A hupehsuchid-mimic mesosaur with a duckbill: YAGM V 1401

Cheng et al. 2019
bring us news of a new armored Early Triassic (250 mya) specimen (YAGM V 1401; Figs. 1,2) they attribute to the armored Early Triassic hupehsuchid, Eretmorhipis carrolldongi (Fig. 5; holotype WGSC V26020; Chen et al. 2015). The holotype specimen lacks a skull. The authors considered the new YAGM specimen, complete with skull, conspecific with the WGSC holotype of Eretmorhipis, noting it had small eyes relative to the body and a duckbill-like rostrum.

Instead
the large reptile tree (LRT, 1389 taxa; Fig. 3) nests the YAGM specimen as a derived mesosaur, 32 steps away from the WGSC holotype of Eretmorhipis in the clade of hupehsuchids. The authors assumed Eremorhipis was a hupehsuchid because it looks like one. It really does. That’s easy to see. They are a close match when you eyeball them. Unfortunately Cheng et al. 2019 did not test that assumption using a phylogenetic analysis that included mesosaurs, which nest basal to hupehsuchids (Fig. 3). Once again, it’s taxon exclusion.

The eyes are actually large relative to the skull,
in the new YAGM specimen (Fig. 2), but the skull is tiny relative to the body. The rostrum is narrow relative to the cranium. Typically that enables binocular vision. The authors did not provide a reconstruction of the skull.

The wide, flat rostrum of the YAGM specimen has an open central area,
like Ornithorhynchus the duckbill platypus (Fig. 4) by convergence. Given that bit of morphology the authors sought to extend the duckbill analog by reporting small eyes relative to the body in the YAGM specimen. That gives them an irrefutable headline, but a little mis-leading given the reconstruction (Fig. 2). The authors suggest Eretmorhipis used mechanoreceptors in the rostrum instead of eyesight. They report, “Apparent similarities include exceptionally small eyes relative to the body, snout ending with crura with a large internasal space, housing a bone reminiscent of os paradoxum, a mysterious bone of platypus, and external grooves along the crura.” That’s pretty awesome! Larry Martin would have enjoyed this list of convergent traits. I have no idea how the ox paradoxum bone fit in the YAGM specimen skull. So it remains a paradox.

Figure 1. Eremorhipis in situ and line drawing from Cheng et al. 2019. Colored here using DGS methods. Some bones are reidentified here. See figure 2 for matching colors.

Figure 1. Eremorhipis in situ and line drawing from Cheng et al. 2019. Colored here using DGS methods. Some bones are reidentified here. See figure 2 for matching colors.

The authors created a chimaera
when they added the hands and feet of the holotype WGSC specimen to the new YAGM specimen in their Nature paper. Since the two specimens are not related, that is going to cause confusion. No matter how sure they were, the authors needed a valid phylogenetic analysis to nest their new specimen, now requiring a new generic and specific name.

Figure 2. Reconstruction of Eretmorhipis skull from figure 1, along with in situ specimen and reconstruction from Cheng et al. 2019. Pectoral and pelvic girdles magnified and colored using DGS methods. The skull appears to provide binocular vision due to the narrow rostrum and wide cranium.

Figure 2. Reconstruction of Eretmorhipis skull from figure 1, along with in situ specimen and reconstruction from Cheng et al. 2019. Pectoral and pelvic girdles magnified and colored using DGS methods. The skull appears to provide binocular vision due to the narrow rostrum and wide cranium.

Traditional paleontologists need to catch up to the LRT
and start including thalattosauriforms and mesosaurs whenever they study basal ichthyopterygians, like hupehsuchids. Basal taxa are all closely related and all three taxa include a wide variety of morphotypes, including some that converge.

Figure 3. Subset of the LRT focusing on Mesosauria, Thalattosauriformes and Ichthyopterygia including two specimens referred to Eretmorhipis nesting here apart from one another.

Figure 3. Subset of the LRT focusing on Mesosauria, Thalattosauriformes and Ichthyopterygia including two specimens referred to Eretmorhipis nesting here apart from one another.

It is worth noting
that many mesosaurs, like the SMF R4710 specimen, lack the long, laterally-oriented, comb-like teeth of Mesosaurus. Most mesosaurs have a typical diapsid skull architecture, distinct from the in-filling of the temporal fenestra that Mesosaurus exhibits. Mesosaurs are common in certain Early Permian strata. That provides plenty of time for the highly derived YAGM specimen to evolve by the Early Triassic.

Figure 4. Ornithorhynchus skull with colors added using DGS methods. Note the large opening in the dorsal view of the rostrum, as in Eretmorhipis.

Figure 4. Ornithorhynchus skull with colors added using DGS methods. Note the large opening in the dorsal view of the rostrum, as in Eretmorhipis, by convergence.

