Pre-elasmosaurs out-competed tanystropheids as passive vertical predators

The hyper-elongate neck developed by convergence three times
(Figs. 1, 2) in the known prehistory of marine tetrapods. On land we have giraffes, langobardisaurs and sauropods, but they are not considered here due to their separate terrestrial environs. Based on the similar necks and diets (fish and squid of these three marine tetrapods, perhaps some of the mystery surrounding these taxa can be resolved.

Figure 1. Albertonectes, Tanystropheus and Dinocephalosaurus to scale.

Figure 1. Albertonectes, Tanystropheus and Dinocephalosaurus to scale. With all the other predators assuming a horizontal pose, maybe the vertical neck of these predators in the midst of schools of fish and squid went unnoticed…until it was too late. Maybe those rocks in the belly of the elasmosaur helped keep it anchored.

The three marine taxa with hyper elongate necks
(Fig. 1) are Albertonectes (Elasmosauridae), Tanystropheus (Tritosauria), and Dinocephalosaurus (Tritosauria). We also know of several specimens closely related to each of these taxa, discussed here, here, and here. They all share more than a hyper-elongate neck in common, but that’s the one thing that predominates. They appear to have all been marine vertical predators, passively extending their neck up into schools of prey, essentially unrecognized because they were not horizontal speedsters, like all the other predators out there.

Figure 2. Skulls of Albertonectes, Dinocephalosaurus and two types of Tanystropheus skulls not to scale.

Figure 2. Skulls of Albertonectes, Dinocephalosaurus and two types of Tanystropheus skulls compared, not to scale. Lots of convergence here, it’s plain to see.

Convergent skull traits in vertical feeders:

  1. Small skull
  2. Long procumbent teeth
  3. Large premaxilla
  4. Upward facing eyes
  5. Dorsally displaced nares
  6. Rostrum wider than tall
  7. Internal naris migrated posteriorly
  8. Flat palate

Renesto 2005 along with Renesto and Saller 2018
presented evidence to show that Tanystropheus had a semi-aquatic horizontal lifestyle.

  1. “Tanystropheus was able to lift the body off the substrate when on land,
  2. Tanystropheus lacked adaptations for continuous swimming, either tail-based or limb-based,
  3. Tanystropheus was able to swim for by rowing with symmetrical strokes of the hind limbs.”

But remember,
Renesto and Saller mistakenly considered Tanystropheus a protorosaur and an archosauromorph. It is neither. In the large reptile tree (LRT, 1175 taxa) Tanystropheus nests with Huehuecuetzpalli and pterosaurs, all in the clade Tritosauria, a clade within Lepidosauria.

Renesto and Saller continue:
“The life style of Tanystropheus,the largest and most bizarre of all tanystropheids, remained uncertain since its discovery… In conclusion, Tanystropheus may have had lived in a shore line environment, where the elongate neck, may have been used to cach preys in shallow water by dashing at the prey propelled by hindlimbs, either starting from the shoreline from a resting positionor, in water, eventually after slowly closing the distance. In water, the long neck would have allowed Tanystropheus to conceal its real size while slowly approaching to fish or squid schools by reducing the disturbance caused by body surrounding water, avoiding to be detected by the prey’s lateral line. When close enough, Tanystropheus may have shifted to fast pursuit for the sudden propulsive final phase, with a series of rapid symmetrical strokes of the hind limbs (Fig. 6).”

Yeah, maybe…
but Renesto and Saller just said Tanystropheus was not a good swimmer. So let’s toss out that shift to fast pursuit.

Imagine a passive predator distinct from
all the other predators assuming a horizontal pose. Maybe the vertical neck of all these predators in the midst of schools of fish and squid went unnoticed by them…until it was too late. Maybe it’s as simple as that. No extant taxa can be used by analogy, so we have to look at extinct taxa with similar traits. We looked at Tanystropheus among the crinoids (Fig. 1), and the evolution to that niche earlier here. The convergent Dinocephalosaurus neck strike hypothesis is from Peters, Demes and Krause 2005. The long-necked limbed ancestors of elasmosaurs were morphologically similar and coeval to long-necked limbed tritosaurs in the Middle Triassic (Fig. 5).

Note added on the vertical neck: 
Peters, Demes and Krause 2005 (actually just Peters, in this case, as there were three comments to the original Dinocephalosaurus paper (Li, Rieppel and LaBarbera 2004), now lumped into one reply) suggested that breathing would have been difficult for long-necked underwater taxa due to changes in pressure with increasing depth, but these taxa could swallow air at the surface then lower the neck to allow the air bubble to rise into the lungs. Just a few degrees of declination would do the trick.

Renesto and Saller report:
“The study focused mostly on the post-dorsal sections of the vertebral column, on the pelvis and hind limb.”  Ignoring the neck in Tanystropheus ignores the biggest clue to its niche. Let’s not do that.

Figure x. The Late Triassic world with the tropical San Giorgio area where Tanystropheus is found highlighted.

Figure 3. The Middle Triassic world with the tropical shallow San Giorgio area where Tanystropheus is found highlighted. Warm waters enabled Tanystropheus and other Triassic reptiles  to stay submerged continually.

Another dinocephalosaur
was reconstructed here (Fig. 3). The neck in this specimen is so gracile, it is difficult to imagine it in any active mode, so the vertical passive pose remains as the only viable alternative. These are not active predators.

Figure 1. The new Dinocephalosaurus has traits the holotype does not, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with an elongate pedal digit 4.

Figure 4. The new Dinocephalosaurus has traits the holotype does not, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with an elongate pedal digit 4.

