis and descendant marine younginiformes.

Figure 3. Spinoaequalis and descendant marine younginiformes. These give rise to plesiosaurs, placodonts, mesosaurs, ichthyosaurs and thalattosuchians. Click to enlarge.

Sobral and Shoch continue:
“Here, we report a new taxon from the Vellberg limestone quarry comprised of skull bones distinct from other diapsids from this locality. It is diagnosed by a long maxilla with a far posteriorly reaching tooth row; a long and stout ventral process of the postfrontal; exclusion of the postorbital from the lower temporal fenestra due to a contact between the anteroventral process of the squamosal and the dorsal process of the jugal; and a tall quadrate + quadratojugal complex.”

“Some anatomical aspects of the new taxon are similar to stem diapsids such as Elachistosuchus huenei from similar deposits of Northern Germany and of uncertain phylogenetic affinity.”

In the LRT Elachistosuchus (Fig. 3) nests certainly between proterosuchids and choristoderes (Fig. 4). Neither are related to aquatic younginiforms.

Figure 1. Elachistosuchus (Janensch 1949, Sobral et al. 2015) is a sister to BPI 2871, a basal choristodere.

Figure 3. Elachistosuchus (Janensch 1949, Sobral et al. 2015) is a sister to BPI 2871, a basal choristodere.

Figure 4. This is where Elachistosuchus nests, next to BPI 2871, at the base of the Choristodera.

Figure 4. This is where Elachistosuchus nests, next to BPI 2871, at the base of the Choristodera.

“A phylogenetic analysis retrieved both taxa in an “ichthyosauromorph” clade, included in an almost exclusively aquatic group. The new taxon, Hupehsuchus, and Elachistosuchus appear as successive sister-taxa to Ichthyopterygia.”

This is not supported by the LRT where Hupehsuchus (Fig. 5) and Elachistosuchus (Fig. 3) are not related  to one another. The outgroups to the Ichthyopterygia (Fig. 1) are the Thalattosuchia, Mesosauria and basal Sauropterygia (pachypleurosaurs).

Figure 2. Basal Ichthyosauria, including Wumengosaurus, Eohupehsuchus, Hupehsuchus and Thaisaurus

Figure 5. Basal Ichthyosauria in the LRT, including Wumengosaurus, Eohupehsuchus, Hupehsuchus and ThesaurusFigure 2. Basal Ichthyosauria, including Wumengosaurus, Eohupehsuchus, Hupehsuchus and Thaisaurus

“It is interesting to note that many of the autapomorphic characters of the new taxon pertain to elements related to the lower temporal fenestra. In particular, the contact between the jugal and squamosal is unusual, but is also found in sauropterygians, saurosphargids, Hupehsuchus, and Wumengosaurus, as well as in rhynchocephalians.”

Oh, why did they have to add rhynchocephalians? They were doing so well! Readers beware, convergence is rampant (= everywhere) in the Reptilia. Don’t rely on one, two or a dozen traits. If you do, you’ll be pulling a Larry Martin. Only rely on the last common ancestor in a valid cladogram to determine relationships.

“Derived ichthyosaurs show the typical jugal-quadratojugal contact, but via an unusual dorsal contact between the two. The jugal–squamosal contact may thus represent a transitional state to the anatomy observed in later ichthyosaurs, reinforcing the interpretation of the ‘ventral cheek embayment’ of basal ‘euryapsids’ as a ventrally open lower temporal fenestra.”

“Thus, the new taxon has implications for the origin of secondarily aquatic groups, and therefore also paleobiogeographic significance. The appearance of placodontians has been traced to central Europe, but ichthyopterygians are believed to have originated in the Eastern Tethys. The new taxon indicates that the earliest evolutionary history of these groups may have occurred in the Western Tethys, associated with the Germanic Basin. This new material emphasizes the importance of sampling small-bodied taxa in the understanding of reptile evolution.”

The Lower Keuper is Carnian, early Late Triassic. Galesphyris is older. It comes from the Late Permian, perhaps representing an early Early Permian genesis.


References
Sobral G and Shoch R 2019. A small diapsid from the Lower Keuper of Germany and the origin of aquatic reptiles. Journal of Vertebrate Paleontology abstracts.

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.”

Figure 1. Mesosaurus origins recovered by the LRT. The fossil record appears to be topsy turvy here with the basal taxa appearing 30 million years later. Fossils are rare and discovery is rarer. Things like this sometimes happen.

