Perhaps the most beautiful ichthyosaur: Eurhinosaurus

Ichthyosaurs are those dolphin-like reptiles of the Triassic, Jurassic and Early Cretaceous. They are derived from Permian mesosaurs, Triassic wumengosaurs, shastasaurs, hupehsuchids and thaisaurs.

Beauty is in the eye of the beholder,
but I present, for your consideration, Eurhinosaurus as, perhaps, the most beautifully proportioned of the ichthyosaurs (Fig. 1).

Figure 1. Eurhinosaurus, a derived ichthyosaur, in several views.

Figure 1. Eurhinosaurus, a derived ichthyosaur, in several views. Click to enlarge. This may be the most beautiful of all ichthyosaurs, despite its mosquito-like skull.

Eurhinosaurus is immediately distinguished
from all other ichthyosaurs by its long rostrum and shorter mandible. The large size of the orbit is also unique as are the slender proportions. A more typically proportioned, yet closely related ichthyosaur, is Leptonectes (Fig. 2), which has another odd autapomorphy: the temporal regions of the skull wrap around the back of the skull.

Figure 2. Leptonectes, a closely related, but more generally proportioned ichthyosaur.

Figure 2. Leptonectes, a closely related, but more generally proportioned ichthyosaur. But note how the temporal regions of the skull have wrapped around the back.

By the way,
there was a tremendous upsurge (3x normal number of visitors) of interest yesterday in the blogpost on pterosaur wing shape. This was on a day that did not have a blogpost. Not sure what that means, but something must be coming up on this subject soon — unless it was some sort of school assignment.

wiki/Eurhinosaurus
wiki/Leptonectes

My, what big flippers you have!

Guizhouichthyosaurus tangae (Cao et Luo in Yin et al., 2000, Late Triassic) is an ichthyosaur with really big flippers derived from a sister to Phalarodon and basal to Shonisaurus popularis.

Figure 1. Guizhouichthyosaurus in situ in ventral aspect. This specimen has some of the biggest flippers among ichthyosaurs, rivaling those belonging to plesiosaurs, which makes one hypothesize a distinct mode of swimming.

Figure 1. Guizhouichthyosaurus in situ in ventral aspect. This specimen has some of the biggest flippers among ichthyosaurs, rivaling those belonging to plesiosaurs, which makes one hypothesize a distinct mode of swimming. The large number of ribs, though, along with the sinuous backbone, suggest that undulation was still used as well.

Known from several specimens, Guizhouichthyosaurus, had a long rostrum and sharp teeth (Fig. 2). When a sea creature has such large flippers the tendency is to imagine that it swam using those paddles/underwater wings. It probably had only a rudimentary tail fin, like Phalarodon or Mixosaurus.

Figure 2. Guizhouichthyosaurus tangae skull preserved in three dimensions.

Figure 2. Guizhouichthyosaurus tangae skull preserved in three dimensions. Tracing from Maisch et al. 2015.

Guizhouichthyosaurus provides clues to the ancestry of the big-fippered Shonisaurus, one of the giants among ichthyosaurs.

Figure 2. Ichthyosaur subset of the large reptile tree.

Figure 3. Ichthyosaur subset of the large reptile tree. 

Guizhouichthyosaurus is also related to the smaller-flippered and misnamed ‘Cymbospondylus’ buchseri (Sander 1989, Fig. 4), which looks a bit like a mosasaur. Now it needs a new generic name. Earlier we looked at other ichthyosaurs more recently misnamed by Sander et al. (2011).

Figure 4. 'Cymbodpondylus' buchseri did not have such large flippers, but did have a long narrow skull and robust elongate torso.

Figure 4. ‘Cymbodpondylus’ buchseri did not have such large flippers, but did have a long narrow skull and robust elongate torso.

I have previously overlooked and ignored most ichthyosaurs because I was more interested in their ancestry among Wumengosaurus, Thaisaurus and beyond to the mesosaurs. But they are a fascinating clade with some odd morphologies worth looking into.