And finally
Note the long neck of the derived nothosaur/pre-elasmosaur, like Wangosaurus (Middle Triassic), preceded the evolution to flippers found in later vertical feeding elasmosaurs (Fig. 5). Like the the coeval vertical predators tanystropheids (Fig. 1) and dinocephalosaurs (Fig. 4) pre-elasmosaurs like Wangosaurus out-competed these similar tritosaur lepidosaurs, which cannot be found after the Triassic. Clearly pre-elasmosaurs were not such great swimmers when they started, and must have been only marginally better swimmers after their small limbs became small flippers. Given this data, the hypothesis of vertical predation of small squid and fish prey in pre-elasmosaurs and their elasmosaur descendants deserves the opportunity to be falsified.

By contrast, pliosaurs,
like Brachauchenius, with their big flippers and large toothy skulls, were excellent horizontal predators and fast swimmers. This contrast is key to the present hypothesis.

Figure 5. Elasmosaurid origins. The long neck preceded the flippers in this clade of vertical feeders.

Figure 5. Elasmosaurid origins. The long neck preceded the flippers in this clade of vertical feeders.

Additional data:
Albertonectes vanderveldei (Kubo et al. 2012; Upper Campanian, Alberta; TMP 2007.022.0002) is a virtually complete elasmosaur 11.2m in length (the longest of any elasmosaur) lacking only the skull. It had a 7m neck of 76 vertebrae, the most of any vertebrate. Stones in the belly might have kept it anchored. The gizzard in birds is located posteriorly, as seen in this elasmosaur.

Tanystropheus longobardicus (Tanystropheus conspicuus von Meyer 1855,  Tribelesodon longobardicus Bassani 1886,  Tanystropheus longobardicus Peyer 1930) Anisian, Middle Triassic, ~240 mya, ~4.5m in length, was considered a pterosaur before Peyer (1930) established that the long bones were neck bones, not wing bones. Derived from a sister to the the T4822 specimen of MacrocnemusTanystropheus was a sister to the much smaller Tanytrachelos and Langobardisaurus, rather than the convergent Dinocephalosaurus. Warm waters enabled Tanystropheus and other Alpine Triassic reptiles  to stay submerged continually.

Dinocephalosaurus orientalis (Li, Rieppel and LaBarbera 2004) Late Ladinian, Middle Triassic ~228 mya, was orginally considered a marine sister to Tanystropheus with limbs nearly transformed into paddles of similar size. Phylogenetic analysis places it closer to a specimen of MacrocemusT2472. Dinocephalosaurus was not a protorosaur, as originally described. Rather Dinocephalosaurus was a tritosaur lepidosaur The skull was described as crushed, but it was actually quite flat in life with dorsally directed orbits. The ribs were also much wider than deep. Both of these are characters found in bottom dwellers, not free-swimmers. The cervical (25) and dorsal (33) counts are the highest among tanystropheids. The limbs were short but the hands and feet were relatively large, paddle-like and probably webbed.

Figure 6. A squad of squid, food for both tanystropheids and elasmosaurs.

Figure 6. A squad of squid, food for both tanystropheids and elasmosaurs.

References
Bassani F 1886. Sui Fossili e sull’ età degli schisti bituminosi triasici di Besano in Lombardia. Atti della Società Italiana di Scienze Naturali 19:15–72.
Li C, Rieppel O and LaBarbera MC 2004. A Triassic aquatic protorosaur with an extremely long neck. Science 305:1931.
Liu, J. et al. 2017. Live birth in an archosauromorph reptile. Nature Communications 8, 14445 doi: 10.1038/ncomms14445
Peters D, Demes B and Krause DW 2005. Suction feeding in Triassic Protorosaur? Science, 308: 1112-1113.
Kubo T, Mitchell MT and Henderson DM 2012. Albertonectes vanderveldei, a new elasmosaur (Reptilia, Sauropterygia) from the Upper Cretaceous of Alberta. Journal of Vertebrate Paleontology 32 (3): 557-572. DOI:10.1080/02724634.2012.658124.
Li C 2007. A juvenile Tanystropheus sp.(Protoro sauria: Tanystropheidae) from the Middle Triassic of Guizhou, China. Vertebrata PalAsiatica 45(1): 37-42.
Li C, Rieppel O and LaBarbera MC 2004. A Triassic aquatic protorosaur with an extremely long neck. Science 305:1931.
Liu, J. et al. 2017. Live birth in an archosauromorph reptile. Nature Communications 8, 14445 doi: 10.1038/ncomms14445
Lockley MG 2006. Observations on the ichnogenus Gwineddichnium and  wyneddichnium-like footprints and trackways from the Upper Triassic of the Western United States. In: Harris JD, Lucas SG, Spielmann JA, Lockley MG, Milner ARG. & Kirkland JI (Eds) – The Triassic-Jurassic Terrestrial Transition. New Mexico Museum of Natural History Science Bulletin 37: 170-175.
Meyer H von 1847–55. Die saurier des Muschelkalkes mit rücksicht auf die saurier aus Buntem Sanstein und Keuper; pp. 1-167 in Zur fauna der Vorwelt, zweite Abteilung. Frankfurt.
Nosotti S 2007. Tanystropheus longobardicus (Reptilia, Protorosauria: Reinterpretations of the anatomy based on new specimens from the Middle Triassic of Besano (Lombardy, Northern Italy). Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, Vol. XXXV – Fascicolo III, pp. 1-88
Peters D, Demes B and Krause DW 2005. Suction feeding in Triassic Protorosaur? Science, 308: 1112-1113.
Peyer B 1931. Tanystropheus longobardicus Bass sp. Die Triasfauna der Tessiner Kalkalpen. Abhandlungen Schweizerische Paläontologie Gesellschaft 50:5-110.
Renesto S 2005. A new specimen Tanystropheus (Reptilia Protorosauria) from the Middle Triassic of Switzerland and the ecology of the genus: Rivista Italiana di Paleontologia e Stratigrafia 111(3): 377-394.
Renesto S and Saller F 2018. Evidences for a semi aquatic life style in the Triassic diapsid reptile Tanystropheus. Rivista Italiana di Paleontologia e Stratigrafia 124(1):23-34.
Rieppel O, Jiang D-Y, Fraser NC, Hao W-C, Motani R, Sun Y-L & Sun ZY 2010. Tanystropheus cf. T. longobardicus from the early Late Triassic of Guizhou Province, southwestern China. Journal of Vertebrate Paleontology 30(4):1082-1089.
Wild R 1973. Die Triasfauna der Tessiner Kalkalpen XXIII. Tanystropheus longobardicus(Bassani) (Neue Ergebnisse). – Schweizerische Paläontologische Abhandlungen 95: 1-162 plus plates.

