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

Laurin and Piñeiro 2017 ‘reassess’ mesosaurs

This paper came with much anticipation
following discussions several years ago with one of the authors (GHP) about mesosaurs (Fig. 2) and their relationship to pachypleurosaurs and thalattosaurs (Fig. 2) in the LRT. Unfortunately only 17 terminal taxa (many suprageneric) were employed by Laurin and Piñeiro 2017 (vs. the 1122 taxa in the large reptile tree, LRT).

Unfortunately,
pachypleurosaurs and thalattosaurs were not among the 17 taxa employed by Laurin and Piñeiro. That makes this study worthless with regard to mesosaur interrelations. Very unfortunate.

From the Laurin and Piñeiro methods:|
“We started from the matrix of Laurin and Reisz (1995), given that this was the matrix that we knew best, that we had confidence in the accuracy of the anatomical scoring, and that we were confident that we could apply the revised scores in a manner coherent with the original scoring.” 

Figure 2. Unfortunately pachypleurosaurs and thalattosaurs are omitted from this cladogram.

Figure 1. Unfortunately pachypleurosaurs and thalattosaurs are omitted from this cladogram from Laurin and Piñeiro 2017. I don’t know of any aquatic basal synapsids or basal captor hinds. Does anyone?

The authors
nested mesosaurs between Synapsida and Captorhinidae (Fig. 1). Neither suprageneric clade include basal members that in any way resemble mesosaurs.

A sampling of mesosaur sister taxa
as recovered by the LRT is shown here (Fig. 2). I challenge the authors to find better sister taxa among the Synapsida or the Captorhinidae.

Figure 2. Click to enlarge. The origin of ichthyosaurs and thalattosaurs from basal diapsids and basal mesosaurs. Relationships are rather apparent when seen in this context.

Figure 2. The origin of ichthyosaurs and thalattosaurs from basal diapsids and basal mesosaurs. Relationships are rather apparent when seen in this context. Chronology is a little mixed up based on earlier radiations and the rarity of fossil formation.

A gradual accumulation of traits
is what we’re all looking for in a cladogram. If you don’t find that using your inclusion set, expand your inclusion set until you do.

Professors Laurin and Reisz
are at the top of the list of professional paleontologists, and have been at the top for decades. Unfortunately they’re holding on to an invalid hypothesis. There is no monophyletic clade ‘Parareptilia.’ Included members don’t look alike and simple expansion of the dataset splits them to other parts of the reptile family tree.

If you find yourself working with top workers
in the field and the results don’t make sense, you may be obligated to follow their lead. That seems to happen all too often. You’ll make more sensible discoveries if you keep a modicum of independence, or complete independence. And you’ll avoid the professional embarrassment of being criticized online instead of being hailed and complimented for ‘finally putting it all together’. This was an opportunity lost, no matter how much detail and data was provided.

This paper was edited by
Holly Woodward (Oklahoma State University) and reviewed by Michael S. Lee (South Australian Museum) and Juliana Sterli (Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina). It’s almost unheard of to see editors and reviewers listed near the titles of papers, btw. More often they are thanked in the Acknowledgements section.

On the plus side
I’m happy to see Piñeiro included a mesosaur skull with every bone colored (Fig. 3). That’s the way to do it nowadays. And if you’re into mesosaurs, this paper does provide a great deal of data about mesosaurs.

Figure 3. Mesosaur skull with bones colored by Laurin and Piñeiro 2017, modified from Piñeiro et al. 2012b.

Figure 3. Mesosaur skull with bones colored by Laurin and Piñeiro 2017, modified from Piñeiro et al. 2012.

References
Laurin M and Piñeiro GH 2017. A reassessment of the taxonomic position of mesosaurs, and a surprising phylogeny of early amniotes. Frontiers in Earth Science, 02 November 2017: 13 pp.  https://doi.org/10.3389/feart.2017.00088
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. 11(5):379-391.

Wumengosaurus and mesosaurs: how can these two NOT be related?

Figure 1. Wumengosaurus specimens from Wu et al. to scale showing size variety.

Figure 1. Wumengosaurus specimens from Wu et al. scaled to show size variety.

