The many faces of Styxosaurus

Styxosaurus snowii (Originally Cimoliasaurus snowii, Williston 1890, KUVP 1301; Welles 1943; Late Cretaceous, Campanian, 80mya; SDSMT 451; Figs. 1, 2) is another giant elasmosaurid with dog-like fangs related to Simolestes in the large reptile tree (LRT, 1438 taxa).

Figure 1. Skull of Styxosaurus (KUVP 1301) from Sachs, Lindgren and Kear 2018, colorized using DGS methods. Broken teeth on this side of the skull repaired based on the dimensions of unbroken teeth on the other side of the skull.

Several new papers
(refs below) have taken another look at the skull of Styxosaurus, now known for about 130 years. Prior freehand drawings of the skull (Fig. 2) seem to overlook certain interesting details, many of which are critical for accurate scoring.

Figure 2. The changing face of Styxosaurus from Welles 1890, Otero 2016 and colorized here. Maybe a little easier to see each bone when colored? 

Overall,
a skeletal reconstruction of Styxosaurus required just a little updating (Fig. 3).

Figure 3. Styxosaurus skeleton as originally drawn and revised here.

A biting animation
(Fig. 4) of Styxosaurus shows the interweaving of the oversized teeth. Note the elongate posterior dentary teeth. As in Tricleidus, these would have made effective foot traps when water was being expelled whenever the jaw closed.

Figure 4. Styxosaurus mandible animated.

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References
Otero RA 2016. Taxonomic reassessment of Hydralmosaurus as Styxosaurus: new insights on the elasmosaurid neck evolution throughout the Cretaceous. PeerJ Figure 3. Styxosaurus skeleton as originally drawn and revised here.
Sachs S, Lindgren J and Kear B 2018. Reassessment of the Styxosaurus snowii (Williston, 1890) holotype specimen and its implications for elasmosaurid plesiosaurian interrelationships. Alcheringa: An Australasian Journal of Palaeontology, DOI: 10.1080/03115518.2018.1508613
Welles SP 1943. Elasmosaurid plesiosaurs with a description of the new material from California and Colorado. University of California Memoirs 13:125-254. figs.1-37., pls.12-29.
Welles SP and Bump J 1949. Alzadasaurus pembertoni, a new elasmosaur from the Upper Cretaceous of South Dakota. Journal of Paleontology 23(5): 521-535.
Williston SW 1890a. Structure of the Plesiosaurian Skull. Science. 16 (405): 262.
Williston SW 1890b. A New Plesiosaur from the Niobrara Cretaceous of Kansas. Transactions of the Annual Meetings of the Kansas Academy of Science. 12: 174–178.

wiki/Styxosaurus

Tricleidus enters the LRT

No surprises here.
Tricleidus seeleyi (Fig. 1; Andrews 1909; Middle Jurassic, Callovian, BMNH R3539) ) enters the large reptile tree (LRT, 1435 taxa) alongside Plesiosaurus.

Figure 1. GIF animation of Tricleidus skull demonstrating both the enormous gape and the filter-feeding teeth that permit water to be pushed out by a constricting throat sac.

Why such long, slender and closely intermeshing teeth?
Not only do those long teeth provide excellent spiky fish traps, but upon closing they act like whale baleen in trapping food inside the mouth cavity while permitting the expulsion of excess water without losing the fish trapped inside.

As in other plesiosaurs
the large jaw muscles filling the posterior skull ensure a strong bite. A small ridge posterior to the jaw joint blocks over-extension of the mandible.

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References
Andrews CW 1909, 1910. A Descriptive Catalogue of the Marine Reptiles of the Oxford Clay, Part I. British Museum (Natural History), London, England: 205 pp.

wiki/Tricleidus

Simolestes enters the LRT as an elasmosaur

Traditionally considered a short-snouted pliosaur,
due to its rosette anterior dentary, Simolestes borax (Lydekker 1877; Mid-Late Jurassic; BMNH R 3319; Fig. 1) nests in the large reptile tree (LRT, 1435 taxa; subset Fig. 2) with the elasmosaurs, Albertonectes and Libonectes.