wiki/Dinocephalosaurus
wiki/Tanystropheus
https://en.wikipedia.org/wiki/Albertonectes

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Leaping lizards in the Late Triassic

Lockley 2006
documented paired, digitigrade, four-toed, accelerating, ‘swimming’ tracks (ichnogenus: Gwyneddichnium; Figs. 1, 2) in the Late Triassic. Tanytrachelos (Figs. 1, 2) was and is considered a good match, but hopping through thinly retreating surf seems to be a better solution. Tanytrachelos is a tritosaur lepidosaur, hence, a ‘leaping lizard’.

Figure 1. Gwyneddichnium tracks (note the acceleration). Along with a to scale Tanytrachelos and Tanytrachelos pes. Note digit 1 does not impress.

Figure 1. Gwyneddichnium tracks CU 159.10 (note the acceleration). Along with a to scale Tanytrachelos and Tanytrachelos pes. Note digit 1 does not impress. As in Cosesaurus, digit 1 impressed only rarely and then only as a point.

Like Cosesaurus and other higher tritosaurs, 
Tanytrachelos was digitigrade and facultatively bipedal. Hopping through thinly retreating surf is more likely based on matching the body to the tracks. So we don’t have to imagine the front half floating or swimming underwater to make the hind feet simultaneously kick to make side-by-side tracks.

Figure 1. Tanytrachelos hopping to match Gwyneddichnium tracks (see figure 2).

Figure 2. Tanytrachelos hopping to match Gwyneddichnium tracks (see figure 1).

Running and arm flapping in Cosesaurus
led to flapping flight in pterosaurs (Fig. 3).

Figure 1. Cosesaurus flapping - fast. There should be a difference in the two speeds. If not, apologies. Also, there should be some bounce in the tail and neck, but that would involve more effort and physics.

Figure 3. Click to enlarge and animate. Cosesaurus flapping – fast. There should be a difference in the two speeds. If not, apologies. Also, there should be some bounce in the tail and neck, but that would involve more effort and physics.

Gwyneddichnium and Rotodactylus tracks
are from trackmakers (Fig. 4)in the same clade of Tritosauria.

Cosesaurus matched to Rotodactylus from Peters 2000.

Figuure 4. Cosesaurus matched to Rotodactylus from Peters 2000.

Another relative of Tanytrachelos, Langobardisaurus,
(Fig. 5) has been considered bipedal by prior authors (Renesto, et al. 2002).

Figure 4. Langobardisaurus bipedal.

Figure 5. Langobardisaurus bipedal.

References
Lockley M 2006. Observations on the ichnogenus Gwyneddichnium and Gwyneddichnium-like footprints and trackways from the Upper Triassic of the western United States. New Mexico Museum of Natural History and Science, Bulletin 37:170–175.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos 7:11-41.
Renesto S, Dalla Vecchia FM and Peters D 2002. Morphological evidence for bipedalism in the Late Triassic Prolacertiform reptile Langobardisaurus. Senckembergiana Lethaea 82(1): 95-106.

 

Revisiting the pes of Pectodens

Earlier we looked at Pectodens, a long-necked tritosaur that nests at the base of the tanystropheids + langobardisaurs and the fenestrasaurs, which ultimately gave rise to pterosaurs.

Metacarpal 5 is the problem (Fig. 1).

Figure 1. The right pes of Pectodens in situ (left), sans the matrix (right), and rearranged to match sister taxa (center). The question is: is the rearrangement valid?

Figure 1. The right pes of Pectodens in situ (left), sans the matrix (right), and rearranged to match sister taxa (center). The question is: is the rearrangement valid?

 

Which bone is metacarpal 5?
Is it the long bone similar to metacarpal 4? That would make sense with most taxa, except Pectodens nests with other long-necked taxa, like Langobardisaurus and Tanystropheus. In those taxa metacarpal 5 is short and pedal 5.1 is metapodial (= very long).

Did taphonomy change things?
Or do we trust phylogenetic bracketing?

One more thing…
If the long bone is the metacarpal, then the phalangeal count matches sister taxa (4 phalanges). If the short bone is the metacarpal, then there is one extra phalanx. Did the preparator add a bone? Or did this taxon have an extra bone?

And take a look
at the width of the tibia + fibula. It’s the right width if the short bone is metacarpal 5. The width is not quite wide enough if the long bone is metacarpal 5.