It’s also worth noting
that the YAGM specimen has a cleithrum and a ventrally broad clavicle along with an interclavicle and other traits found in mesosaurs, but lacking in hupehsuchids.

Figure 1. The holotype specimen of Eretmorhipis carrolldongi WGSC V26020 compared to the figure drawn form Cheng et al. 2019.

Figure 5. The holotype specimen of Eretmorhipis carrolldongi WGSC V26020 compared to scale to the figure drawn form Cheng et al. 2019 for specimen YAGM V 1401. Cheng et al. created a chimaera when they added the WGSC specimen hands and feet to the new YAGM specimen without first nesting them together in a cladogram. These two specimens do not nest together in the LRT despite the massive convergence. Don’t try to eyeball taxa. Let the software take the bias out of it.

A word to workers: Don’t try to ‘eyeball’ taxa.
Let the phylogenetic software take the bias out of making a taxonomic determination. We’ve seen professional workers make this mistake before by combining diphyletic turtles, whales, seals, and by miss-nesting Vancleavea, Lagerpeton, Chilesaurus, Daemonosaurus by taxon exclusion. Let’s not forget those who keep insisting that pterosaurs are archosaurs (virtually all traditional workers), again by omitting pertinent 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 6. 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. The YAGM specimen is large, like Mesosaurus, but later (at 250 mya) than Thadeosaurus.

References
Chen X-H, Motani R, Cheng L, Jiang D-Y and Rieppel O 2015. A new specimen of Carroll’s mystery hupehsuchian from the Lower Triassic of China. PLoS One 10, e0126024, https://doi.org/10.1371/journal.pone.0126024 (2015).
Cheng L, Motani R, Jiang D-Y, Yan C-B, Tintori A and Rieppel O 2019. Early Triassic marine reptile representing the oldest record of unusually small eyes in reptiles indicating non-visual prey detection. Nature Scientific Reports Published online January 24, 2019.

 

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

Update on the Rossman 2002 “Brazilosaurus”

Figure 1. The nesting of Rossman's "Brazilosaurus" (the PIMUZ AIII 0192 specimen) at the base of the Thalattosauria is confirmed with the addition of post-cranial and new cranial data.

Figure 1. Click to enlarge. The nesting of Rossman’s “Brazilosaurus” (the PIMUZ AIII 0192 specimen) at the base of the Thalattosauria is confirmed with the addition of post-cranial and new cranial data.

Today: Some new data and a new reconstruction of the skull and postcrania of the intriguing and potentially important postcrania of PIMUZ A/III 0192. This mesosaur-ish reptile was attributed by Rossman 2002 to Brazilosaurus sanpauloensis. 

Earlier the only data I had on this specimen was a line drawing of a skull from Rossman 2002. This data resulted in a nesting outside of Brazilosaurus + Stererosternum + Mesosaurus, at the base of the Thalattosauria. Notably, with the additional data, the nesting did not change.

Crushed skulls are often the best Because all the parts are crushed into a single plane and you can rebuild that “house of cards” or “crushed eggshell” in many views. Some parts are visible through the orbit. The occiput often flips to the side.

Mesosaurs are key
Workers have attempted to nest mesosaurs based on its lack of temporal fenestration — and they (Modesto 2006) end up with pareiasaurs and millerettids and procolophonoids. But with it’s hyper-long teeth, Mesosaurus is clearly a derived form. What we’re looking for is a basal taxon that looks like Mesosaurus with small plesiomorphic teeth. Then perhaps we’ll see more evidence for the diapsid skull morphology. And that’s exactly what we find.

Figure 2. Click to enlarge. The Rossman "Brazilosaurus" PIMUZ AIII 0192. This is a basal thalattosaur and a derived mesosaur. Due to severe crushing elements from the other side of the skull made it appear that the skull lacked fenestration following the generally accepted but mistaken hypothesis that all mesosaurs lacked temporal fenestra.

Figure 2. Click to enlarge. The Rossman “Brazilosaurus” PIMUZ AIII 0192. This turns out to be THE  basal thalattosaur as well as a basal mesosaur. Due to severe crushing elements from the other side of the skull made it appear that the skull lacked fenestration following the generally accepted but mistaken hypothesis that all mesosaurs lacked temporal fenestra. Note the tiny forelimbs and deep tail chevrons. Low dorsal spines and gracile ribs are also noteworthy.

Here (Fig. 1) the Rossman (2002) “Brazilosaurus” nests at the base of the Thalattosauria, whether using the Rossman data as is, or pulling slightly different traits out (Fig. 2).

Rossman (2002) presented photos of several interesting mesosaurs. Among them was the SMF-R-4710 specimen attributed to Stereosternum (Fig. 3). This one does nest with Stereosternum, but with more open temporal fenestrae, its nests at the base of the mesosaurs, close to the base of the ichthyosaurs and thalattosaurs.