Figure 1. Simolestes in several views and colorized using DGS methods compared to the elasmosaur, Libonectes.

I have looked for,
but not seen any post-crania for Simolestes, despite reports that post-crania exists. And I’m willing to let the data decide.

Given that the two elasmosaur skulls are the same size
(Fig. 1) lends weight to the hypothesis that Simolestes is an elasmosaur, not a pliosaur. Distinct from Libonectes, Simolestes has a single upper and lower fang, like a dog, with smaller teeth elsewhere.

Figure 2. Subset of the LRT focusing on Eusauropterygians (pachypleurosaurs, nothosaurs, plesiosaurs and kin).

And another eusauropterygian,
Peloneustes (Fig. 3) also enters the LRT, nesting, with no surprise, close to Dolichorhynchops.

Figure 3. Peloneustes fossil on display.

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References
Godefroit P 1994.  Simolestes keileni sp. nov., un Pliosaure (Plesiosauria, Reptilia) du Bajocien supérieur de Lorraine (France). Bulletin des Académie et Société Lorraines des sciences, ISSN 0567-6576, 1994, tome 33, n°2, p. 77-95. 33. .
Lydekker  R 1877. Notices of new and other Vertebrata from Indian Tertiary and Secondary rocks. Records of the Geological Survey of India 10(1):30-43

 

 

Dolichorhynchops enters the LRT with some overlooked bones

To no one’s surprise
Late Cretaceous Dolichorhynchops (Fig. 1) nests with the similar and coeval Trinacromerum (Fig. 2) in the large reptile tree (LRT, 1432 taxa, subset Fig. 4). Both nest between plesiosaurs and pliosaurs in the LRT, all derived from a sister to Early Jurassic Rhomaleosaurus.

Figure 1. The FHSM VP 404 specimen of Dolichorhynchops with modification to the pectoral girdle and hypothetical addition of chevrons. Gastalia are not shown, but were probably present based on phylogenetic bracketing.

O’Keefe 2004 reports,
“For most of the twentieth century the polycotylids were classified as true pliosaurs, because they possessed short necks, large heads, and other proportional differences in common with Jurassic pliosauroids such as Peloneustes and Liopleurodon (O’Keefe, 2002; see O’Keefe, 2001 for taxonomic review). In 1997, however, Carpenter questioned the monophyly of the Pliosauroidea as traditionally defined and instead posited a sister-group relationship between the Polycotylidae and the Elasmosauridae, a view also championed by Bakker (1993). O’Keefe (2001) performed a cladistic analysis of the clade Plesiosauria, and found that the traditionally-defined Pliosauroidea were indeed polyphyletic, although a sister-group relationship with the Elasmosauridae was not supported. Instead, O’Keefe found that the Polycotylidae were a derived group of cryptocleidoid plesiosauroids, most closely related to Jurassic taxa such as Tricleidus and Cryptoclidus. The novel phylogenetic position of the Polycotylidae found by O’Keefe (2001) renders the Pliosauroidea polyphyletic as traditionally defined.”

At present
the LRT nests only two polycotylids and they nest together between plesiosaurs and pliosaurs, as superficial appearances suggest. Other poycotylids have not been tested.

From O’Keefe 2004:
POLYCOTYLIDAE Williston 1908 revised definition: A taxon including Polycotylus, Edgarosaurus, Dolichorhynchops, Trinacromerum, their most recent common ancestor, and all descendants.

Figure 2. The plesiosaur Trinacromerum has a parietal crest, but it was laterally packed with jaw muscles.

Dolychorhynchops osborni (Williston 1902; Late Cretaceous, 80mya; FHSM VP-404) is a sister to Trinacromerum with a narrow, flat skull. Below, the skull was inappropriately made wider and taller and several bones were overlooked by O’Keefe. Note the palatal extension of the premaxilla to the internal nares, as in other eusauropterygians, overlooked by prior workers. Here hypothetical hemals were added. Gastralia were likely present.