Sometimes
you have to make a decision in paleontology. Sometimes you have to point your finger at a preparator’s mistake. Sometimes you make the mistake when you use your brain OR when you accept the data as presented.

What to do… what to do…

Figure 1. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017.

Figure 2. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017.

Here’s what I wrote a while back
at ReptileEvolution.com:

Pectodens zhenyuensis (Li et al. 2017; IVPP V18578; Anisian, Middle Triassic; 38cm in length) was originally considered to be a diapsid and a possible protorosaur. Here Pectodens nests between Macrocnemus and Langobardisaurus. Originally the interclavicle, sternum and quadratojugal were overlooked. Note the large orbit, the long metarsal 5 and the perforated pubis. The elongate caudal transverse processes anchor powerful leg muscles.

Figure 2. Pectodens skull traced using DGS techniques and reassembled below.

Figure 3. Pectodens skull traced using DGS techniques and reassembled.

With the short metacarpal 5
Pectodens cleanly nests with fewer autapomorphies at the base of the Langonbardisaurus/Tanystropheus clade.

References
Li C, Fraser NC, Rieppel O, Zhao L-J and Wang L-T 2017. A new diapsid from the Middle Triassic of southern China. Journal of Paleontology.7 pp. doi: 10.1017/jpa.2017.12

Another long-necked embryo tritosaur: Li et al. in press

This appears to be
yet another Tanystropheus-like and Dinocephalosaurus-like taxon, yet not closely related to either. Earlier we looked at another similar embryo, still within its mother.

Li, Rieppel and Fraser in press (2017)
bring us a new curled up (as if in an egg, but without a shell) embryo from the Guanling Formation (Anisian), Yunnan province, China (Figs. 1, 2). The specimen is unnamed and not numbered. It appears to combine the large head and eyes of langobardisaurs with the short limbs and many cervical vertebrae of Dinocephalosaurus. Please remember, in this clade, juveniles do not have a short rostrum and large eyes unless their parents also had these traits.

Figure 1. The unnamed and not numbered Triassic embryo Li et al. assign to a new species close to Dinocephalosaurus.

Figure 1. The unnamed and not numbered Triassic embryo Li et al. assign to a new species close to Dinocephalosaurus. At 72 dpi monitor resolution, this image is 2.5x life size. Here bones are colorized, something Li et al. could have done, but avoided. I’m happy to report that the line drawing was traced by Li et al. on their own photo. The two are a perfect match.

Unfortunately
Li et al. have no idea what they’re dealing with phylogenetically. They relied on old invalidated hypotheses of relationships. They report the specimen:

  1.  is a marine protorosaur and an archosauromorph – actually it is a marine tritosaur lepidosaur. Taxon exclusion and traditional bias hampered the opinion of Li et al. They did not perform a phylogenetic analysis.
  2. is closely related to Dinocephalosaurus – actually it is more closely related to the much smaller, but longer-legged Pectodens (Figs. 4, 5). In the large reptile tree (LRT, 1036 taxa) 8 steps are added when the embryo is force-nested with Dinocephalosaurus. The embryo is distinct enough that the new specimen deserves a new genus.
  3. confirms viviparity – probably not (but see below). The specimen is confined within an elliptical shape (Fig. 1), as if bound by an eggshell or membrane, which was not preserved. Perhaps, as in pterosaurs and many other lepidosaurs, the embryo was held within the mother’s body until just before hatching, within the thinnest of egg shells and/or membranes.
  4. is too immature to describe it as a new taxon – not so. Tritosaur lepidosaurs (from Huehuecuetzpalli to Pterodaustro) develop isometrically. Thus, full-term embryos and hatchlings have adult proportions.
Figure 2. The specimen from figure 1 unrolled for clarity. This specimen most closely resembles the basal langobardisaur, Pectodens, not Dinocephalosaurus. Remember, tritosaurs develop isometrically. Embryos closely resemble adults. That's why three scale bars are included.

Figure 2. The specimen from figure 1 unrolled for clarity. This specimen most closely resembles the basal langobardisaur, Pectodens, not Dinocephalosaurus. Remember, tritosaurs develop isometrically. Embryos closely resemble adults. That’s why three scale bars are included. This specimen has feeble limbs but a strong swimming tail, distinct from that of Dinocephalosaurus.

Li et al. report
“In the fossil record only oviparity and viviparity can be distinguished, Ovoviviparity of different intermediate stages, which is often observed in modern squamates would then be referred to the category of viviparity, whatever the stages of maturity and nutritional patterns are.” Yes, they correctly report ovoviviparity in squamates, which are the closet living relatives of tritosaur lepidosaurs. That’s exactly what we have here.

Figure 1. The new Dinocephalosaurus has traits the holotype does not, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with an elongate pedal digit 4.

Figure 3. The new Dinocephalosaurus has traits the holotype does not, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with an elongate pedal digit 4.

Li et al. report,
“[The] skeleton is preserved tightly curled so as to produce an almost perfect circular outline, which is strongly indicative of an embryonic position constrained by an uncalcified egg membrane.”

Figure 2. Pectodens skull traced using DGS techniques and reassembled below.

Figure 4. Pectodens skull traced using DGS techniques and reassembled below. No sclerotic ring here. 

Distinct from Pectodens the new genus embryo has:

  1. 24 cervicals
  2. 29 dorsals
  3. 2 sacrals
  4. and about 64 caudals
Figure 1. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017.

Figure 5. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017. The skull shown here is the original reconstruction. Compare it to figure 4.