Figure 3. The SMF-R-4710 specimen attributed to Stereosternum, but with larger temporal fenestrae, smaller teeth and other distinct traits.

Figure 3. Click to enlarge. The SMF-R-4710 specimen attributed to Stereosternum, but with larger temporal fenestrae, smaller teeth and other distinct traits. The high unfused dorsal processes are also seen on Hupehsuchus and Utatsusaurus.

If you think this Stereosternum specimen is starting to look a lot like Wumengosaurus, you’d be right. And Wumengosaurus now nests at the base of Hupesuchus + Ichthyosaurs, so we’re getting closer and closer to the common origin of both. One keeps its teeth, the other does not.

Figure 3. Stereosternum SMF-R-4710 reconstructed from traced image (in color below). Here temporal fenestration is clearly diapsid.

Figure 3. Stereosternum SMF-R-4710 reconstructed from traced image (in color below). Here temporal fenestration is clearly diapsid.

Lest you doubt
Running the large reptile tree with all temporal fenestration traits deleted recovers a single tree unchanged from the full character list tree. If anyone wants to come up with better data or a better tracing, please use specimen numbers.

There are so many specimens of mesosaurs
and so many variations and only three names for them (Mesosaurus, Brazilosaurus, Stereosternum). Someone needs to put it all together and catalog some of the important specimens (e.g. the holotypes). We’ll need new generic names for the specimens above.

If someone has a good photo of the Brazilosaurus holotype (Shikama and Ozaki 1966), please let me know.

References
Cope ED 1886. A contribution to the vertebrate paleontology of Brazil. Stereosternum tumidum, gen. et sp. nov. Proceedings of the American Philosophical Society 23(121):1-21.
Modesto S 2006.
 The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society, 2006, 146, 345–368.
Shikama T and Ozaki T 1966. On a Reptilian Skeleton from the Palaeozoic Formation of San Paulo, Brazil” Transactions and Proceedings of the Palaeontological Society of Japan, New Series 64: 351–358.

 

Brazilosaurus and something like it at the base of the Thalattosauria

Brazilosaurus holotype – Shikama (1966)
Brazilosaurus (Fig. 1) was described by Shikama (1966) as a sort of mesosaur with a longer neck, shorter teeth and other minor differences. From the available online pdf (created from photocopies), I can only read the line drawing, but photos were published originally (if anyone has them, I’d like to see them, they’re too dark on the pdf).

Figure 2. The holotype Brazilosaurus from Shikama (1966, above) and bones colorized (below) based on sister taxa. The apparent giant cheek region may represent the occiput crushed into the bedding plane as it is the right size and shape and no sister taxa have anything similar. The gracile jugal indicates the presences of a lateral temporal fenestra, as in mesosaurus.

Figure 2. The holotype Brazilosaurus from Shikama (1966, above) and bones colorized (below) based on sister taxa. The apparent giant cheek region may represent the occiput crushed into the bedding plane as it is the right size and shape and no sister taxa have anything similar. The gracile jugal indicates the presences of a lateral temporal fenestra, as in mesosaurus. These are the problems working with line drawings rather than fossils or photos. No giant teeth here, as in Mesosaurus.

The illustrated cheek region is much too large compared to all sister taxa, so it may represent the occiput, which it is the right size to be. The jugal looks similar to those in Stereosternum and Mesosaurus, so likewise probably framed a lateral temporal fenestra. Guesswork at this point.

Brazilosaurus referred specimen – Rossman (2002) 
published this image (Fig. 1) of a specimen he attributed to Brazilosaurus. While not too far off from each other phylogenetically, no one would ever confuse the two. They’re just too different. Note the scale bars. Those tells us the Rossman specimen is not just a short-rostrum juvenile. Overall it’s the same size but has a shorter/taller rostrum and larger orbit.

My re-interpretation of the Rossman bones is in color. The reinterpretation is closer to the interpretation of sister taxa. The parietal goes on top of the head, not posterior to the orbit, for instance. And an unidentified bone sticks up from the cranium and that bone has what appears to be half of a pineal / parietal opening. When I see hirez photos I’ll revisit this if necessary.

Figure 1. Specimen referred to Brazilosaurus by Rossman (2002, above), colorized with bones reidentified below. Although sharing many traits with Brazilosaurus (Fig. 2), it also shares many traits with Xinpusaurus suni (Fig. 3).

Figure 2. Specimen referred to Brazilosaurus by Rossman (2002, above), colorized with bones reidentified below. Although sharing many traits with Brazilosaurus (Fig. 2), it also shares many traits with Xinpusaurus suni (Fig. 3). Apparently this diapsid resealed its temporal fenestrae, but a good look at the fossil itself is needed to confirm.