Wumengosaurus, the basal enaliosaur, is known from several sizes (Fig 1). Sometimes that’s not readily apparent when all the images are published at the same size, not to the same scale.

What happens to the skull of Wumengosaurus as it matures (Fig. 2)?

Figure 2. Wumengosaurus in small and large varieties along with Stereosternum and Mesosaurus to scale.

Figure 2. Wumengosaurus in small and large varieties along with Stereosternum and Mesosaurus to scale.

The rostrum doesn’t get relatively longer. The skull becomes relatively smaller.

The relatively small Stereosternum is a sister to Wumengosaurus in the large reptile tree. Their skulls document their similarities, overlooked by Wu et al. (2011) and Jiang et al. (2008). So what if the temporal fenestra disappear in certain (not all) Mesosaurus? That’s just a small number of characters out of a suite of synapomorphies.

References
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.
Wu X-C, Cheng Y-N, Li C, Zhao L-J and Sato T 2011. New Information onWumengosaurus 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.

wiki/Wumengosaurus

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.

What is Hupehsuchus? Another “Platypus of the Triassic”

Yesterday we looked at a new “platypus of the Triassic,” the basal thalattosaur, Concavispina and noted some traits shared with Hupehsuchus (Fig. 1), another “platypus of the Triassic,” by convergence.

Nesting Problems
Paleontologists have had trouble figuring out what Hupehsuchus (Middle Triassic) was. This marine reptile nearly stymied Carroll and Dong (1991) who found it shared 32 derived traits with ichthyosaurs and 29 with the completely unrelated mosasaurs, 26 with plesiosaurs and 22 with nothosaurs. Mesosaurs were not included, but they are further removed than ichthyosaurs. Wumengosaurus, the current outgroup taxon, was unknown at the time.

Figure 1. Click to enlarge. Hupesuchus, a close relative of Concavispina and ichthyosaurs, derived from Wumengosaurus and Stereosternum.

Figure 1. Click to enlarge. Hupesuchus, a close relative of Concavispina and ichthyosaurs, derived from Wumengosaurus and Stereosternum.

Wikipedia (Feb. 2013) reported, “It is unknown exactly what Hupehsuchus is related to. It is fairly clear that it shares a close relationship with Nanchangosaurus, but other relations are unknown. Many features, including the discovery of polydactyly, suggest that Hupehsuchus is related to the ichthyosaurs, but the fact that Hupehsuchus’ extra digits include more bones in the hand, rather than just the fingers as in the ichthyosaurs, may discredit that theory. It along withNanchangosaurus seem to be so different from any other reptile that a new order has been constructed for the two genera called Hupehsuchia.”

Motani (1999) correctly nested Hupehsuchus at the base of the Ichthyosauria, but nothing beyond the base, leaving that a great unknown.

This is lunacy. Or lethargy.
All it takes is a phylogenetic analysis to figure out what Hupehsuchus is and where it nests.  The large reptile tree nested Hupehsuchus at the base of the Ichthyosauria. Both taxa were sisters to Thalattosauria. All three were derived from a sister to Wumengosaurus and the mesosaur, Stereosternum. You can trace the lineage all the way back to the first tetrapods, but we’ll stop here with Petrolacosaurus (Fig. 2).

One of the big problems that the large reptile tree overcame
was the nesting of mesosaurs within the Diapsida. Mesosaurs largely, but not completely, roofed over their temporal fenestrae, which caused them to be seriously mislabeled with the pareiasaurs and millerettids. By focusing on a single trait (temporal fenestrae) while ignoring a similar absence of a lateral temporal fenestra in a known diapsid, Araeoscelis, AND ignoring a suite of other traits, paleontologists essentially painted themselves into a corner they then could not escape from.

(The same sort of academic blindfold also exists with poposaur ankles.)

 

Figure 2. Click to enlarge. The origin of ichthyosaurs and thalattosaurs from basal diapsids and basal mesosaurs. Relationships are rather apparent when seen in this context.

Figure 2. Click to enlarge. The origin of ichthyosaurs and thalattosaurs from basal diapsids and basal mesosaurs. Relationships are rather apparent when seen in this context.