Li et al overlooked:

  1. strap-like coracoids, strip-like clavicle and T-shaped interclavicle
  2. scattered manual elements
  3. pelvic girdle
  4. ectopterygoid, jugal, articular, angular, surangular

Li et al. report:
“The fewer cervical vertebrae (24 as opposed to 33 (based on an undescribed specimen kept in the IVPP)), and the presence of sclerotic plates are features inconsistent with Dinocephalosaurus.This embryo therefore documents the presence of at least one additional dinocephalosaur-like species swimming in the Middle Triassic of the Eastern Tethys Sea.

“Scleral ossicles have previously not been described in any protorosaur.”
– but they are common in tritosaur lepidosaurs, like pterosaurs.

Figure 6. Pectodens adult compared to today's embryo and its 8x larger adult counterpart after isometric scaling.

Figure 6. Pectodens adult compared to today’s embryo and its 8x larger adult counterpart after isometric scaling. Looks more like Pectodens than Dinocephalosaurus, doesn’t it? See taxon inclusion WORKS! Sclerotic rings were omitted here to show skull bones. The ring would have had a smaller diameter if if were surrounding a sphere, rather than crushed flat. 

A word to traditional paleontologists:
Don’t keep digging yourself deeper into invalidated hypotheses and paradigms. Use the LRT, at least for options.

Don’t give up on naming embryos
and adding them to phylogenetic analysis, especially if they are tritosaur lepidosaurs. Hatchlings nest with adults so you can used hatchlings in analysis.

Don’t avoid creating reconstructions.
That’s a great way to discover little splinters of bone, like clavicles and coracoids, that would have been otherwise overlooked.

The LRT is here for you.
BETTER to check this catalog prior to submission rather than have your work criticized for being unaware of the latest discoveries or overlooking pertinent taxa AFTER publication.

References
Li C, Rieppel O, Fraser N C, in press. Viviparity in a Triassic marine archosauromorph reptile. Vertebrata PalAsiatica, online here.

Pectodens: basal to tanystropheids and pterosaurs

It’s always good
to see another tritosaur. That’s the lineage that gave rise to a menagerie of taxa, including pterosaurs. That’s a heretical hypothesis of relationships recovered by the large reptile tree (LRT, 997 taxa).

Figure 1. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017.

Figure 1. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017.

Li et al. 2017 conclude:
“A new, small terrestrial tetrapod is described from the Middle Triassic of Yunnan, China. Pectodens zhenyuensis n. gen. n. sp. bears very characteristic elongate teeth forming a comb-like marginal dentition. The elongate cervicals of Pectodens zhenyuensis n. gen. n. sp. with low neural spines together with the morphology of the cervical ribs are features consistent with protorosaurs, such as Macrocnemus. However, the imperforate puboischiadic plate, simple rounded proximal tarsals, and a straight 5th metatarsal are primitive characteristics. Unlike tanystropheids, but in common with Protorosaurus (personal observation, N.C. Fraser, 2013), both lack a thyroid fenestra in the pelvis.”

Figure 2. Pectodens skull traced using DGS techniques and reassembled below.

Figure 2. Pectodens skull traced using DGS techniques and reassembled below. Here a quadratojugal process of the jugal is identified and other parts are assembled with greater accuracy than a freehand sketch (Fig. 1).

Pectodens zhenyuensis (Li et al. 2017; IVPP V18578; Anisian, Middle Triassic; 38cm in length) was originally considered a diapsid and a possible protorosaur. Here Pectodens nests between Macrocnemus and Langobardisaurus (Fig. 3). Originally the interclavicle, sternum and quadratojugal were overlooked.

Note the large orbit, the long metarsal 5 and the perforated pubis. The elongate caudal transverse processes anchor powerful leg muscles.

Occasionally within the Tritosauria
metatarsal 5 is not short, but elongate. It is always axially twisted. The pubis and ischium typically angle away from one another, but sometimes produce a thyroid fenestra. Tritosaurs have a sternum, like many other lepidosaurs do. Protorosaurs do not have a sternum.

Li et al. did not attempt a phylogenetic analysis.
Instead they made educated guesses as to the affinities of Pectodens, overlooking the variation present in related taxa revealed in a cladogram. Pulling a Larry Martin (highlighting or letting yourself get confused by one or two traits) is never a good idea. Better to let hundreds of traits determine the exact nesting of a taxon without bias. Let the taxa nest themselves. Let the convergent traits simply be convergent traits.

Earlier we looked at the pectoral girdle and sternum of Langobardisaurus, Huehuecuetzpalli and other tritosaurs. Pectodens fits right in.

The posterior maxillary teeth in Pectodens
are wider at their base presaging the grinding teeth found in Cosesaurus, basal pterosaurs and Langobardisaurus.

Note the way the fingers and toes
bend anteriorly during use. That’s a lepidosaur trait. Pectodens would have had sprawingling hind limbs given its simple femoral head. Tracks matching such curved toes are known from the Middle Triassic.

Li et al. considered Pectodens to be the first terrestrial taxon
from the its locality. And that’s definitely a probability. However, given that Tanystropheus and others may have been underwater bipedal predators (squid parts were found in their torso), let’s leave open the possibility that Pectodens was maybe dipping its toe in the water.

Figure 1. Subset of the LRT focusing on Tritosauria. Pectodens nests here basal to the Characiopoda (Tanystropheids + Fenestrasauria including pterosaurs).

Figure 1. Subset of the LRT focusing on Tritosauria. Pectodens nests here basal to the Characiopoda (Tanystropheids + Fenestrasauria including pterosaurs).