The referred specimen is likewise known to me only as a drawing with flaws (Fig. 2). Again, I wish I had access to the original specimen, or to a good photo. Or to the post-crania. All attempts at contacting Rossman have failed. I understand he is in poor health.

Xinpusaurus suni (Fig. 3) is a basal thalattosaur (close to ichthyosaurs like Utatsusaurus) sharing many traits with the Rossman specimen. It has no upper temporal fenestra, but it has a large lateral temporal fenestra. So these fenestra appear to come and go, as they do in basal diapsids like Araeoscelis.

Figure 3. Xinpusaurus suni, a basal thalattosaur sharing many traits with the Rossman specimen.

Figure 3. Xinpusaurus suni, a basal thalattosaur sharing many traits with the Rossman specimen. This specimen suggests the parietal of Rossman might be the postorbital. Here the septomaxilla are no longer indicated. The tiny bones above the nares are reduced nasals. It is important to see the original materials to solve these problems.

Adding Brazilosaurus to the large reptile tree nests it with Mesosaurus. Adding the Rossman specimen nests it basal to all thalattosaurs and therefore close to Xinpusaurus. Hupehsuchus + Utatsusaurus are outgroups.

So, one more mesosaur – thalattosaur connection.

References
Rossmann T 2002. Studien an Mesosauriern (Amniota inc. sed., Mesosauridae): 3. Neue Aspekte zur Anatmie, Erhaltung und Paläoökologie aufgrund der Exemplare im Paläontologischen Institut der Universität Zurich. Neues Jahrbuch fur Geologie und PaläontologieAbhandlungen 224, 197-221.
Shikama T 1966. On a reptilian skeleton from the Palaeozoic formation of San Paulo, Brazil. Transactions and Proceedings of the Palaeontological Society of Japan, New Series 64:351-358.

Mesosaur-oid Skulls

It’s a Unfortunate Tradition in Paleontology
Mesosaurus has been left pretty much alone in the fossil record, with few to no relatives close in morphology. That’s why it has been associated with such mismatches as pareiasaurs, captorhinids and other oddballs among the Anapsida (= basal Reptilia).

Here (Fig. 1) in a heretical presentation you haven’t seen in any prior publications, are a few mesosaur-oid cousins recovered from the large reptile tree. Here Mesosaurus nests among the basal enaliosauria (sea reptiles) and its quite derived, not primitive. Even so, a sister to Mesosaurus gave rise to several large clades of paddle-finned reptiles.

It’s best to let the evidence to the talking. Gosh they sure look like cousins, don’t they? Mesosaurus was not alone. Closing of the upper temporal fenestra was common in this clade, but not universal. Earlier we looked at evidence for an open lateral temporal fenestra.

Mesosaurus and its kin

Figure 1. Mesosaurus and its kin among the ichthyosaurs and thalattosaurs. Please note the similarities in the placement of the naris, the skull sutures and the general reduction (in many cases, not all) of the teeth. The closure of the upper temporal fenestra in Mesosaurus and thalattosaurs was by convergence with different sutural patterns. That’s evolution for ya!

Compare and contrast
Sometimes you have to get the entire family into the same picture to see the similarities and family ties. That’s the case here as I illustrate the relationships recovered by the large reptile tree simply by putting several similar skulls together following the patterns recovered by phylogenetic analysis. Other closely related forms (not included in the figure) reduced the rostrum, modified their teeth and the post-crania likewise evolved in different directions. These include Sinosaurosphargis and the placodonts and Vancleavea among the thalattosaurs. Mesosaurus itself was more specialized than Stereosternum and whatever unknown protomesosaurs derived from a sister to Claudiosaurus await their eventual discovery.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Fenestration in the Mesosaurus Skull – Piñeiro et al. 2012

Not an Anapsid After All
Traditionally Mesosaurus (Gervais P 1865) has been considered a basal reptile, an anapsid (Laurin and Reisz 1995, Modesto 1999) lacking any temporal fenestration. Most skulls are crushed and slightly scattered, requiring reassembly. It’s been confusing. Now a new paper by Piñeiro et al. (2012) provides strong evidence of a lower termporal fenestra in undisturbed material.

Graciela Piñeiro,

Figure 1. Graciela Piñeiro, lead author on the new mesosaurus paper.