FIgure 3. Skull of Dolichorhynchops along with tracing by Carpenter and reconstruction by O’Keeefe, which seems overblown in width and height, given a constant length. Both workers overlooked the palatal extension of the premaxilla to the internal nares, as in other eusauropterygians. Always best to check the fossils because workers sometimes overlook bones, sutures, etc.

In figure three above,
several bones were overlooked by prior workers. Others have new outlines/sutures. Of particular interest, the premaxilla in palatal view (yellow) extends to the internal nares (red), as in other eusauropterygians.

Figure 4. Subset of the LRT focusing on Eusauropterygians (pachypleurosaurs, nothosaurs, plesiosaurs and kin).

Luckily,
someone made a Dolichorhynchops fantasy/documentary, initially designed for 3D IMAX theatres, I think. Here it is on YouTube:

While watching…
ask yourself if the creators/producers/animators actually used the cuter-looking Trinacromerum (Fig. 2), or the inflated O’Keefe restoration (Fig. 3), rather than the flat-headed FHSM VP404 holotype of Dolichorhynchops (Fig.1), which has the cuter name and nickname “Dolly”.

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References
Carpenter K 1996. A review of short-necked plesiosaurs of the Western Interior, North America. Neues Jahrbuch fur Geologie und Palaontologie, Abhandlungen 201(2):259-287.
O’Keefe FR 2001. A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). Acta Zoologica Fennica 213:1-63.
O’Keefe FR 2004. On the cranial anatomy of the polycotylid plesiosaurs, including new material of Polycotylus latipinnis Cope, from Alabama. Journal of Vertebrate Paleontology. 24 (2): 326–340.
Williston SW 1902. Restoration of Dolichorhynchops osborni, a new Cretaceous plesiosaur. Kansas University Science Bulletin 1 (9): 241–244.
Williston SW 1908. North American Plesiosaurs: Trinacromerum. Journal of Geology 16(8): http://www.jstor.org/stable/30068152

wiki/Trinacromerum
wiki/Dolichorhynchops

Splitting the frontals in pliosaurs

Pliosaurs are like derived pterosaurs in very few ways,
but this one stands out: The premaxilla extends all the way back to the parietal (Fig. 1) in both clades.

Figure 1. Kronosaurus dorsal skull colorized from originals in McHenry 2009. Note the premaxilla/parietal contact. Here the nasals appear to fuse to the maxillae, something added here in pink that was not intended by McHenry 2009. n = naris. o = orbit.

Other sauropterygian taxa split the nasals
as the premaxilla extends to the frontals. Pliosaurs also split the frontals with the invading premaxilla (Fig. 1).

By contrast,
in pterosaurs the premaxillae more or less sits on top of the nasals and frontals whenever the premaxillae extend posteriorly.

If you’ve ever had an interest in the giant pliosaur, Kronosaurus
you will be thrilled to read McHenry 2009. It’s chock full of details like this (Fig. 1).

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References
McHenry CR 2009. ‘Devourer of Gods’ The palaecology of the Cretaceous pliosaur Kronosaurus queenslandicus. PhD dissertation, U of Newcastle.

Sachicasaurus: the first giant nothosaur, not a pliosaur

Páramo-Fonseca, Benavides-Cabra and Gutiérrez 2018
described Sachicasaurus (Figs. 1-3, MP111209-1, Barremian (Early Cretaceous) Columbia; estimated 10m in length, 2m skull length), a taxon they thought was a giant pliosaur related to Brauchauchenius (Fig. 2).

Unfortunately
the authors did not consider comparing their discovery to Nothosaurus. The short flippers are the first clue that perhaps they should have done so. Misidentifying several bones was a problem. The large reptile tree (LRT, 1420 taxa) tests each new taxon against all prior taxa, thereby largely avoiding the paleo-sin of taxon exclusion.

Figure 1. Sachicasaurus skull from Páramo-Fonseca et al. 2018, colors added. Some skull bones restored in color. Note the differences in the preorbital region of the skull between the original interpretation drawing and the DGS color applied to the skull photo.