Figure 1. Mesosaurus origins recovered by the LRT. The fossil record appears to be topsy turvy here with the basal taxa appearing 30 million years later. Fossils are rare and discovery is rarer. Things like this sometimes happen. None of these taxa appear to be fully aquatic, but related thalatttosaurs and ichthyosaurs definitely were.

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

Hovasaurus tarsus ontogeny animation

Caldwell 1995
provided a series of growth stages of the tarsus of Hovasaurus that chronicle the appearance of the ankle bones. Here is an animation of the same (Fig. 1).

Figure 1. From Caldwell 1995, an ontogenetic series showing the growth of the carpus in the basal diapsid Hovasaurus.

Figure 1. From Caldwell 1995, an ontogenetic series showing the growth of the  tarsus in the basal diapsid Hovasaurus. Scale bar = 1 cm.  Since these specimens were not found as part of a family assemblage, there are some specimens that appear to diverge from others in terms of carpal element shapes. Plus, we see here a certain amount of individual variation, the driving force behind evolution. There is a large discontinuity between K and L due to a lack of fossils at that stage of growth. C = calcaneum. A = Astragalus. c = centralia.

Unfortunately
Caldwell was under the impression that the basal diapsid Hovasaurus was close to the ancestry of extant lepidosaurs. The large reptile tree (LRT, 1028 taxa) invalidates that hypothesis with the addition and inclusion of more taxa.

According to the LRT
Hovasaurus
is a marine younginiform, basal to those diapsids that ultimately produced members of the Enaliosauria, a large clade of marine (new) archosauromorphs. Lepidosaurs had a separate origin going back to basalmost amniotes (= reptiles) like Gephyrostegus.

Figure 1. Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Figure 2 Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Hovasaurus is interesting
in that it developed a plesiosaur-style pectoral girdle without being directly related to plesiosaurs. Hovasaurus and Tangasaurus (Fig. 2) look like  they are missing a scapula. In related Thadeosaurus the scapula has been reported only on juvenile taxa (gray box).

References
Caldwell MW 1995. Developmental constraints and limb evolution in Permian and extant  lepidosauromorph diapsids.

What?? No scapula??

In a few marine younginiforms,
Tangasaurus (Haughton 1924, Currie 1982), Hovasaurus (Piveteau 1926, Currie 1981) and Thadeosaurus (Carroll 1981, Currie 1984) the scapula cannot be found (Fig. 1). But in a young thadeosaur (if conspecific), a scapula is present (in gray). These are all currently sisters in their own clade in the large reptile tree, The lack of a scapula is not currently a scored trait in the large reptile tree.

Figure 1. Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Figure 1. Tangasaurus, Hovasaurus and some specimens of Thadeosaurus, three marine younginiformes, apparently have no scapula. Click to enlarge. The young Thadeosaurus, if that is indeed what it is (in gray box) shows what a scapula should look like.

When you first encounter these specimens
you scratch your head and search, looking for the scapulae to no avail. Then, when you realize these three sisters share this trait — it still is difficult to accept. The coracoids and sternae + interclavicle form a chest plate. What holds that pectoral girdle in place? What locks the humerus down?  It is hard to look at those naked anterior ribs. Usually something is there to cover them~ Maybe I just missed it…

It is at this node in the evolution of marine younginiforms
that they were moving from a terrestrial niche into an aquatic one. From such Late Permian taxa we get plesiosaurs, placodonts, mesosaurs, thalattosaurs and ichthyosaurs, along with the widely varied sinosaurosphargids including Atopodentatus. So the change in niche is echoed and sometimes amplified in the morphology of descendant taxa, starting with these three (Fig. 1).

References
Carroll RL 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society London B 293: 315-383
Currie PJ 1984. Ontogenetic changes in the eosuchian reptile Thadeosaurus. Journal of Vertebrate Paleontology 4(1 ): 68-84.
Currie PJ 1981. Hovasaurus boulei, an aquatic eosuchian from the Upper Permian of Madagascar. Palaeontologica Africana, 24:99-163.
Currie P 1982. The osteology and relationships of Tangasaurus mennelli Haughton. Annals of The South African Museum 86:247-265. http://biostor.org/reference/111508
Haughton SH 1924. On Reptilian Remains from the Karroo Beds of East Africa. Quarterly Journal of the Geological Society 80 (317): 1–11.
Piveteau J 1926. Paleontologie de Madagascar XIII. Amphibiens et reptiles permiens. Annls Paleont. 15: 53-180.
Reisz RR, Modesto SP and Scott DM 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society B 278 (1725): 3731–3737.

wiki/Hovasaurus
wiki/Tangasaurus