References
Maisch M et al. 2015. Cranial osteology of Guizhouichthyosaurus tangae (Reptilia: Ichthyosauria) from the Upper Triassic of China. Journal of Vertebrate Paleontology 26(3): 588-597.
Yin G-Z, Zhou X, Cao Y, Yu Y and Lu Y 2000. A preliminary study on the early Late Triassic marine reptiles from Guanling, Guizhou, China. Geology-Geochemisty 28(3):1–23 (Chinese with English abstract).
Sander PM 1989. The large ichthyosaur Cymbospondylus buchseri sp. nov., from the Middle Triassic of Monte San Giorgio (Switzerland), with a survey of the genus in Europe. Journal of Vertebrate Paleontology 9(2): 163-173.
Sander PM, Chen X-C, Cheng L and Wang X-F 2011. Short-snouted toothless ichthyosaur from China suggests Late Triassic diversification of suction feeding ichthyosaurs. PlosOne DOI: 10.1371/journal.pone.0019480

Cymbospondylus – primitive or derived?

Cymbospondylus petrinus is a 20-30 foot (up to 8.33 m) long Middle Triassic ichthyosaur with a long, low toothy skull, short broad paddles and a long low, tail (Fig. 1).

Figure 1. Cymbospondylus skull and overall in lateral view.

Figure 1. Cymbospondylus skull and overall in lateral view.

The question is: 
Is Cymbospondylus primitive and derived from Chaohusaurus and Grippia (as per Motani 1999)? Or is Cymbodpondylus derived and derived from Mixosaurus (as per Maisch and Matzke (2000, 2003) and the large reptile tree)?

Cymbospondylus appears to be primitive.
It has the long snaky body of basal ichthyosaurs, like Utatsusaurus and Thaisaurus.

However, if Cymbospondylus nests between Grippia and Mixosaurus
it is a giant nesting between two relatively small to tiny ichthyosaurs.

Figure 2. Cymbospondlyus compared to sister taxa according to the large reptile tree to length (above) and to scale (below). Shown in gray is Shonisaurus popularis, which is compared to to Shonisaurus sikanniensis.

Figure 2. Cymbospondlyus compared to sister taxa according to the large reptile tree to length (above) and to scale (below). Shown in gray is Shonisaurus popularis, which is compared to to Shonisaurus sikanniensis.

Motani (1999)
nested Cymbospondylus at the base of the Ichthyosauria between Chaohusaurus + Grippia and Mixosauria (Mixosaurus and all higher ichthyosaurs (Merriamosauria).

Maisch and Matzke (2000, 2003)
nested Cymbospondylus a little higher, between Mixosauria and Merriamosauria.

Figure 2. Ichthyosaur subset of the large reptile tree.

Figure 2. Ichthyosaur subset of the large reptile tree.

The large reptile tree (Fig. 2) nested Cymbspondylus petrinus between Mixosaurus and the toothless Guanlingsaurus liangae YGMR SPC V03017 + the possibly toothless giant Shonisaurus sikanniensis (apart from ‘Cymbospondylus’ buchseri, which here (Fig 2) nests with Shonisaurus popularis in a distinct clade). So we should expect several taxa transitional between Mixosaurus and these giants and near giants.

Despite their long, snaky look, Cymbospondylus and kin are not primitive, but may have reverted to that morphology as they grew to larger and larger size.

References
Leidy J 1868. Notice of some reptilian remains from Nevada: Proceedings of the American Philosophical Society 20:177-178.
Merriam JC 1908. Triassic ichthyosauria with special references to the American forms. Memoirs of the University of California 1: 1-196.
Yin G-Z, Zhou X, Cao Y, Yu Y and Luo Y 2000. A preliminary study on the earlyLate Triassic marine reptiles from Guanling, Guizhou, China. Geology-Geochemisty 28(3):1–23 (Chinese with English abstract).

Ichthyosaur skulls in phylogenetic order (so far…)

Figure 1. Ichthyosaur skulls in phylogenetic order (top to bottom). Those below the red line have not been ordered yet. All of those below the red line have a naris/lacrimal contact and many do not have a naris/maxilla contact. They are mostly Jurassic and Cretaceous taxa. Boxed specimens are not yet tested. Many illustrations from Maisch and Matzke 2000. Click to enlarge. Not to scale.

Figure 1. Ichthyosaur skulls in phylogenetic order (top to bottom). Many illustrations from Maisch and Matzke 2000. Not to scale.