Sachicasaurus vitae (Páramo-Fonseca, Benavides-Cabra and Gutiérrez 2018, 10m in length) was originally considered a short-flippered pliosaur related to long-flippered Brachauchenius characterized by two autopomorphic characters: a very short mandibular symphysis ending at the mid length of the fourth mandibular alveoli and reduced number of mandibular teeth (17-18). Here this giant nests with Nothosaurus (above). Originially several bones were misidentified. The ilium is uniquely bifurcated with a dorsal and posterior process. In dorsal view the mandibles are convex while the maxillae are concave, leaving quite a gap between them.

Figure 2. Sachicasaurus was the size of the pliosaurs, Kronosaurus and Brauchenia, but was related to Nothosaurus. This is the first known giant nothosaur.

Several bones were originally misidentified.
The former left ‘scapula’ is really the clavicle. The former right ‘scapula’ is really the coracoid. The former sock-shaped ‘radius’ is the tiny scapula.

Figure 3. Sachisaurus pectoral girdle and flippers reconstructed with new identities provided here. Pectoral elements are digitally duplicate and flipped left to right.

Real plesiosaurs have long flippers
with more than the usual number of phalanges per digit. By contrast, Sachicasaurus does not have long flippers and It has the plesiomorphic number of phalanges. Yes, the skull is huge and the neck is short. In the LRT those pliosaur-like traits are not enough to attract Sachicasaurus toward the pliosaurs. Note the different interpretations of the skull bones presented here (Fig. 1). The nasals, in particular, are nothosaurian, not pliosaurian.

Figure 4. Data for Nothosaurus for comparison with Sachicasaurus. The interclavicle could easily be lost. Note the plesiomorphic number of phalanges on both the manus and pes. Compare these to those in figure 3.

From the same Early Cretaceous formation
three real pliosaurs have been discovered. This would have been the fourth one, except it’s a nothosaur with pliosaur size and proportions by convergence. Earlier we looked at a similar convergence between toothed whales and baleen whales.

Figure 5. Subset of the LRT focusing on the Eusauropterygia, including Sachicasaurus.

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References
Páramo-Fonseca ME, Benavides-Cabra CD and Gutiérrez IE 2018. A new large Pliosaurid from the Barremian (Lower Cretaceous) of Sáchica, Boyacá, Colombia. Earth Sciences Research Journal 220(4):223-238. eISSN 2339-3459. Print ISSN 1794-6190.

wiki/Sauropterygia
wiki/Sachicasaurus

Rhomaleosaurus enters the LRT

Rhomaleosaurus is a big basal plesiosaur. 
Some specimens reached 7m in length.

Figure 1. Two species attributed to the genus Rhomaleosaurus.

In the large reptile tree (LRT, 1413 taxa) Rhomaleosaurus nests basal to plesiosaurs + pliosaurs, all derived from Late Triassic Yunguisaurus and kin. Smith 2007, Druckenmiller 2006 and O’Keefe 2001 nested Rhomaleosaurus with pliosaurs. The skull is larger than in traditional plesiosaurs and yunguisaurs, but not in more primitive and Early Jurassic Hauffiosaurus, which nested with Anningasaura a plesiosaur-mimic, evolving hyerphalangeal flippers in convergence with traditional plesiosaurs.

Rhomaleosaurus cramptoni (originally Plesiosaurus Carte and Bailey 1863, Seeley 1974; Early Jurassic, Toarcian, 180 mya; 7m long; NMING F8785) nests basal to Plesiosaurus, the elasmosaurids and pliosaurids in the LRT.

Smsith 2007 reported, “The Rhomaleosauridae ranges throughout the Lower Jurassic and extends into the Lower part of the Middle Jurassic. The latter part of the Middle Jurassic sees the emergence of the first pliosaurids and it is possible that the rhomaleosaurids were out competed by these very short-necked predators during the Middle Jurassic.”