Let’s not continue to nest tanystropheids
with protorosaurs. Sure they share several traits by convergence, but they are not related to one another as determined by a large gamut analysis, the LRT.

References
Li C, Fraser NC, Rieppel O, Zhao L-J and Wang L-T 2017. A new diapsid from the Middle Triassic of southern China. Journal of Paleontology.7 pp. doi: 10.1017/jpa.2017.12

 

Live birth in ‘Dinocephalosaurus’? Maybe. Maybe not.

Yesterday Liu et al. 2017 reported on
a pregnant Dinocephalosaurus (Figs. 1-5). This is wonderful and exciting news. However, the embryo is NOT in the process of passing through the cloaca, as we’ve seen in ichthyosaurs. The embryo is much higher in the abdomen, still in the uterus. So the headline “Live birth in an archosauromorph reptile” is… at best… premature. Live birth is still a possibility. A critical examination of the data reveals a few more major and minor problems.

Dinocephalosaurus in resting, feeding and breathing modes.

Figure 1. The holotype (not the new specimen) of Dinocephalosaurus in resting, feeding and breathing modes. In breathing mode the throat sac would capture air that would not be inhaled until the neck was horizontal at the bottom of the shallow sea. Orbits on top of the skull support this hypothesis. Image from Peters 2005. The new specimen has a longer neck, a more robust tail, and a different pedal morphology.

Unfortunately
the authors nested Dinocephalosaurus within the Archosauromorpha (Fig. 2). That is incorrect. Dinocephalosaurus nests within the new Lepidosauromorpha in the large reptile tree (LRT, 952 taxa), which minimizes the taxon exclusion problem suffered by the much smaller taxon list in the Liu et al. 2017 tree.

Figure 6. Cladogram from Liu et al. 2017 with colors added based on results from the LRT. Taxon exclusion is a major problem here.

Figure 2. Cladogram from Liu et al. 2017 with colors added based on results from the LRT. Taxon exclusion is a major problem here. Note in the Liu et al. cladogram members of the Protorosauria are divided into three clades. In sympathy, members of the Tritosauria and Protorosauria do indeed converge with one another. More taxa clears up the problem shown here of cherry-picking taxa.

Dinocephalosaurus actually nests
within the lepidosaur clade, Tritosauria, a clade that also includes Tanystropheus, pterosaurs and several other taxa (Fig. 7) that had been mistaken for protorosaur relatives in the Liu et al and other prior studies.

As a lepidosaur, 
Dinocephalosaurus would have been able to retain embryos within the mother far longer that in extant archosauromorphs. And based on the extreme thinness of pterosaur eggshells (closest known relatives with embryos, Fig. 7), those leathery eggshells only develop just prior to egg laying. So live birth is only one of a spectrum of options for Dinocephalosaurus. As in pterosaurs, the eggs could have hatched shortly after the female laid them on the shoreline.

Dinocephalosaurus. Note the very narrow cranial portion of the skull and the very wide cheeks. That, by it self, opens the orbits dorsally. Sure there's some lateral exposure, but those eyes are looking up!

Figure 3. The holotyype of Dinocephalosaurus. Although extremely similar, the new specimen is different in several ways. See below.

Liu et al. report that live birth is unknown in the Archosauromorpha.
However, in the LRT mammals and enaliosaurs (sauropterygians + ichthyosaurs) are both archosauromorphs that experience live birth. Hyphalosaurus, a choristodere archosauromorph, had extremely thin eggshells and retained developing embryos inside the mother until laying those eggs.

Figure 5. Hypothetical Tanystropheus embryo compared to Dinocephalosaurus embryo.

Figure 4. Hypothetical Tanystropheus embryo compared to part of an embryo of the new specimen attributed to Dinocephalosaurus.

More about that embryo
What little is preserved of the Dinocephalosaurus embryo (Fig. 4) is curled up in its amniotic sac, as one would expect for any reptile embryo still in utero. For comparison, note the hypothetical Tanystropheus embryo alongside it. That long neck has to go somewhere and Dinocephalosaurus provides further evidence that juvenile tritosaurs were isometric duplicates of their adult parents. That long neck did not develop with maturity. Among other tritosaurs we see juveniles similar in proportion to adults in the basal form, Huehuecuetzpalliand all pterosaur embryos.

Liu et al. further report. “Despite the complexity of this transition, viviparity has evolved at least 115 times in extant squamates (lizards and snakes), in addition to a single time in the common ancestor of therian mammals. Moreover, viviparity is a common reproductive mode in extinct aquatic reptiles including eosauropterygians, ichthyosaurs, mosasauroids, some choristoderans and likely mesosaurs.” Since mosasauroids are extinct squamates that makes at least 116 times for lepidosaurs.    Some living squamates produce eggs that hatch shortly after they are expelled, a sort of transition from oviparity to viviparity. That’s where pterosaurs fall and perhaps Dinocephalosaurus.

More cladogram issues
The Liu et al. figure 1 cladogram shows a polytomy of most reptilian clades arising during the Permian. No such polytomy appears in the LRT in which Archosauromorpha diverged from the Lepidosauromorpha tens of millions of years earlier in the Viséan (Lower Carboniferous). Liu et al. mistakenly report that trilophosaurs, rhynchosaurs and pterosaurs are archosauromorph reptiles. They are lepidosauromorph reptiles in the LRT.

Figure 1. The new Dinocephalosaurus has traits the holotype does not, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with an elongate pedal digit 4.

