Stylinodon may be a giant herbivorous mink

In 1873 
O. C. Marsh 1874) found an extinct Eocene (50.3 to 40.4 Ma) mammal “of great interest. The lower molar teeth, all essentially alike, and inserted in deep sockets” were the most striking feature. He named it Stylodon mirus (Figs. 1,2). All the teeth grew with “persistent pulps” and had a thin layer of enamel. The specimen was considered close to Toxodon with some edentate affinities (Marsh 1897). Stylinodon was placed under the family Stylinodontidae and the order Tillodontia. According to Schoch 1986 (first issue of JVP!) its ancestors were like Onychodectes.

Stylinodon mirus (Marsh 1874; middle Eocene, 45 mya; Figs. 1-2) was originally considered a taeniodont, perhaps derived from Onychodectes. Here it nests with Mustela, the living European mink, among the Carnivora. There were twice as many molars (4), each with a single root, as in the two double rooted molars of the mink. Large claws and certain forelimb traits indicate that Stylinodon was a digger, not a cursor.

The present nesting
of Stylinodon mirus (YPM VP 011095, Marsh 1874; Figs. 1, 2) in the Carnivora occurred when I realized it was a poor fit at the base of the Condylarthra/Paenungulata, despite its herbivorous dentition and tusk-like teeth (canines, not incisors).

Figure 1. Stylinodon skull. Note the transverse premaxilla, a trait of the Carnivora.

Figure 1. Stylinodon skull. Note the transverse premaxilla, a trait of the Carnivora.

Distinct from condylarths
Stylinodon has a transverse premaxilla, essentially invisible in lateral view. The lower canine is the anteriormost tooth on the dentary. These traits are shared with other members of the Carnivora. In the present taxon list Stylinodon shares more traits with Mustela, the European mink (Fig. 1) despite the loss of molar cusps and increase in size. They both were diggers. Together they nest with Phoca, the seal, and Palaeosinopa, the amphibious piscivore, all derived from a sister to Procyon, the omnivorous raccoon (Fig. 2).

Figure 1. Stylinodon compared to Mustela, the European mink to scale.

Figure 2 Stylinodon compared to Mustela, the European mink to scale.

As in the earlier issue
with indricotheres, related taxa can have distinctively different types of teeth, one more reason to not weight dental traits too heavily, unless that’s all you have.

Figure 2. Mustela the European mink is an extant relative to Stylinodon.

Figure 3. Mustela the European mink is an extant relative to Stylinodon.

Mustela lutreola (Linneaus 1761; extant European mink; up to 43cm in length) is a fast and agile animal related to weasels and polecats. Mustela lives in a burrow, but it also swims and dives skilfully. It is able to run along stream beds, and stay underwater for one to two minutes. Mustela is derived from a sister to Phoca and other seals, all derived from a sister to Procyon. With this close relationship, Stylinodon (Fig. 2 a giant weasel with simple teeth.

Schoch and Lucas 1981
and Schoch 1983 considered Stylinodon and kin derived from a sister to the long-legged basal condylarth, Onychodectes. The large reptile tree (LRT, Fig. 2) does not support that nesting. Onychodectes has a long premaxilla lacking in taeniodonts.

Figure 2. Subset of the LRT showing the Carnivora nesting at the base of the Eutheria (placental mammals).

Figure 4. Subset of the LRT showing the Carnivora nesting at the base of the Eutheria (placental mammals).

Schoch and Lucas 1981
determined that Stylinodon had two upper incisors (one lower), a giant canine, four premolars and three molars, as in Onychodectes. That may be so, but the premolars and molars look alike.

 

Figure 6. Wortmania as drawn freehand by Schoch compared to bones Photoshopped together.

Figure 5 Wortmania as drawn freehand by Schoch compared to bones Photoshopped together.

Wortmania (Hay 1899, Williamson and Brusatte 2013; above) and Psittacotherium (Cope 1862; below) are related to Stylinodon. All are among the largest taxa in the early post-Cretaceous, derived from smaller weael-like basal mammals in the Cretaceous.

Figure 7. Psittacotherium in various views.

Figure 6.  Psittacotherium in various views. Overall it is elongated to more closely match related taxa.

It is rare but not unheard of
for members of the Carnivora to become omnivores and herbivores. Think of the giant panda and certain viverrids. Now the stylinodontid taeniodonts join their ranks.