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References
Carte A, Bailey WH 1863. Description of a new species of Plesiosaurus, from the Lias, near Whitby, Yorkshire. J R Dublin Soc 4:160–170.
Conybeare W 1824. On the Discovery of an almost perfect Skeleton of the Plesiosaurus. Geological Society of London. Retrieved 2010-1-15.
Cruickshank ARI 1994. Cranial anatomy of the Lower Jurassic pliosaur Rhomaleosaurus megacephalus (Stutchbury) (Reptilia: Plesiosauria). Philosophical Transactions of the Royal Society Lond Ser B 343:247–260.
Seeley HG 1874. Note on some of the generic modifications of the plesiosaurian pectoral arch. Quarterly Journal of the Geological Society Londong 30:436–449.
Smith AS 2007. Anatomy and systematics of the Rhomaleosauridae (Sauropterygia, Plesiosauria). Ph.D. thesis, University CollegeDublin.
Smith AS and Dyk GJ 2008. The skull of the giant predatory pliosaur Rhomaleosaurus cramptoni: implications for plesiosaur phylogenetics. Naturwissenschaften. 95: 975–980. doi:10.1007/s00114-008-0402-

wiki/Plesiosaurus
wiki/Rhomaleosaurus

Libonectes enters the LRT

After applying colors to
the bones in a photograph of the skull of Libonectes (Fig. 1, Turonian, early Late Cretaceous, Welles 1949, originally Elasmosaurus morgani), the Carpenter 1997 drawing was added to gauge similarities and difference. A transparent GIF makes this easy. Comparisons to the earlier (Late Triassic) Yunguisaurus and Thalassiodracon are instructional. These taxa also rotate the orbits anteriorly, providing binocular vision. The pterygoid (dark green) pops out slightly behind the jugal.

Figure 1. The skull of Libonectes. Freehand drawing from Carpenter 1997. DGS colors added here. Some parts of the original fossil may be restored. The fossil may be more fully prepared than this now. Note the slight differences between the fossil and drawing. The orbits appear to permit binocular vision.

Libonectes morgani, (Welles 1949, Elasmosaurus morgani, Carpenter 1997) an elasmosaur of the Turonian, early Late Cretaceous. In the large reptile tree (LRT, 1399 taxa) this skull nests with the skull-less Albertonectes (Fig. 2) and Plesiosaurus (Fig. 3) at first with no resolution owing to the lack of common traits between the skull-only and skull-less taxa.

Figure 2. Plesiosaurus skull in several views representing two specimens alongside the pectoral girdle. Data comes only from this drawing, not the fossil itself, which I have not yet seen.

Later the post-crania of Libonectes is added
and the two elasmosaurs now nest together sharing fore limbs slightly longer than hind limbs (Fig. 3) among several other less obvious traits. Neck length, much longer with more vertebrae than in Plesiosaurus, scores the same, “Presacral vertebrae, 31 or more” in the LRT.

Figure 3. Libonectes flippers. 2nd frame shows PILs. Terrestrial tetrapods flex and extend along continuous PILs. The in vivo misalignment of phalanges in Libonectes largely prevents flexion and extension, creating a large stiff flipper at misaligned PILs. Those that are more proximal are continuous, permitting a limited amount of flexion and extension.

Sachs and Kears 2017
bring us images and descriptions of the post-crania of Libonectes, a Late Cretaceous elasmosaur, one of the sauropterygian plesiosaurs, similar in most respects to the other tested elasmosaur, Albertonectes, which we looked at earlier here.

Distinct from terrestrial tetrapods
that flex and extend their phalanges along continuous PILs, the in vivo misalignment of phalanges in Libonectes largely prevents flexion and extension, creating a stiffer flipper at the misaligned PILs. Note, those that are more proximal are continuous, permitting more flexion and extension.

PILs were first documented
in Peters 2000. Many taxa may be distinguished by their fore and hind PIL patterns as also shown for pterosaurs in Peters 2011.

It is worth noting (and scoring)
that the forelimbs are slightly larger than the hind limbs in elasmosaurs, distinct from other sauropterygians, convergent with many ichthyosaurs, sea turtles and perhaps other taxa I am overlooking presently (overlooking some birds and all bats and pterosaurs for the moment, because they fly).

Figure 4. Elasmosaurid origins. The long neck preceded the flippers in this clade of vertical feeders.

We looked at hypothetical elasmosaur swimming techniques
a few months ago here.