The Piñeiro et al. (2012) Abstract
The Early Permian mesosaurids are the oldest known aquatic amniotes with an exclusively Gondwanan distribution. Although several hundred of complete skeletons have been discovered and intensively studied, the anatomy and taxonomic composition of the group, as well as its phylogenetic relationships remain controversial. Several well-preserved
mesosaurid specimens found in Uruguay justify a new anatomical reconstruction of the skull of Mesosaurus tenuidens, differing from earlier ones especially in the presence of a lower temporal fenestra. The significance of this structure for the evolution of temporal fenestration in amniotes is evaluated according to the two most recent phylogenetic hypotheses, in which mesosaurids are basalmost sauropsids or basalmost parareptiles. A synapsid-like fenestration may be the primitive condition for Amniota, and it may be also a basal condition for parareptiles, because recent phylogenies suggest a basal position for mesosaurids and lanthanosuchoids within that group, and both possess a lower temporal fenestra. Our results also give a moderately strengthened support for diapsid affinities of turtles.

Mesosaurus skull with lateral temporal fenestra.

Figure 2. Mesosaurus skull with lateral temporal fenestra. From Piñeiro et al. 2012.

The New Mesosaurus Skull
The lateral temporal fenestra of this Mesosaurus (Fig. 2) occurs between embayments of the jugal and squamosal. Some skulls may have closed off this fenestra. Others may produce a slightly different fenestra shape. This trait may be universal or not. It’s hard to tell, but at least some specimens of Mesosaurus had this trait.

Vindication
Earlier I reconstructed the skull of Mesosaurus with a lateral termporal fenestra based on reassembling  in situ tracings by Modesto (2006). With or without this trait, mesosaurs nested between Claudiosaurus and kin and enaliosaurs (sauropterygians, ichthyosaurs and thalattosaurs). That was not the first time someone proposed a lateral temporal fenestra (reference escapes me at the moment [von Huene 1941), but finding a lateral temporal fenestra broke with current paradigm. Further study of the more primitive and shorter-toothed Stereosternum will hopefully vindicate the appearance of an upper temporal fenestra as well.

Missing Sister Taxa 
Pineiro et al. (2012) attempted to nest Mesosaurus in a pseudoclade of select taxa virtually all of which possessed a lateral temporal fenestra, whether bound ventrally by a jugal/ quadratojugal/ quadrate bar or not. This tree assumed the lateral temporal fenestra appeared only once without convergence. Turtles (a taxon without temporal fenestration) were also included. Sauropterygians and several basal, mesosaur-like, aquatic forms with temporal fenestration, including Claudiosaurus, Hovasaurus, Thadeosaurus and Wumengosaurus were not included. Also missing were Adelosaurus and Acerosodontosaurus closer to the araeoscelids. Importantly, the large reptile tree found these six taxa to be the closest sisters to mesosaurs (represented by Stereosternum). Excluding these taxa is an unfortunate oversight that could have taken the study to the next level because these six taxa displayed various forms of temporal fenestration and mesosaurs nested in the midst of them.

Disagreements on the Phylogenetic Placement
Piñeiro et al. (2012) placed (not nested) Mesosauridae between Synapsida (Eothyris at the base) and Sauropsida (Romeria, etc. at the base), several nodes away from Petrolacosaurus and Ichthyosauria. Piñeiro et al. (2012) also placed Mesosauridae between Milleretidae and the aforementioned Romeria, etc.) following traditional topologies by Laurin and Reisz (1995) for basal amniotes, Reisz et  al. (2007) for “parareptiles,” and Laurin (2004) for other taxa, from Modesto (1999) for mesosaurids. Missing from the above studies were more undulating aquatic reptiles (listed above) like mesosaurs.

What is Happening in This Clade?
The examples of sister taxa in the large reptile tree indicate that the initial appearance of the diapsid configuration in taxa like Eudibamus and Petrolacosaurus, was modified in derived taxa like Araeoscelis (loss of lateral temporal fenestra), Mesosaurus (loss of upper temporal fenestra) and Wumengosaurus and sauropterygians (loss of lower temporal bar).

What the Large Reptile Tree Indicates
The large reptile tree indicates that the diapsid configuration appeared at least twice by convergence and variations thereof also appear by convergence. The lower temporal bar appeared in certain lines and disappeared in others.

Why were turtles included in this study?
Turtles are anapsids derived from diadectomorphs like Stephanospondylus and have nothing to do with diapsids like Petrolacosaurus or Mesosaurus, according to the recovered tree. Rather than elevating mesosaurs to the ranks of derived taxa, the authors proposed lowering the synapsid-like fenestra trait to the basal ranks (see abstract above), suggesting (to them) that turtles lost their diapsid configuration during their evolution. That hypothesis is not supported by the present tree based on a magnitude more taxa that encompasses the entire Reptilia.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Gervais P 1865. Du Mesosaurus tenuidens, reptile fossile de l’Afrique australe. Comptes Rendus de l’Académie de Sciences 60:950–955.
Laurin M and Reisz RR 1995. 
A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society 113:165-223.
Modesto SP 1999. Observations on the structure of the Early Permian reptile Stereosternum tumidum Cope. Palaeontol. Afr. 35, 7–19.
Modesto SP 2006. The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society 146 (3): 345–368. doi:10.1111/j.1096-3642.2006.00205.x.
Modesto SP 2010.
 The postcranial skeleton of the aquatic parareptile Mesosaurus tenuidensfrom the Gondwanan Permian. Journal of Vertebrate Paleontology 30 (5): 1378–1395. doi:10.1080/02724634.2010.501443.
Piñeiro G, Ferigolo J, Ramos A and Laurin M 2012.
Cranial morphology
of the Early Permian mesosaurid *Mesosaurus tenuidens* and the evolution of
the lower temporal fenestration reassessed. Comptes Rendus Palevol.
von Huene F 1941. Osteologie und systematische Stellung von Mesosaurus. Palaeontogr. Abt. A. 92, 45–58.