References
Linneaus C von 1761. xxx
Marsh OC 1874. Notice of new Tertiary mammals 3. American Journal of Science. (3) 7i: 531-534.|
Marsh OC 1897. The Stylinodontia, a suborder of Eocene Edentates. The American Journal of Science Series 4 Vol. 3:137-146.
Rook DL and Hunter JP 2013. Rooting Around the Eutherian Family Tree: the Origin and Relations of the Taeniodonta. Journal of Mammalian Evolution: 1–17.
Schoch RM and Lucas SG 1981. The systematics of Stylinodon, an Eocene Taeniodont (Mammalia) from western North America. Journal of Vertebrate Paleontology 1(2):175-183.
Schoch RM 1983. Systematics, functional morphology and macroevolution of the extinct mammalian order Taeniodonta. Peabody Museum of Natural History Bulletin 42: 307pp. 60 figs. 65 pls.

 

wiki/Stylinodon
wiki/Mustela

Laquintasaura: verrrry basal ceratopsian from the Early Jurassic

Figure 2. Phytodinosauria with a focus on Stegosauria (yellow green).

Figure 1. Subset of the LRT focusing on the Phytodinosauria. Here Laqunitasaura nests at the base of the Ceratopsia.

I still hold to the hypothesis|
that a phylogenetic analysis that is able to lump and separate taxa is better than one that cannot do this. In the large reptile tree (LRT, 989 taxa), Laquintasaura venezuelae (Barrett et al. 2014; Early Jurassic, 200mya ~1m in overall length; Fig. 2) nests at the base of the ceratopsia (outside of Hexinlusaurus and Yinlong) and not far from the base of the Ornithopoda (outside of Changchunsaurus). It is very plesiomorphic and very early even for an ornithischian, let alone a ceratopsian.

Figure 1. Laquintasaura and tooth from Barrett et al. 2014. The early and plesiomorphic ornithischian has a naris shifted dorsally and other traits that nest it between the base of the onithopoda (Changchunsaurus) and the base of the ceratopidae (Hexinlusaurus).

Figure 2. Laquintasaura and tooth from Barrett et al. 2014. The early and plesiomorphic ornithischian has a naris shifted dorsally and other traits that nest it between the base of the onithopoda (Changchunsaurus) and the base of the ceratopidae (Hexinlusaurus). Compare to premaxillary teeth in figure 3.

Barrett et al. were not so sure where Laquintasaura nested
as they reported, “A strict consensus of these 2160 MPTs places Laquintasaura in an unresolved polytomy with the major ornithischian clades Heterodontosauridae, Neornithischia and Thyreophora along with other early ornithischian taxa, such as Lesothosaurus.”

The Barrett et al. diagnosis reports:
“Laquintasaura can be differentiated from other early ornithischians by the following autapomorphic combination  of dental characters: cheek tooth crowns have isosceles-shaped outlines, which are apicobasally elongate, taper apically, are mesiodistally widest immediately apical to the root/crown junction, possess coarse marginal denticles extending for the full lengths of the crown margins, and possess prominent apicobasally extending striations on their labial and lingual surfaces. Postcranial autapomorphies include: sharply inflected dorsal margin of ischium dorsal to the obturator process; femoral fibula epicondyle medially inset in posterior or ventral views; and astragalus with a deep, broad, ‘U’-shaped notch in anterior surface.”

I had no access to the fossil(s).
And I had to trust the drawing produced by Barrett et al. (Fig. 1) for my data. Contra the Barrett et all. analysis, there was no loss of resolution with Laquintasaura in the LRT.

Figure 2. The skull of Yinlong a basal certatopsian.

Figure 3 The skull of Yinlong a basal certatopsian. Those premaxillary teeth are quite similar to those figure in Barrett et al. for Laquintasaura. Note the dorsal naris, horizontal ventral premaxilla.

References
Barrett PM, Butler RJ, Mundil R, Scheyer TM, Irmis RB, Sánchez-Villagra MR. 2014. A palaeoequatorial ornithischian and new constraints on early dinosaur diversification. Proceedings of the Royal Society B 281:20141147. http://dx.doi.org/10.1098/rspb.2014.1147

Basal tetrapods revised with more taxa

Full resolution during a Heuristic search was not enough.
Full resolution with high Bootstrap scores was the goal. Reexamination of the data would hopefully get to that goal, as it did so many times before. Sometimes it takes awhile. It’s a learning process, and I learned a lot over the last several weeks, sometimes from difficult and scrappy data. Here’s the result:

Figure 1. Subset of the large reptile tree (LRT) focusing on basal tetrapods.