Ichthyosaur phylogeny has been examined by Motani (1999), Maisch and Matzke (2000) and Maisch (2010). The large reptile tree (Fig. 2) offers yet another solution and finally have the correct outgroup taxa included. All four of these studies are broadly similar, but do differ from each other in detail.

Figure 1. Subset of the LRT focusing on the clade Ichthyosauria.

Figure 1. Subset of the LRT focusing on the clade Ichthyosauria.

Note (Fig. 2) the two Shonisaurus specimens do not nest together. Neither do the two Cymbospondylus specimens. Earlier we talked about all the specimens attributed to Shastasaurus.

This is a continuing study.

References
Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity 3: 151-214.
Maisch MW and Matzke AT 2000. “The Ichthyosauria”. Stuttgarter Beiträge zur Naturkunde: Serie B 298: 159.
Motani R 1999. Phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology 19(3):473-496.’

Shonisaurus popularis vs. ‘Shonisaurus’ sikanniensis

Earlier we looked at the mistaken renaming of ‘Shonisaurus sikanniensis’ by Sander et al. 2011 to Shasatasaurus sikanniensis. S. sikanniensis and Shastsaurus don’t nest together, and share relatively few traits, so they can’t be the same genus.

Nicholls and Manabe (2004) described Shonisaurus’ sikanniensis (Fig. 1) as a 21m monster, the largest known ichthyosaur.

Figure 7. The giant sixth putative Shastasaurus, S. sikanniensis.

Figure 1. The giant sixth putative Shastasaurus, S. sikanniensis.

Unfortunately
their scale bars (Fig. 1) don’t confirm that length, but suggest one closer to 18 meters. That includes the 1 meter of missing distal tail they presume.

Worse yet
‘Shonisaurus sikanniensis’ shares very few traits with Shonisaurus popularis (Camp 1976, 1080, Kosch 1990; Fig. 2), the holotype for the genus. S. popularis nests with Guizhouichthyosaurus. S. sikanniensis nests with Cymbospondylus and YGMR SPC V3107, a specimen formerly attributed to Shastasaurus linagae by Sander et al. 2011. Like   S. sikanniensis, the 3107 specimen has a skull twice as wide as tall and a large orbit.

Figure 2. Shonisaurus populars compared to 'Shonisaurus' sikanniensis to scale.  Note the distinct skull and pectoral girdle morphologies.

Figure 2. Shonisaurus populars compared to ‘Shonisaurus’ sikanniensis to scale. Note the distinct skull and pectoral girdle morphologies. Click to enlarge. The torso is not so deep in S. popular is when they are angled back, as shown in most skeletons.

Interestingly,
no teeth are found in adult Shonisaurus popularis, only juveniles. Both Shonisaurus have expanded rib tips. Both are giants. Both may be toothless as adults.

Figure 3. Two ichthyosaurs once considered Shastasaurus suction feeders. The 3107 specimen nests with S. sikanniensis and both taxa need a new genus name. The 3108 specimen is very primitive and nests with Mikadocephalus.

Figure 3. Two ichthyosaurs once considered Shastasaurus suction feeders. The 3107 specimen nests with S. sikanniensis and both taxa need a new genus name. The 3108 specimen is very primitive and nests with Mikadocephalus.

I’m not sure how
and why my trees differ in detail from previously published work, but during the course of this study I’ve found prior data that did not agree with one another. So, evidently the data can be interpreted more than one way. And too often, I’m stuck with using published tracings as data without a photo to confirm. On the other hand, we’re in close agreement on many taxa and sister taxa recovered by the large reptile tree do resemble one another and make sense with regards to evolutionary patterns. Putting the reconstructions together, side-by-side, continues to be an important way to uncover prior and current mistakes.

Figure 4. Cladogram with Shonisaurus popular is added. Bootstrap scores shown.

Figure 4. Cladogram with Shonisaurus popular added. Bootstrap scores shown. Note the two Shonisaurus specimens do not nest together, nor do they share many traits.