Permian marine reptiles
Evidently there was a huge and rapid diversification of reptiles following the return of mesosaurs to the water. We’re just now getting twigs from that bush. That’s why Hupehsuchus looks so different form Wumengosaurus and Utatsusaurus. Then again, it looks even _less_ like anything else on the large reptile tree, and that tells the tale.

So what was behind that increase in marine diversification?
Were mesosaur descendants competing with any other tetrapods in the water? Some mesosaurs were able to handle hyper-saline waters. Living amphibians like fresh waters. But temnospondyls, the big amphibians of the Carboniferous, Permian and Triassic, are found in both fresh and coastal marine sediments. So the amphibians were there first and they were bigger.

Mesosaurs, plesiosaurs and ichythyosaurs were all live bearers, so that may have been a factor. Amphibians were all still laying eggs in water. So mother mesosaurs protected her embryos until birth, but the young were fewer in number as amphibians typically produce large amounts of eggs.

Mesosaurs, plesiosaurs and ichythyosaurs are not found in coal deposits, but in sediments that once formed sea floors. So perhaps more open seas further from shore gave early Permo/Triassic marine reptiles a playing field in which to evolve quickly and successfully, away from the Early and Middle Triassic amphibians.

Of course the great Permo-Triassic extinction event might have helped.

Remember, when we find a fossil taxon, it can be millions of years older than the original specimen of that species, having spread and multiplied, thereby multiplying our chances of finding it. Then again, the species and its fossil could be just a flash in the pan, of its own time only. Phylogenetic analysis helps in this regard, finding specimens millions of years younger than their phylogenetic descendants, or not, helps determine the longevity of a species. But I digress.

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.

By the way the Feb. 5 blog on phylogenetic analysis was hugely popular despite its farcical nature. 

References
Carroll RL and Dong Z-M 1991. Hupehsuchus, an enigmatic aquatic reptile from the Triassic of China, and the problem of establishing relationships. Philosophical Transactions of the Royal Society London B 28 331:131-153.
Motani R 1999. Phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology 19(3):473-496.
Young C-C and Dong Z-M 1972. On the aquatic reptiles of the Triassic in China. Vertebrate Paleontology Memoirs. 9-1-34.

wiki/Hupehsuchus

A Giant Mesosaur

Cymbospondylus is a primitive Triassic ichthyosaur of enormous length, approximately ten meters. It is also, due to its geneology a giant mesosaur. All ichthyosaurs and thalattosaurs are derived from mesosaurs. Cymbospondylus is one of the few ichthyosaurs to retain the long, sinuous body shape of mesosaurs.

Figure 1. Cymbospondylus overall in situ.

Figure 1. Cymbospondylus overall in situ. Overall, a very similar morphology to any basal mesosaur with the addition of flippers transformed from limbs.

Despite the Size Difference
Actually an order or two of magnitude larger in size, the giant ichthyosaur Cymbospondylus (Leidy 1868) kept the basic proportions of Stereosternum, but with a shorter neck and limbs transformed into flippers.

Stereosternum, a basal mesosaur

Figure 2. Stereosternum, a basal mesosaur

A comparison of skulls helps make the point.
The long premaxilla, the posteriorly shifted nares, the size of the supratemporal are obvious shared traits with Wumengosaurus acting as a transitional taxon. Details at reptileevolution.com.

Figure 2. A comparison of mesosaur skulls. Stereosternum at the base. Wumengosaurus with a very distinct upper temporal fenestra. And Cymbospondylus with upper temporal fenestra more dorsally oriented.

Figure 2. A comparison of mesosaur skulls. Stereosternum at the base. Wumengosaurus with a very distinct upper temporal fenestra. And giant Cymbospondylus (not to scale) with upper temporal fenestra more dorsally oriented, not quite visible in lateral view.

No other taxa are closer in the large reptile tree to thalattosaurs and ichthyosaurs than mesosaurs.

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
Leidy J 1868. Notice of some reptilian remains from Nevada: Proceedings of the American Philosophical Society, v. 20, p. 177-178.