A Tip of the Hat to Augusta (1964)

Just a short note and a tip of the hat to Dr. Josef Augusta (1964) who wrote: “Ichthyosaurs are very closely related to Mesosaurs…” 

This was prior to the publications of the most primitive and mesosaur-like ichthyosaurs, like Utatsusaurus, and prior to the publication of aquatic mesosaur ancestors like Claudiosaurus and several other sisters. See the full tree here. Among modern paleontologists, almost none have tested this relationship with the exception of Maisch (2010) who still wasn’t sure. He also considered procolophonids as possible ancestors.

References
Augusta J and Burian Z 1964. Prehistoric Sea Monsters. Paul Hamlyn, London. 63pp.
Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity 3: 151-214.

Mesosaurus Embryos!

Good friend, Dr. Graciela Piñeiro et al. (2012) just described the oldest known amniotic embryos. They belong to mesosaurs (Gervais 1865). From their abstract: “The earliest undisputed crown-group amniotes date back to the Late Carboniferous, but the fossil record of amniotic eggs and embryos is very sparse, with the oldest described examples being from the Triassic. Here, we report exceptional, well preserved amniotic mesosaur embryos from the Early Permian of Uruguay and Brazil. These embryos provide the earliest direct evidence of reproductive biology in Paleozoic amniotes. The absence of a recognisable eggshell and the occurrence of a partially articulated, but well-preserved embryo within an adult individual suggest that mesosaurs were viviparous or that they laid eggs in advanced stages of development. Our finds represent the only known documentation of amniotic embryos in the Paleozoic and the earliest known case of viviparity, thus extending the record of these reproductive strategies by 90 and 60 Ma, respectively.”

Embryo Mesosaurus

Figure 1. Embryo Mesosaurus curled up within its ellipsoid amniotic membrane, but no egg shell was preserved. Left: specimen; Middle: tracing of specimen; Right: restoration of specimen.

The embryo was ~10% the size of an average adult and coiled as if in an ellipsoid egg. The snout was relatively short. The head was relatively large. Despite the elliptical shape of the embryos, no shell was preserved. Some embryos were found within their mother. Only one and rarely two were carried at a time. These data support the large reptile family tree that recovered a mesosaur/thalattosaur/ichthyosaur relationship in which ichthyosaurs are known to exhibit live birth (viviparity) emerging from their amniotic sac prior to birth.

Lizards typically carry the embryo within the uterus for extended periods. Many exhibit viviparity, but lizards and mesosaurs are not related.

Ichthyosaurs and sauropterygians also exhibit viviparity and are closer to mesosaurs. All three are distantly related to both therapsids (basal mammals lay eggs) and archosaurs (both birds and crocs lay eggs). So viviparity in this clade seems to have had its genesis in mesosaurs.

There’s more big news on mesosaurs to come.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Gervais P 1865. Du Mesosaurus tenuidens, reptile fossile de l’Afrique australe. Comptes Rendus de l’Académie de Sciences 60:950–955.
Piñeiro G, Ferigolo J, Meneghel M and Laurin M 2012. The oldest known amniotic embryos suggest viviparity in mesosaurs. Historical Biology: An International Journal of Paleobiology, DOI:10.1080/08912963.2012.662230

Thalattosaurs: Wet, Wild and Largely Ignored.

Today’s Post Introduces the Thalattosauria.
Rarely, if ever have the thalattosauria been portrayed or illustrated as a clade. Thalattosaurs are not the most popular extinct reptiles. They’re often overlooked in cladistic analyses. Some thalattosaurs (I’m thinking of Vancleavea and Helveticosaurus at the moment) have been wrongly considered members of other clades. Wumengosaurus  was considered an aberrant sauropterygian. Here I’ll attempt to remedy that situation with a single image of all the thalattosaurs (and their ancestors) to scale currently listed in reptileevolution.com and a short description of each taxon.