References
Camp CL 1976. Vorläufige Mitteilungüber grosse Ichthyosaurier aus der oberen Trias von Nevada. Österreichische Akademie der Wissenschaften, Mathematisch-Naturwissenschaftliche Klasse, Sitzungsberichte, Abteilung I 185:125-134.
Camp CL 1981. Child of the rocks – The story of the Berlin-Ichthyosaur State Park. Nevada Bureau of Mines and Geology, Special Publication 5, 36 pp.
Kosch, BF 1990. A revision of the skeletal reconstruction of Shonisaurus popularis (Reptilia: Ichthyosauria). Journal of Vertebrate Paleontology 10 (4): 512.
Nicholls EL, Manabe M 2004. Giant ichthyosaurs of the Triassic – a new species of Shonisaurus from the Pardonet Formation (Norian: Late Triassic) of British Columbia. Journal of Vertebrate Paleontology 24 (3): 838–849.
Sander PM, Chen X-C, Cheng L and Wang X-F 2011. Short-snouted toothless ichthyosaur from China suggests Late Triassic diversification of suction feeding ichthyosaurs. PlosOne DOI: 10.1371/journal.pone.0019480

Xinminosaurus, yet another basal ichthyopterygian

I had no idea
so many basal ichthyopterygians were out there. Oddly, their original authors suspected the same, but did not put forth cladograms to support their hunches. Plus, some were Middle Triassic in age, while more derived taxa are found in Early Triassic strata. Finally, the proximal outgroups for ichthyosaurs (Fig. 2) were not recognized.

Figure 1. Xinminosaurus in situ and with DGS reconstructed.

Figure 1. Xinminosaurus in situ and with DGS reconstructed.

Xinminosaurus catactes (Jiang et al. 2008, Middle Triassic, GMPKU-P-1071, 1.6m). is another basalmost ichthyopterygian known for over 7 years now. Distinct from its closest kin, the teeth of Xinminosaurus were large squarish blocks. The paddles were short and broad with just a few extra phalanges (3-5-5-5-2) on the manus.

Figure 2. Cladogram of ichthyosaurs and kin with five putative Shastasaurus specimens in pink.

Figure 2. Cladogram of ichthyosaurs and kin with Xinminosaurus nesting close to the base of the Ichthyopterygia or as a transitional taxon proximal to that clade. 

Xinminosaurus had smaller cervicals than in Thaisaurus. The humerus was shorter. The scapula was not as tall. The hind limbs were shorter, more paddle-like. All these traits are more ichthyosaurian. So these taxa (Fig. 2), together with Wumengosaurus, provide a gradual accumulation of ichthyosaurian traits.

The origin of ichthyosaurs
is not such a mystery when you employ 530 taxa, but this topology was recovered when only half the current number of taxa were known, when Stereosternum was the sister to the Ichthyopterygia. The rest have been added over the last four years.

Whenever basal ichthyosaurs are mentioned,
Cartorhynchus and Omphalosaurus are considered. The large reptile tree found Cartorhynchus nested close to the pachypleurosaur, Qianxisaurus. Omphalosaurus is known by too few bones to be included in the large reptile tree, but earlier, it was considered close to Sinosaurosphargis.

References
Jiang D, Motani R, Hao W, Schmitz L, Rieppel O, Sun, Sun Z 2008. New primitive ichthyosaurian (Reptilia, Diapsida) from the Middle Triassic of Panxian, Guizhou, southwestern China and its position in the Triassic biotic recovery. Progress in Natural Science 18 (10): 1315.

The ‘Shastasaurus’ wastebasket

Last night I actually read Ji et al. 2013 and discovered I was confirming their earlier findings on Sander et al. 2011 — not by matching their tree topology, which matches certain nodes and not others — but in disputing the Sander et al. lumping of several ichthyosaurs under Shastasaurus. Even so, Ji et al. lumped those several ichthyosaurs together in the same clade as Shastasaurus, which the large reptile tree cannot confirm. I also learned that Shang and Li 2009 reassigned Guizhouichthyosaurus tangae (Cao et Luo in Yin et al. 2000) to Shastasaurus, which was an error on their part. Updates have been made.

Sometimes paleontologists like to name new species.
Other times paleontologists consider their latest discoveries variations on old themes. So they lump them together and don’t give them new generic names, perhaps only new species names.