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.

Mesosaurus Mistake?

The recent interpretation of a lateral temporal fenestra in Mesosaurus (Piñeiro et al. 2012, Fig. 1) was accompanied by a new interpretation of the premaxilla/maxilla suture. Added to this problem, the published restorations of the Mesosaurus skull in lateral and dorsal views don’t match (Fig. 1). Note the shifting of the fenestrae among other sutural changes.  Unfortunately, no sister taxa have the premaxilla/nasal configuration that Piñeiro et al. (2012) propose.

Mesosaurus skull with lateral temporal fenestra.

Figure 1. Mesosaurus skull with lateral temporal fenestra. From Piñeiro et al. 2012. Note the many differences in bone shape not attributable to the two distinct views. More rigorous attention would have aligned the fenestra and sutures in both views of these apparently freehand sketches.

Mesosaurus skull reconstructed based on data from Modesto (2006).

Figure 2. Mesosaurus skull reconstructed based on data from Modesto (2006).

Modesto
had it right.

Modesto (2006) delineated and identified the premaxilla and nasal of Mesosaurus (Fig. 2) and these sutures also agree with those of sister taxa, as recovered by the large reptile tree. With or without the skull, mesosaurs don’t shift within the tree.

Rostrum of Mesosaurus (dorsal view).

Figure 3. New tracings of the Piñeiro et al. (2012) rostral skull of Mesosaurus. On the left the bones are colored in the new interpretation. At right blue indicates published interpretation of nasals, unlike those of any other sister.

The split or separation of the nasals seen in thalattosaurs and sauropterygians (by convergence) has its origin in mesosaurs. In these taxa the elongated premaxilla contacts (sometimes just barely) the frontals. In ichthyosaurs, like wumengosaurs and mesosaurs, the premaxilla does not contact the frontals. Even so, it does split the nasals at least anteriorly.
Don’t Forget the von Huene (1940) Interpretation.
von Huene (1940) spent a considerable amount of time examining several Mesosaurus specimens and he was the first to interpret a lateral temporal fenestra (Fig. 4). He also interpreted nasals extending anteriorly a short distance beyond the nares as in the Piñeiro (2012) interpretation.

Almost overlooked, von Huene’s interpretations of the rostrum are much narrower than those of Piñeiro et al. (2012) indicating some variation in the specimens they were looking at. Certainly these are distinct species. Perhaps distinct genera?

Mesosaurus according to von Huene 1940.

Figure 4. Mesosaurus according to von Huene 1940. Color added to nasals in blue and premaxilla in yellow. This interpretation of the premaxilla/nasal suture matches that of Piñeiro et al. (2012).

Which is Correct?
Neither Piñeiro et al. (2012) nor von Huene (1940) compared their specimens to sisters recovered by the large reptile tree in which the premaxilla bisects the nasals. The image below is recolorized from Piñeiro (2012) showing two interpretations: 1) a longer premaxilla and 2) a possible interdigitating nasal and premaxilla. Hard to say given the data (Fig. 2).

Two reinterpretations of the premaxilla/nasal suture in a recolorized restoration of Mesosaurus

Figure 5. Two reinterpretations of the premaxilla/nasal suture in a recolorized restoration of Mesosaurus by Piñeiro et al. (2012). See Fig. 1 for the original. Also more squamosal has been restored on one side. It is not only possible, but likely that there is variation in several aspects of Mesosaurus from quarry to quarry and from species to species.

What’s Interesting
Here you see variation in interpretation between three paleontologists (one being the Piñeiro team). Look around and you’ll see the maxilla extends beyond mid orbit in two interpretations, but not the Modesto (2004) one. The tooth count differs in all three. The rostral proportions differ. The lateral temporal fenestra differs. The premaxilla/nasal suture differs. The frontal/nasal suture differs. Granted, they are not all looking at the same specimens. And now you can see why it is so important in phylogenetic analysis to use specimens for taxa, even if incomplete.

The Good Thing Is…
Mesosaurus will never again be lumped with basal anapsids like pareiasaurs and captorhinids. Hopefully…

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.

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.