 

Figure 2. The Thalattosauria nesting within the Enaliosauria

Figure 2. The Thalattosauria nesting within the Enaliosauria

How Thalattosaurs Nest
Here (Fig. 2) thalattosaurs nested as sisters to ichthyosaurs and mesosaurs. Specifically Stereosternum and Wumengosaurus were their common ancestors.

Xinpusaurus
Despite its many unique traits, Xinpusaurus nested at the base of the Thalattosauria, close to the Ichythosauria. With fins rather than feet and a long, essentially toothless, sword-like rostrum, Xinpusaurus is really off in a clade all by itself. However to move it into the Ichthyosauria takes at least 14 extra steps.

Askeptosaurus
More on the main line of thalattosaur evolution (without transforming its feet into flippers), Askeptosaurus was a larger version of Wumengosaurus with a longer temporal region, more gracile dorsal ribs and relatively shorter limbs.  Two clades arose from this taxon.

Figure 1. The Thalattosauria to scale.

Figure 1. The Thalattosauria to scale.

The Short-Faced Thalattosaurs
Miodentosaurus, EusaurosphargisHelveticosaurus and Vancleavea were the short-faced thalattosaurs. They encompass a wide gamut of morphologies with distinct tooth patterns, vertebral counts and overall sizes. Miodentosaurus lost most of its teeth and became the largest thalattosaur by enlarging the post-crania without enlarging the skull. Eusaurophargis had an upturned jawline, short teeth, a wide, low body and fewer but longer dorsal vertebrae. Helveticosaurus had huge teeth, some so long and closely packed that they must have strained seawater. Vancleavea had a carnivorous dentition, was armored with bony scutes and had a deep tail ideal for sculling.

The Long(er)-Faced Thalattosaurs
Endennasaurus, Clarazia and Thalattosaurus were the long-faced thalattosaurs. Endennasaurus was much smaller than Askeptosaurus and lost its teeth. Clarazia had a shorter rostrum that extended beyond the mandibles; short, blunt teeth and a shorter neck. Thalattosaurus had sharp teeth and blunt teeth and a smaller postorbital region.

Learn more about thalattosaurs and check out the references at reptileevolution.com.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Cheng L 2003. A new species of Triassic Thalattosauria from Guanling, Guizhou. Geological Bulletin of China 22:274–277.
Cheng YN, Wu XC, Li C, Sato T 2007. A new thalattosaurian (Reptilia: Diapsida) from the Upper Triassic of Guizhou, China. Vertebrata PalAsiatica 45: 246–260.
Jiang D-A, Maisch MW, Sun S-L, Matzke AT and Hao WC 2004. A new species of Xinpusaurus (Thalattosauria) from the Upper Triassic of China. Journal of Vertebrate Paleontology 24:80–88. BioOne
Jiang D-Y, Rieppel O, Motani R, Hao W-C, Sun Y-I, Schmitz L and Sun Z-Y. 2008. A new middle Triassic eosauropterygian (Reptilia, Sauropterygia) from southwestern China. Journal of Vertebrate Paleontology 28:1055–1062.
Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity 3: 151-214.
Merriam JC 1904. A new marine reptile from the Triassic of California. University of California Publications, Bulletin of the Department of Geology, 3, 419–421.
Merriam JC 1905. The Thalattosauria, a group of marine reptiles from the Triassic of California. Memoirs of the California Academy of Sciences, 5, 52 pp.
Nicholls EL 1999. A reexamination of Thalattosaurus and Nectosaurus and the relationships of the Thalattosauria (Reptilia, Diapsida). Paleobios 19:1–29.
Müller J, Renesto S and Evans SE 2005. The marine diapsid reptile Endennasaurus(Reptilia: Thalattosauriformes) from the Late Triassic of Italy. Palaeontology 48:15-30.
Nesbitt SJ, Stocker MR, Small BJ and Downs A 2009. The osteology and relationships of Vancleavea campi (Reptilia: Archosauriformes). Zoological Journal of the Linnean Society 157 (4): 814–864. doi:10.1111/j.1096-3642.2009.00530.x.
Nopcsa F 1925. Askeptosaurus, ein neues reptil der Trias von Besano: Centralblatt für Mineralogie, Geologie und Paläontologie, p. 265-267.
Nosotti S and Rieppel O 2003. Eusaurosphargis dalsassoi n.gen. n.sp., a new, unusual diapsid reptile from the Middle Triassic of Besano (Lombardy, N Italy). Memories of the Italian Society of Natural Science and the Museum of Natural History in Milan, XXXI (II).
Parker WG and Barton B 2008. New information on the Upper Triassic archosauriform Vancleavea campi based on new material from the Chinle Formation of Arizona. Palaeontologia Electronica 11 (3): 20p.
Peyer B 1936. Die Triasfauna der Tessiner Kalkalpen. X. Clarazia schinzi nov. gen. nov. spec. Abhandlungen der Schweizerischen Pala¨ontologischen Gesellschaft, 57, 1–61.
Peyer B 1955. Die Triasfauna der Tessiner Kalkalpen. XVIII. Helveticosaurus zollingeri, n.g. n.sp. Schweizerische Paläontologische Abhandlungen 72:3-50.
Renesto S 1992. The anatomy and relationships of Endennasaurus acutirostris (Reptilia: Neodiapsida) from the Norian (Late Triassic) of Lombardy. Rivista Italiana di Paleontologia e Stratigrafia, 97:409-430
Rieppel O 1987. Clarazia and Hescheleria; a reinvestigation of two problematic reptiles from the Middle Triassic of Monte San Giorgio (Switzerland). Palaeontographica, A, 195, 101–129.
Rieppel O 1989. Helveticosaurus zollingeri Peyer (Reptilia, Diapsida): skeletal paedomorphosis; functional anatomy and systematic affinities. Palaeontographica A 208:123-152.
Wu XC, Cheng YN, Sato T, Shan HY 2009. Miodentosaurus brevis Cheng et al., 2007 (Diapsida: Thalattosauria): its postcranial skeleton and phylogenetic relationships. Vertebrata PalAsiatica 47: 1–20.
Wu X-C, Cheng Y-N, Li C, Zhao L-J and Sato T 2011. New Information on Wumengosaurus delicatomandibularis Jiang et al., 2008, (Diapsida: Sauropterygia), with a Revision of the Osteology and Phylogeny of the Taxon. Journal of Vertebrate Paleontology 31(1):70–83.
Yin G-Z, Zhuo X-G, Cao Z-T, Yu Y-Y and Luo Y-M 2000. A preliminary study on the early Late Triassic marine reptiles from Gunanling, Guizhou, China. Geology, Geochemistry 28(3):1–22.
Zhao LJ, Sato T, Liu T, Li JC, Wu XC. 2010. A new skeleton of Miodentosaurus brevis (Diapsida:Thalattosauria) with a further study of the taxon. Vertebrata Palasiatica 48: 1–10.