Sander et al. (2011) introduced two short-snouted 33-foot (10 m) ichthyosaurs they suggested were suction feeders possibly tied to a Late Triassic minimum in atmospheric oxygen (fewer fish = more squid). Suction feeding was hypothetically accomplished by rapid retraction of the tongue in a tube-like snout, like a modern-day beaked whale. This news was covered by Brian Switek writing for Smithosonian magazine online here. Previously one of these was described under the name Guanlingsaurus linage (Fig. 1). These two toothless ichthyosaurs were lumped by Sander et al. (2011) with the holotype of Shastasaurus pacificus (Merriam 1895, 1902, 1908; UCMP 9017, Figs. 1, 4) from California. The UCMP specimen did not preserve a rostrum, so whether or not it had teeth or even a short rostrum was considered unknown.

Figure 1. Two Shastasaurus specimens once considered suction feeders. The 3108 specimen nests with the very primitive Mikadocephalus. The 3107 specimen nests with the Cymbospondylus and S. sikkannensis.

Figure 1. Two Shastasaurus specimens once considered suction feeders. The 3108 specimen nests with the very primitive Mikadocephalus. The 3107 specimen nests with the Cymbospondylus and S. sikkannensis.

The Sander et al. phylogenetic analysis nested the three specimens together (Fig. 2) despite their apparent differences, giving them or retaining their individual species names.

Figure 6. Phylogenetic relationships of Shastasaurus. This cladogram represents the strict consensus of 72 most parsimonious trees. Differences in topology among MPTs are mainly found among the outgroup taxa and the basal Merriamosauria. Derived Parvipelvia were part of the analysis but were omitted for clarity.

Figure 2. From Sander et al. 2011, phylogenetic relationships of Shastasaurus.
This cladogram represents the strict consensus of 72 most parsimonious trees. Differences in topology among MPTs are mainly found among the outgroup taxa and the basal Merriamosauria. Derived Parvipelvia were part of the analysis but were omitted for clarity.

On a side note,
Motani et al. (2013) formerly dismissed/retracted the suction-feeding hypothesis with Sander on the list of authors. These researchers found that ichthyosaurs did not possess the neccesary rostral features, like elaborate hyoids and a tube-like snout, that allow suction-feeding to work. That story was again covered by Brian Switek, but this time for NatGeo here.

Figure 6. The fifth putative Shastasaurus, S. tangae, IVPP V11853.

Figure 3. A fifth putative Shastasaurus, S. tangae, IVPP V11853.was erroneously reassigned by Shang and Li 2009. Originally named Guizhouichthyosaurus tangae by Cao et Lu in Yin et al. 2000, it nests close to Ichthyosaurus in the large reptile tree.

A fourth putative Shastasaurus
S. tangae was reassigned by Shang and Li (2009; Fig. 5, originally named Guizhouichthyosaurus tangae by Cao et Lu in Yin et al. 2000,). Distinct from the others it had a long toothy rostrum. More than 60 presacral vertebrae were present and the tail was ventrally bent.

Figure 7. The giant sixth putative Shastasaurus, S. sikanniensis.

Figure 4. The giant sixth putative Shastasaurus, S. (originally Shonisaurus) sikanniensis (renamed by Sander et al 2011). It nests with the toothless 3107 specimen (Fig. 3).

A fifth putative Shastasaurus,
S. skanniensis (Nicholls and Manabe 2004, originally Shonisaurus, renamed by Sander et al. 2011) was the giant of the group at 21 meters (69 feet; Fig. 6). It did not preserve a rostrum, but no teeth were found with what remained of the giant jaws.

Figure 5. Shastasaurus

Figure 5. Shastasaurus

The sixth (but by no means final) Shastasaurus
S. alexandrae (Merriam 1902) is a large ichthyosaur known from a 3D jumbled specimen preserving the material between the nostrils and pectoral girdle only.

Figure 1. Subset of the LRT focusing on the clade Ichthyosauria.

Figure 1. Subset of the LRT focusing on the clade Ichthyosauria.

Adding these six Shastasaurus specimens
to a selection of basal ichthyosaurs splits them into four clades. The first two specimens, S. alexandrae and S. pacficus surprisingly nested basal to hupehsuchids, drawing these odd-little fellows into the midst of the Ichthyosauria (or not depending on your definition).

The 3108 specimen nests with the basal ichthyosaur Mikadocephalus.

