A basal hupehsuchid with a duckbill: YAGM V 1401

Updated September 30, 2021
with new tracings and nesting the YAGM specimen between Wumengosaurus and the rest of the Ichthyopterygia, basal to hupehsuchids.

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 then 1944 taxa now; Fig. 3) nests the YAGM specimen basal to the clade of hupehsuchids, close to Wumengosaurus.

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. Eretmorhipis 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. Eretmorhipis 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 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 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.

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.

 

The “lost” quadratojugal and supratemporal of Wumengosaurus

In basal enaliosaurs, the quadratojugal tends to be ignored or misidentified. Claudiosaurus is one such example. That’s because the qj generally shifts during taphonomy.  Wumengosaurus is another such taxon based on several specimens (Fig. 1). There is also a misidentified supratemporal here.

Figure 1. The unidentified supratemporal (in yellow/green) and quadratojugal (in lavender) in two Wumengosaurus specimens. Postorbital in violet. Squamosal in magenta. this is the ichthyosaur temporal region pattern, losing the lateral temporal fenestra, but gaining a secondary lateral temporal fenestra by the raising of the quadratojugal.

Figure 1. The unidentified supratemporal (in yellow/green) and quadratojugal (in lavender) in two Wumengosaurus specimens. Postorbital in violet. Squamosal in magenta. this is the ichthyosaur temporal region pattern, losing the lateral temporal fenestra, but gaining a secondary lateral temporal fenestra by the raising of the quadratojugal.

Why look for a bone that’s not identified?
Because sister taxa (Stereosternum, Mesosaurus, ichthyosaurs)  have a supratemporal and quadratojugal.

The fact that the lateral temporal fenestra disappears between the quadratojugal and squamosal in ichthyosaurs, yet both are arched posterior to the jugal means ichthyosaurs have a secondary lateral temporal fenestra, convergent with the typical lateral temporal fenestra of basal diapsids and protorosaurs + archosauriforms. And that’s how ichthyosaurs need to be scored in phylogenetic analysis.

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.
Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity 3: 151-214.
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

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.