Figure 1. Subset of the large reptile tree (LRT) focusing on basal tetrapods.

Some interesting results here. 

  1. Large temnospondyls are now split in two  (with, as before, many former small temnospondyls joining the equally small lepospondyls).
  2. Ichthyostega, now not so primitive, nests closer to Reptilomorpha.
  3. New reconstructions are offered for some taxa, like Tuditanus and Utaherpeton.
  4. Basal diplocaulids, like Keraterapeton, were added.
  5. Two taxa known as Trematosaurus, one with a shorter rostrum, one with a longer one, are split apart on the tree. Gavial-like snouts are not monophyletic at present, but long-nouted forms do not have long snouts as juveniles. This is a well-known quagmire I may get into later.

Look for more basal tetrapods with legs, not fins in the Late Devonian.
Not sure where they are, but they are out there. Apparently there were several ventures onto land, not just one fin-to-finger transition.

In a few days
I’ll start with some of the interesting details as time allows, but basically this completes the task, the tree, and the broad strokes that hypothetically echo the origin of reptiles and the variation that followed thereafter.

 

Trimerorhachis: a late survivor of the fin/finger transition?

Figure 1. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition.

Figure 1. Flattened Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition, not within the Temnospondyli. Both are late survivors of a Devonian radiation.

Wikipedia reports:
Trimerorhachis (Early Permian, (Cope 1878, Case 1935, Schoch 2013; up to 1m in length) is an extinct genus of dvinosaurian temnospondyl within the family Trimerorhachidae. The trunk is long and the limbs are relatively short. Many bones are poorly ossified, indicating that Trimerorhachis was poorly suited for movement on land. The presence of a branchial apparatus indicates that Trimerorhachis had external gills in life. The body of Trimerorhachis is also completely covered by small and very thin osteoderms, which overlap and can be up to 20 layers thick. The scales were more similar to fish scales than they were to reptile scales, according to Colbert 1955. However, Olson 1979 disputed that interpretation. Specimens are often preserved as masses of bones that are mixed together and densely packed in slabs of rock”

Figure 2. Trimerorhachis forelimb and hind limb in situ and reconstructed.

Figure 2. Trimerorhachis forelimb and hind limb in situ and reconstructed. Pawley 1979 did not report metacarpals or a pubis. It is possible and perhaps likely that only 4 metacarpals were present along with two phalanges, but its worth exploring all possibilities. 

As a late (Early Permian) survivor of a Late Devonian radiation
Trimerorhachis evolved by convergence certain traits found in other more derived tetrapods, like a longer femur and open palate (narrow, bowed pterygoids). Testing all possibilities while minimizing assumptions is the most valuable benefit of a large gamut phylogenetic analysis conducted by unbiased software. Workers used to eyeball specimens in the pre-computer days.

Figure 2. Trimerorhachis pelvis. The pubis is not ossified.

Figure 3. Trimerorhachis pelvis. The pubis is not ossified here, according to Pawley 1979, but see Fig. 1.

Like other workers,
Pawley 1979 considered Trimerorhachis close to Dvinosaurus (Fig. 7) and both thought to be derived from the basal temnospondyl Balanerpeton and Dendrerpeton. The large reptile tree (LRT) nests both taxa at the base of the Lepodpondyli, not closely related to Trimerorhachis and distinct from Temnospondyli. Pawley supports the hypothesis that aquatic ‘temnospondyls,’ like Trimerorhachis, had terrestrial ancestors. By contrast, the LRT nests Trimerorhachis with weak-limbed taxa more primitive than any temnospondyl.

Additionally
the LRT nests Batrachosaurus and Gerrothorax in the Dvinosaurus / Trimerorhachis clade. This clade features horizontally opposed dorsal ribs and an equally flattened skull. Another flattened taxon, Ossinodus, is closely related. I have not seen limb material for any of these taxa. Acanthostega is the closest taxon that preserves limbs.

Figure 3. Trimerorhachis hind limb and pes from Pawley 1979.

Figure 4. Trimerorhachis hind limb and pes from Pawley 1979 and reconstructed here.

Pawley 1979 noted,
“The vast majority of the [Trimerorhachis] specimens consists of ornamental cranial and pectoral girdle bones, intercentra, and larger elements of the appendicular skeleton. Neural arches, pleurocentra, ribs and distal limb elements are rare.” No sacrals were found by Pawley. No dorsal ribs had uncinate processes (like those in Ichthyostega and Eryops). The chevrons were long and tapered distally (creating a fin?). The interclavicle was diamond-shaped with a longer anterior portion.