S. tangae nests with the derived Ichthyosaurus and Ophthalmosaurus.

The toothless 3107 specimen and S. sikanniensis nested with the toothy Cymbospondylus.

So toothlessness did not arise only once or twice within the Ichthyosauria, but several more times. The hupehsuchidae may not be as odd and isolated as was once believed. Some of these taxa need new generic names.

A look at the 228th character trait, related to size, indicates that small taxa appeared at the base of each major radiation of ichthyosaurs and proto-ichthyosaurs, as in pterosaurs and other major clades that experienced phylogenetic miniaturization.

Of course,
I’m not using ichthyosaur-specific character traits here, but relying on the same 228 characters that lumped and split the rest of the 530 taxa now populating the large reptile tree. But the high bootstrap scores are encouraging.

Others who have produced cladograms
of ichthyosaur relationships have not employed mesosaurs and Wumengosaurus as outgroups. I also find it odd that Sander et al. did not recover a closer relationship between S. pacificus and Hupehsuchus, despite their many similarities. Perhaps it was their resistance to employing Thaisaurus, a basal ichthyosaur.

References
 Ji C, Jiang, DY, Motani R, Hao W-C, Sun ZY, and Cai T 2013. A new juvenile specimen of Guanlingsaurus (Ichthyosauria, Shastasauridae) from the Upper Triassic of southwestern China. Journal of Vertebrate Paleontology 33 (2): 340.
McGowan C 1996. A new and typically Jurassic ichthyosaur from the Upper Triassic of British Columbia. Canadian Journal of Earth Sciences 33: 24–32.
Merriam JC 1895. On some reptilian remains from the Triassic of northern California. Am J Sci, 50(3): 55-57.
Merriam JC 1902. Triassic Ichthyopterygia from California and Nevada. Univ Calif Publ, Bull Dept Geol, 3(4): 63-108.
Merriam JC 1908. Triassic Ichthyosauria, with special reference to the American forms. Mem Univ Calif, 1: 1-196.
Motani R, Ji C, Tomita T, Kelley N, Maxwell E., Jiang D., Sander P 2013Absence of suction feeding ichthyosaurs and its implications for Triassic mesopelagic paleoecologyPLoS ONE. 8, 12: e66075. doi:10.1371/journal.pone.0066075.
Nicholls EL, Manabe M 2004. Giant ichthyosaurs of the Triassic – a new species of Shonisaurus from the Pardonet Formation (Norian: Late Triassic) of British Columbia. Journal of Vertebrate Paleontology 24 (3): 838–849.
Sander PM, Chen X-C, Cheng L and Wang X-F 2011. Short-snouted toothless ichthyosaur from China suggests Late Triassic diversification of suction feeding ichthyosaurs. PlosOne DOI: 10.1371/journal.pone.0019480
Shang Q-H and Li C 2009. On the occurrence of the ichthyosaur Shastasaurus in the Guanling biota (Late Triassic), Guizhou, China. Vertebrata PalAsiatica 2009(7):178-193.
Yin G-Z, Zhou  X, Cao Y, Yu Y and Luo Y 2000. A preliminary study on the earlyLate Triassic marine reptiles from Guanling, Guizhou, China. Geology-Geochemisty 28(3):1–23 (Chinese with English abstract).

Thaisaurus and the origin of the Ichthyosauria

Updated April 13, 2015 with a revised subset of the large reptile tree (Fig. 2).

Earlier we looked at Mikdadocephalus as the basalmost ichthyosaur. Today, a more primitive taxon is presented.

Thaisaurus chonglakmanii (Mazin et al. 1991; Early Triassic; Fig. 1.) was considered the most basal ichthyosaur by Maisch (2010). That is confirmed in the large reptile tree where Thaisaurus nests between Wumengosaurus and the remainder of the Ichthyosauria (sensu Maisch 2010, Fig. 2).

Figure 1. Thaisaurus in situ, traced using DGS, elements of tracing shifted using DGS and restored.

Figure 1. Thaisaurus in situ, traced using DGS, elements of tracing shifted using DGS and restored. Click to enlarge. Confirming Maisch 2010, this is a basal ichthyosaur, transitional between Wumengosaurus and the remainder of the Ichthyosauria. Many of the bones are missing but their impressions remain.