Figure 5. The new Dinocephalosaurus has traits the holotype does not have, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with a more elongate pedal digit 4. The partial embryo is in magenta at left.

The new specimen looks like a Dinocephalosaurus, but is it one?
Distinct from the holotype, the new specimen has a deep robust tail with deep chevrons (Fig. 5) as in Litorosuchus (Fig. 6), also from China, but from higher strata. They all share a common ancestor in one of the highly variable Macrocnemus specimens (Fig. 7). The toes of the new specimen are more asymmetric. The neck probably has more vertebrae (several are lost, but note the longest ones are NOT at the base of the neck in the holotype). Unfortunately little more can be said with so much of the mother lacking at present. We’ve already seen a Chinese Tanystropheus similar to, but not identical to the European Tanystropheus. We can imagine even greater variation within the available gamut of the present sparse fossil evidence.

Figure 3. Litorosuchus compared to Macrocnemus and kin at two scales.

Figure 6. Litorosuchus compared to Macrocnemus and kin at two scales. Litorosuchus has deep chevrons and a robust tail, as in the new specimen, lacking in Dinocephalosaurus.

Figure 2. Given the scrappy, skull-less data, the putative Dinocephalosaurus either nests with its namesake or by convergence with Litorosuchus which shares deep chevrons and a robust tail, among other traits.

Figure 7. Given the scrappy, skull-less data, the putative Dinocephalosaurus either nests with its namesake or by convergence with Litorosuchus which shares deep chevrons and a robust tail, among other traits.

It really is too much
to expect identical specimens to come from distinct fossil bearing strata. So variation within Dinocephalosaurus is a possibility. But so is convergence from the lineage of Litorosuchus.

Next steps
The paleo-community needs to include more specimen-based taxa in their cladograms or the Liu et al. problem (not restricted to them!) is going to continue ad infinitum. I know that’s a lot of work. But it can be done (I’ve done it!) and it needs to be done. Just start with a large gamut analysis and keep adding taxa to it. That will make the current phylogenetic problems go away.

Final note
Images of tanystropheids and dinocephalosaurs swimming horizontally through open waters (Liu et al. 2017 their figure 3) may not be an accurate portrayals of their daily lives. Other options have been published (Fig. 1) or appear online (Fig. 8). Odd-looking tetrapods often have uncommon niches and atypical behaviors.

Tanystropheus underwater among tall crinoids and small squids.

Figure 8. Tanystropheus in a vertical strike elevating the neck and raising its blood pressure in order to keep circulation around its brain and another system to keep blood from pooling in its hind limb and tail.

References
Li C, Rieppel O and LaBarbera MC 2004. A Triassic aquatic protorosaur with an extremely long neck. Science 305:1931.
Liu, J. et al. 2017. Live birth in an archosauromorph reptile. Nature Communications 8, 14445 doi: 10.1038/ncomms14445
Peters D, Demes B and Krause DW 2005. Suction feeding in Triassic Protorosaur? Science, 308: 1112-1113.

 

What is Triopticus? (It’s not what they think it is…)

Updated July 13, 2017 with new bone identifications for Triopticus. These further cement the sisterhood to Tanytrachelos. 

It’s been a long time
since an interesting ‘reptile’ showed up in the literature. Especially an ‘enigma’ like this one.

A recent paper by Stocker et al. 2016
reports on a domed and expanded Late Triassic cranium that they identify as an archosaur, but it’s unlike that of any other archosaur. Triopticus primus was named for its three eyes, with a big one on top (Fig. 1). The authors compared the domed appearance of the cranium in Triopticus with a Cretaceous dome-headed ornithischian dinosaur, Stegoceras.  They also discussed convergence in general and provided a CT scan brain endocast of Triopticus.

Unfortunately 
the authors employed a prior cladogram (Fig. 2) by Nesbit et al. 2015, expanded from Pritchard et al. 2015. that was shown to not recover sister taxa that looked alike and did not provide a gradual accumulation of derived traits at several nodes. In their cladogram Triopticus nested without resolution among basal archosauriforms like Proterosuchus, which looks nothing like it. By contrast, the LRT was able nest and fully resolve Triopticus elsewhere.

Figure 1. In round 1 I added characters shown here to the LRT in two passes. One recovered a sisterhood with mesosaurs. The other nested with Tanytrachelos, among the tanystropheid tritosaur lepidosaurs. Both shown here for comparison.

Figure 1. In round 1 I added characters shown here to the LRT in two passes. One recovered a sisterhood with mesosaurs. The other nested with Tanytrachelos, among the tanystropheid tritosaur lepidosaurs. Both shown here for comparison. Triopticus would be 2x the size of the giant Tanyrachelos from New Mexico.

From the Stocker et al. abstract:  “Exemplifying this extreme morphological convergence, we present here a new dome-headed taxon from the assemblage, which further illustrates the extraordinary range of morphological disparity present early in the Late Triassic.” That ‘extraordinary range’ should be — and will be — chopped down substantially with the right sister taxa.

A few problems with the archosauriform hypothesis include:

  1. No other archosauriforms, until you get to pachycelphalosaurs in the Cretaceous, expand the cranium deleting the upper temporal fenestra.
  2. The entire rostrum and mandible is absent, so no naris, antorbital fenestra or teeth are known, even in part.
  3. They dubiously identified an antorbital fenestra and fossa at the edge of the fossil.
  4. …and they were not aware that Tanytrachelos and kin, including pterosaurs within the – Lepidosauria -, also have an antorbital fenestra, but without a fossa.
  5. A large pineal opening is present, but never present at such a size in archosauriforms.
  6. The extreme angle of the rostrum coupled with the large orbit are traits not found in basal archosauriforms that typically have a long boxy rostrum.
Figure 2. Stocker et al. 2106 cladogram nesting Triopticus uncertainly within a set of unresolved basal archosauriforms. The LRT completely resolves that node.