References
Carpenter K 1999. Revision of North American elasmosaurs from the Cretaceous of the western interior. Paludicola, 2(2): 148-173.
Sachs S and Kear BP 2017. Redescription of the elasmosaurid plesiosaurian Libonectes atlasense from the Upper Cretaceous of Morocco. Cretaceous Research 74:205–222.
Welles SP 1949. A new elasmosaur from the Eagle Ford Shale of Texas. Fondren
Science Series, Southern Methodist University 1: 1-28.

wiki/Albertonectes
wiki/Libonectes

Hauffiosaurus: convergent with later plesiosaurs

Updated March 28, 2019
with a new old engraving of Anningasaura.

Two misfit plesiosaurs nest together in the LRT
Earlier we looked at Anningsaura (Fig. 6). Vincent and Benson (2012) reported, “In general morphology, NHMUK OR49202 does not resemble any known plesiosaurian taxon.”

Figure 1. The sisters of Anningsaura, Simosaurus and Pistosaurus. Until today, these provided the only clues as to the post-crania of Anningsaura, of which only the first eight cervicals are known.

Anningasaura 
(originally Plesiosaurus macrocephalus, Lydekker 1889; NHMUK OR49202) represents a completely ‘new’ branch of the plesiosauria in which the orbits virtually cannot be seen in dorsal view and the jugals bend down posteriorly to produce an angled temporal arch (Fig. 1). Moreover the premaxillae were thought to not contact the frontals and the nasals were absent. Benson et al. (2012) created a phylogenetic analysis that nested Anningsasaura at the base of the pliosaur/plesiosaur split with Bobosaurus as the outgroup.

Figure 2. Hauffiosaurus from Vincent 2011 with colors and reconstructions added.

Hauffiosaurus zanoni 
(O’Keefe 2001; Vincent 2011; Early Jurassic; 3.4m long; uncatalogued Hauff museum) is another plesiosaur that, according to Vincent 2011, “does not resemble any known plesiosaurian taxon.” This genus was considered a basal pliosauroid. Here (Fig. 3) the large reptile tree (LRT, 1392 taxa) nests between Anningsaura and Pistosaurus. Benson et al 2012 nested Hauffiosaurus one or two nodes apart from Anningsaura. No taxa in those nodes is currently in the LRT. So the LRT is a close match!

As you might imagine,
the characters in the LRT are not the same as those found in Benson et al. 2012, yet the tree topologies, so much as they can be compared, are nearly identical. This was done without first-hand access to the fossils. So, this methodology works.

Figure 3. Subset of the LRT. Here the clade Eosauropterygia nests Anningsaura with Hauffiosaurus. This nesting demonstrates an early convergence with later pliosaurids.

The skull of Hauffiosaurus is exposed in palatal view
(Fig. 4) and as such gives clear data on the often hidden palatal elements. Overlooked by Vincent 2011, the premaxilla extends to the internal naris, as in other taxa (Fig. 5), like Pliosaurus, also an overlooked connection.

Figure 4. Hauffiosaurus skull in palatal view from Vincent 2011, colors added. Overlooked by Vincent, the premaxilla (yellow) contacts the internal naris

DGS is able to document traits
overlooked by those with first-hand access to the fossils themselves (Figs. 4, 5).

Figure 5. Pliosaurus kevani palate, from Benson et al. 2013, also has an overlooked premaxilla-internal naris contact. Red ellipses encircle the internal nares, probably too small for respiration.

Figure 6. Anningasaura colorized from an old engraving. No other aquatic taxon has such bizarrely curved teeth. This taxon is closely related to Hauffiosaurus, so it provides insight into the lateral view of Hauffiosaurus.