Diagnosis (according to Maisch 2010) “Autapomorphies are the macroscopically smooth, conical and slender tooth crowns (convergent to the Leptonectidae), and a postfrontal that remains separated from the fenestra supratemporalis. Plesiomorphies aiding in identification are: humerus without lamina anterior, humerus, femur and zeugopodials very elongate and slender, metatarsal five long and slender, as big as metatarsal one.”

Figure 2. Subset of the large reptile tree focusing on the Ichthyosauria. Note the basal position of Thaisaurus between Wumengosaurus and the remainder of the Ichthyosauria. Low bootstrap score around the base of the hupesuchids represent two skull-only taxa nested with a skull less taxon, Parahupehsuchus. Note the shift in position of the hupehsuchids as well as the various nodes at which the various specimens attributed to Shastaaurus nest.

Figure 2. Subset of the large reptile tree focusing on the Ichthyosauria. Note the basal position of Thaisaurus between Wumengosaurus and the remainder of the Ichthyosauria. Low bootstrap score around the base of the hupesuchids represent two skull-only taxa nested with a skull less taxon, Parahupehsuchus. Note the shift in position of the hupehsuchids as well as the various nodes at which the various specimens attributed to Shastaaurus nest.

The small size of Thaisaurus (Fig. 3) brings up the subject, once again, of phylogenetic miniaturization at the genesis of major clades. We’ve seen this before with mammals, birds, reptiles, pterosaurs, dinosaurs and others.

Figure 3. Basal ichthyosauria to scale. Here Wumengosaurus, Thaisaurus, Mikadocephalus and a specimen attributed to Shastasaurus are illustrated. Note the phylogenetic miniaturization shown by Thaisaurus, a trait often seen at the origin of major clades.

Figure 3. Basal ichthyosauria to scale. Here Wumengosaurus, Thaisaurus, Mikadocephalus and a specimen attributed to Shastasaurus are illustrated. Note the phylogenetic miniaturization shown by Thaisaurus, a trait often seen at the origin of major clades.

Apparently, and this should come as no surprise, the fore limbs of basal ichthyosaurs transformed into flippers prior to the hind limbs.

Apparently the high neural spines of Wumengosaurus were shorter in Thaisaurus, but these are poorly preserved.

Apparently the extreme reduction and multiplication of the cervicals of Wumengosaurus was an autapomorphy because outgroup taxa, like Stereosternum, do not have this trait.The elongation of metatarsal V is also a trait shared between Thaisaurus and Stereosternum.

Note the putative basal ichthyosaur, Cartorhynchus, nests instead with basal pachypleurosaurs and explained here.

More on Thaisaurus and other basal ichthyosaurs later.

References
Maisch MW 2010. Phylogeny, systematics, and the origin of the Ichthyosauria – the state of the art. Palaeodiversity 3:151-214.
Mazin J-M et al. 1991. Preliminary description of Thaisaurus chonglakmanii n. g. n. sp. a new ichthyopterygian (Reptilia) from the Early Triassic of Thailand. – Comptes- Rendus des Séances de l’Académie de Sciences Paris, Série II, 313: 1207-1212.

 

Besanosaurus skull and flippers reconstructed

Figure 1. Besanosaurus in situ (below) and skull reconstructed (above).

Figure 1. Besanosaurus in situ (below) and skull reconstructed (above). Left flippers are reconstructed here from scattered phalanges.

Besanosaurus leptoryhnchus (Dal Sasso and Pinna 1996, Fig. 1) was a large Middle Triassic ichthyosaur with a small skull and slender flippers. The authors nested Besanosaurus between Shonisaurus + Himalayasaurus and Shastasaurinae (Merriamia, Pessosaurus, Californosaurus, Shastasaurus). Unfortunately none of those genera are presently included in the large reptile tree.  Besanosaurus nests here (Fig. 2) between Chaohusaurus and Qianichthyosaurus, two taxa not included in Dal Sasso and Pinna. Perhaps over the upcoming weekend more ichthyosaurs can be added to the large reptile tree. We nested Mikadocephalus at the base of the ichthyosaurs recently here.

Figure 2. Family tree of basal ichthyosaurs. Several taxa (listed above) are not yet included.