Figure 2. Stocker et al. 2106 cladogram nesting Triopticus uncertainly within a set of unresolved basal archosauriforms and far from the Tanystropheidae. The LRT completely resolves all nodes. Note how this cladogram mixes Lepidosauromorpha with Archosauromorpha and separates the protorosaurus, Protorosaurus and Prolacerta.

This is a perfect problem
for the large reptile tree (LRT) which now provides then 820, now 1036 opportunities for Triopticus to nest in. With that large number of taxa, unfortunately I had to split the matrix in two, even for a simple Heuristic Search. By contrast, the Stocker et al. matrix included 30 taxa and 247 characters.

Stocker et al. report,
“We chose this dataset because the following combination of character states in Triopticus are also present in some archosauromorph taxa:

  1. presence of a single occipital condyle;
  2. ossified laterosphenoid;
  3. presence of a metotic strut of the otoccipital;
  4. presence of upper and lower temporal fenestrae;
  5. presence of an antorbital fenestra and fossa formed by the lacrimal.”

They provided no reconstructions of included taxa.

First,
I tested Triopticus against basal tetrapods and the new Lepidosaurmorpha and found that Triopticus nested with the aquatic, long-necked tritosaur Tanytrachelos (Fig. 1), large specimens of which were recently found in New Mexico (Fig. 3). Like Triopticus the rostrum descends at a high angle from a tall cranium in Tanytrachelos, which also shares a large orbit and a large pineal foramen (at present known only from sister taxa). Like related fenestrasaurs and langobardisaurs, Tanytrachelos also had a small antorbital fenestra without a fossa, but that would have been beyond the rim of the broken skull in Triopticus (Fig. 5).

Figure 2. A large incomplete Tanytrachelos from New Mexico compared to the smaller more complete East Coast specimen. Triopticus would be twice as large as the New Mexico specimen.

Figure 3. A large incomplete Tanytrachelos from New Mexico compared to the smaller more complete East Coast specimen. Triopticus would be twice as large as the New Mexico specimen.

Second,
I tested Triopticus with the rest of the matrix, the new Archosauromorpha, and found that Triopticus nested with the mesosaurs (Fig. 4), an aquatic enaliosaur clade close to thalattosaurs and ichthyosaurs, all derived from basal pachypleurosaurs. It did not nest with archosauriforms. While basal mesosaurs have typical diapsid temporal regions, Mesosaurus, like Triopticus, closes up the upper temporal fenestra, then the lateral temporal fenestra with bone expansion.  Mesosaurs also retain a relatively large pineal foramen and have large eyes, but they don’t have a sharply descending preorbital region.

Mesosaurus

Figure 4. Mesosaurus, like Triopticus, has a large pineal foramen and expands the skull bones to obliterate former temporal fenestrae.

Digital Graphic Segregation
was applied to the cranial lump that is Triopticus (Fig. 5) and the skull suture patterns, perhaps overlooked by those with firsthand access due to the expansion of the cranial bones, revealed a Tanytrachelos-like morphology (Fig. 2). I illustrate this interpretation here with the hope that this hypothesis can be either confirmed or falsified. This is a tough assignment.

Figure 5. Triopticus reconstructed along the bauplan of Tanytrachelos.  The upper temporal fenestra is the top half of a divided lateral temporal fenestra. At 72 dpi this is 90 percent of actual size.

Figure 5. Triopticus reconstructed along the bauplan of Tanytrachelos. The upper temporal fenestra is the top half of a divided lateral temporal fenestra. At 72 dpi this is 90 percent of actual size.

Tanystropheids
have been reported from the Hayden Quarry of northern New Mexico (Chinle Formation) far from the west central Texas location of Otis Chalk. Stocker et al. included Tanytrachelos in their study, even though they have not provided a reconstruction of it, so it is difficult to imagine how they interpreted it. Tanystropheids, in general, have widely varying skull shapes. Triopticus appears to have expanded the morphospace just a little, not a lot.

The loss of ventral material in the Triopticus fossil
appears to have occurred at the roof the narial/oral opening.

So what other long-necked animal
expands the cranium like Triopticus? Giraffa, the giraffe. Maybe it will turn out to be a better analogy than short-necked Stegoceras?

References
Nesbitt SJ, Flynn JJ, Pritchard AC, Parrish JM, Ranivoharimanana L and Wyss AR 2015. Postcranial osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its systematic position among stem archosaur reptiles. Bulletin of the American Museum of Natural History 899, 1-125.
Pritchard AC, Turner AH, Nesbitt SJ, Irmis RB and Smith ND 2015. Late Triassic tanystropheid (Reptilia, Archosauromorpha) remains from northern New Mexico (Petrified Forest Member, Chinle Formation): insights into distribution, morphology, and paleoecology of Tanystropheidae. Journal of Vertebrate Paleontology, 10.1080/02724634.02722014.02911186.
Stocker MR, NesbittSJ, Criswell KE, Parker WG, Witmer LM, Rowe TB, Ridgely R  Brown MA 2016. A Dome-Headed Stem Archosaur Exemplifies Convergence among Dinosaurs and Their Distant Relatives. Current Biology (advance online publication)DOI: http://dx.doi.org/10.1016/j.cub.2016.07.066   pdf