wiki/Askeptosaurus
wiki/Endennasaurus
wiki/Eusaurosphargis
wiki/Helveticosaurus
wiki/Miodentosaurus
wiki/Wumengosaurus
wiki/Thalattosaur
wiki/Vancleavea

What is Helveticosaurus?

Helveticosaurus: an Enigma From the ’50s
Helveticosaurus is a large marine reptile from the Middle Triassic known from a complete and largely articulated skeleton with a very badly crushed, short-snouted skull with giant teeth. Originally described by Peyer (1955) as a placodont, Helveticosaurus doesn’t share many of the key characters shared by all other placodonts. Wikipedia reports that the affinities of Helveticosaurus with other diapsids remains largely unknown.

 

Figure 1. Helveticosaurus, a short snouted thalattosaur

Figure 1. Helveticosaurus, a short snouted thalattosaur

Results of the Large Study
Here, in the large study, Helveticosaurus nested as a thalattosaur, a clade of marine reptiles close to ichthyosaurs, but with a huge variety of skull and tooth shapes. The skull of Helveticosaurus was different from most other thalattosaurs, most of which had a long snout and shorter teeth. The only other thalattosaur with a similar skull was Vancleavea, which was originally described as an archosauriform. Eusaurophargis also has a short high rostrum, but its teeth were short and it had far fewer dorsal vertebrae. Miodentosaurus was also similar, but had very few, very short teeth. The hands and feet of these thalattosaurs were all quite similar.

Putting Humpty Together Again
The big problem with figuring out what Helveticosaurus was, was a lack of a good skull reconstruction. That crushed skull proved too intimidating for half a century. All the parts are there. No one wanted to step forward and put the skull back together again. Here the parts are identified and reconstructed.

What’s With Those TEETH!
The premaxilla of Helveticosaurus has giant sabertooth fangs, perfect for inflicting wounds on large prey or, possibly, dislodging prey/plants from the sea floor. These carnivorous weapons were followed by a curtain of hyper-elongated teeth in the maxilla. These would have been useless in dismembering, crushing or slicing prey items. They are so long they look they would break. The maxillary teeth were somewhat similar to baleen in that they might have been able to strain food while allowing water to exit… only a guess following the process of elimination. Not sure yet what the diet of Helveticosaurus was.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Peyer B 1955. Die Triasfauna der Tessiner Kalkalpen. XVIII. Helveticosaurus zollingeri, n.g. n.sp. Schweizerische Paläontologische Abhandlungen 72:3-50.
Rieppel O 1989. Helveticosaurus zollingeri Peyer (Reptilia, Diapsida): skeletal paedomorphosis; functional anatomy and systematic affinities. Palaeontographica A 208:123-152.

wiki/Helveticosaurus