Figure 2. Family tree of basal ichthyosaurs. Several taxa (listed above) are not yet included.

Below are a series of ichthyosaur skulls to show how Besanosaurus nests with them. Gray bones below Besanosaurus may be hyoids.

Figure 3. How Besanosaurus nests among the ichthyosaurs listed.

Figure 3. How Besanosaurus nests among the ichthyosaurs listed.

The original identification of the skull bones is shown below (Fig. 4). A few changes were made here (Fig. 1).

Figure 4. Original identification of bones in Besanosaurus.

Figure 4. Original identification of bones in Besanosaurus by Dal Sasso and Pinna 1996.

References
Dal Sasso C and Pinna G 1996. Besanosaurus leptorhynchus n. gen. n. sp., a new shastasaurid ichthyosaur from the Middle Triassic of Besano (Lombardy, N. Italy). Paleontologia Lombardia 4:23 pp.

 

Which ichthyosaur is the most primitive? Part 2

Updated April 13, 2015 with a flatter cranium reconstruction.

Among the Triassic ichthyosaurs I have tested, this one, Mikadocephalus, now nests as the most primitive. Note the plesiomorphic skull bones: a parietal without a crest or trough,  and an overall similarity to the outgroup taxa described below.

Figure 1. Mikadocephalus (Middle Triassic) now nests as the basalmost ichthyosaur, next to Eohupehsuchus and Wumengosaurus on one side and Utatsusaurus and Grippia on the other. Here the former premaxilla is the dentary and a few other bones are reidentified. And it's big, which means there are more primitive ichthyosaurs yet to be discovered in the Early Triassic.

Figure 1. Mikadocephalus (Middle Triassic) now nests as the basalmost ichthyosaur, next to Eohupehsuchus and Wumengosaurus on one side and Utatsusaurus and Grippia on the other. Here the former premaxilla is the dentary and a few other bones are reidentified. And it’s big, which means there are more primitive ichthyosaurs yet to be discovered in the Early Triassic.

Mikadocephalus gracilirostric (Maisch and Matzke 1997) was described from “an almost complete skull” from the Anisian-Ladinian of Switzerland that “does not fit into any of the currently recognized families of Triassic ichthyosaurs.” 

It is a large ichthyosaur with a skull around 50 cm long. That size is probably due to its late appearance in the Middle Triassic. Undiscovered Early Triassic sisters were probably much smaller.

Oddly,
the largest bones of the skull, the dentaries, were originally thought to be absent. Here the former premaxillae are identified as paired dentaries (yellow) and the premaxillae are smaller bones found elsewhere, splintered and separated into dorsal and tooth-bearing portions. Colorizing and reconstructing the bones using DGS made these identifications possible.

Figure 2. Wumengosaurus, a proximal outgroup taxon to hupehsuchids + ichthyosaurs.

Figure 2. Wumengosaurus, a proximal outgroup taxon to hupehsuchids + ichthyosaurs has a skull similar to that of Mikadocephalus.

Yesterday we looked at a number of basal ichthyosaur skull temple regions and it was quite apparent that this area underwent great changes that are not apparent in Mikadocephalus. Rather Mikadocephalus has a more primitive skull, like that of Eohupehsuchus (Fig. 3) and Wumengosaurus (Fig. 2).

Figure 2. Eohupehsuchus has a skull similar to that of the basalmost ichthyosaur, Mikadocephalus.

Figure 3. Toothless Eohupehsuchus is a hupehsuchid that has a skull similar to that of the basalmost ichthyosaur, Mikadocephalus.

It is unfortunate
the post-crania of Mikadocephalus is unknown. In 1997 Eohupehsuchus and Wumengosaurus were unknown. Even so, had a reconstruction of Mikadocephalus been created in 1997 I think we would have known about its special status a long time ago. Or at least the status of the dentaries would have been realized. The authors nested Mikadocephalus between Cymbospondylus and Temnodontosaurus out of an list of eight ichthyosaurs.

References
Maisch M and Matzke AT 1997. Mikadocephalus gracilirostris n. gen., n. sp., a new ichthyosaur from the Grenzbitumenzone (Anisian-Ladinian) of Monte San Giorgio (Switzerland). Paläontologische Zeitschrift 71(3/4):267-289.