References
Benson RBJ, Evans M, Druckenmiller PS 2012. Lalueza-Fox, Carles. ed. ”High Diversity, Low Disparity and Small Body Size in Plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary”. PLoS ONE 7 (3): e31838. doi:10.1371/journal.pone.0031838
Benson RBJ, et al. (6 co-authors) 2013. A giant pliosaurid skull from the Late Jurassic of England. PLoS ONE 8(5): e65989. doi:10.1371/journal.pone.0065989
Dalla Vecchia FM 2006. A new sauropterygian reptile with plesiosaurian affinity from the Late Triassic of Italy. Rivista Italiana di Paleontologia e Stratigrafia 112 (2): 207–225.
O’Keefe RF 2001. A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). Acta Zoologica Fennica 213:1–63.
Vincent P 2011. A re-examination of Hauffiosaurus zanoni, a pliosauroid from the Toarcian (Early Jurassic) of Germany. Journal of Vertebrate Paleontology 31(2): 340–351.
Vincent P and Benson RBJ 2012. Anningasaura, a basal plesiosaurian (Reptilia, Plesiosauria) from the Lower Jurassic of Lyme Regis, United Kingdom, Journal of Vertebrate Paleontology, 32:5, 1049-1063.

wiki/Anningasaura
wiki/Hauffiosaurus

 

Paludidraco and Cymatosaurus in the LRT

It’s been awhile since we looked at anything wet.
A new robust-ribbed sauropterygian, Paludidraco ( Fig. 1, Middle Triassic) does indeed share many traits with Simosaurus, as described by Chaves et al. 2018.

A welcome confirmation!
Due to its tiny dentition, Paludidraco was originally considered a likely filter feeder, distinct from related, long-toothed nothosaurs and plesiosaurs. Simosaurus also has relatively tiny teeth, but on a larger skull and fewer in number. That’s evolution at work!

Isn’t it great to see these two related taxa together? Doesn’t it make compare and contrast so much easier? See the evolution of the human ear bones from primitive jaw bones illustration here for another great example of comparative anatomy.

Figure 1. Simosaurus compared to Paludidraco. Isn’t it great to see these two related taxa together? Doesn’t it make compare and contrast so much easier? 

Chaves et al. 2018 provided
a cladogram of marine reptile relationships (Fig. 2). Most of these taxa are also included in the large reptile tree ( LRT, 1261 taxa, subsets Figs. 3, 4), which includes many times more taxa and more marine reptiles. Missing from the Chavez team cladogram (Fig. 2) is the genus/taxon Anningsaura, which links nothosaurs to pistosaurs + plesiosaurs in the LRT. The Chaves et al. cladogram, nests Cymatosaurus (Fig. 4) and Corosaurus basal to Pistosaurus + plesiosaurs.

Figure 2. Paludidraco cladogram from Chaves et al. 2018 with arrows showing how taxa nest in the LRT. Taxon exclusion is the problem here. See figure 3.

The Chaves et al. 2018 cladogram
(Fig. 2) excludes many pertinent taxa, so much so that important interrelationships were missed, based on the authority of the LRT (Fig. 3), which minimizes taxon exclusion due to its wider gamut of taxon inclusion. Several taxa in the Chaves et all cladogram would shift positions when tested with more taxa (arrows in Fig. 2) as the LRT shows (Fig. 3).

Figure 3. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria). Paludidraco was not added when this graphic was created, but has since been added. Sharp-eyed readers will see Vancleavea here.

Cymatosaurus
had to be added to the LRT (Fig. 4) to test it fairly against the Chavez team cladogram (Fig. 2). Only the skull is known (AFAIK) from three different species.

FIgure 4. The addition of Cymatosaurus is more of an insertion, that changes nothing else in the tree topology. Here it nests on the nothosaur side of Simosaurus, not close to plesiosaurs.

Despite the many offshoot traits
found in Anningsaura, the rest of its traits nest it firmly at the base of the pistosaurs + plesiosaurs, where Chaves et al. nests Cymatosaurus. In the LRT Cymatosaurus nests close to Paludidraco, but more on the nothosaur side than the plesiosaur side.

References
Chaves C de M, Ortega F and Pérez‐García A 2018. New highly pachyostotic nothosauroid interpreted as a filter-feeding Triassic marine reptile. Biology Letters. 14 (8): 20180130.
Maisch MW 2014. A well preserved skull of Cymatosaurus (Reptilia: Sauropterygia) from the uppermost Buntsandstein (Middle Triassic) of Germany. Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen. 272 (2): 213–224.

wiki/Paludidraco