Figure 4. Trimerorhachis humerus changes during ontogeny

Figure 5. Trimerorhachis humerus changes during ontogeny

The humerus
(Fig. 5) was  L-shaped and the degree of torsion varied between specimens from 45º to 90º. The distal end always exhibited a low degree of ossification.

Figure 6. Trimerorhachis cladogram. Gray area is the Temnospondyli clade.

Figure 6. Trimerorhachis cladogram. Gray area is the Temnospondyli clade.

Pawley considered
Trimerorhachis a secondarily adapted aquatic temnospondyl. All workers have noted the wide open palate vacuities that characterize most, but not all members of the Temnospondyli. By contrast, the LRT nests Trimerorhachis with taxa that had not yet left the water completely and shared a flat morphology with Tiktaalik and Panderichthys.

This is the second time
elongate limbs and digits have appeared by convergence in basal tetrapods. Earlier Pholidogaster and kin provided the first exceptions to the rule. Note that all known specimens of Trimerorhachis are Early Permian, some tens of millions of years later than the Late Devonian radiation of that clade. The Ichthyostega line is the one that ultimately produced crown Tetrapoda via a sister to Eucritta.

FIgure 8. Dvinosaurus nests with Trimerorhachis and also has ceratobranchial (gill) bones.

FIgure 7. Dvinosaurus nests with Trimerorhachis and also has ceratobranchial (gill) bones. The loss of the intertemoral is shown here in light green merging to the postorbital in orange. 

If these nestings are not correct
and Trimerorhachis ultimately nests higher on the basal tetrapod tree, then we’re witnessing massive convergence of another sort, convergence that allies Trimerorhachis with tetrapods at the fin/finger transition. I’d like to see limbs for Gerrothorax or any other plagiosaur, if available.

Figure 9. Ossinodus is a close relative of Trimerorhachis in the LRT.

Figure 8. Ossinodus is a close relative of Trimerorhachis in the LRT. 

By the way, I find this fascinating…
week after week, far and away the most popular page(s) on this blog continue to be on the origin of bats.

References
Berman DS and Reisz RR 1980. A new species of Trimerorhachis (Amphibia, Temnospondyli) from the Lower Permian Abo Formation of New Mexico, with discussion of Permian faunal distributions in that state. Annals of the Carnegie Museum. 49: 455–485.
Case EC 1935. Description of a collection of associated skeletons of Trimerorhachis. University of Michigan Contributions from the Museum of Paleontology. 4 (13): 227–274.
Colbert EH 1955. Scales in the Permian amphibian Trimerorhachis. American Museum Novitates. 1740: 1–17.
Olson EC 1979. Aspects of the biology of Trimerorhachis (Amphibia: Temnospondyli). Journal of Paleontology. 53 (1): 1–17.
Pawley K 2007. The postcranial skeleton of Trimerorhachis insignis Cope, 1878 (Temnospondyli: Trimerorhachidae): a plesiomorphic temnospondyl from the Lower Permian of North America. Journal of Paleontology. 81 (5):
Williston SW 1915. Trimerorhachis, a Permian temnospondyl amphibian. The Journal of Geology. 23 (3): 246–255.
Williston SW 1916. The skeleton of Trimerorhachis. The Journal of Geology. 24 (3): 291–297.

wiki/Trimerorhachis

Distribution of ‘key’ traits in basal tetrapods

Before the advent of phylogenetic analysis,
paleontologists attempted to define clades with a short list of synapomorphies. In this way they were getting close to the dangers of pulling a Larry Martin. Many taxa, like pterosaurs and Vancleavea were (and are) considered enigmas because they seemed to appear suddenly in the fossil record with a short suite of traits that did not appear in other reptiles. That was only true back then because paleontologists were only considering short lists of traits.

After the advent of phylogenetic analysis
considering long lists of traits, the rule of maximum parsimony allowed clades to include members that do not have a short list of key traits. For instance some reptiles, like snakes, do not have limbs, but that’s okay based on the rule of maximum parsimony as demonstrated in the large reptile tree (LRT, 977 taxa, subsets shown in Figs. 1-5).

Before the advent of phylogenetic analysis
Carroll (1988) divided basal tetrapods into labyrinthodonts and lepospondyls and presented short lists of key traits.

Labyrinthodonts

  1. evolved directly from rhipidistian fish
  2. labyrinthine infolding of the dentine
  3. palate fangs and replacement pits
  4. vertebral centra composed of more than one element
  5. otic notch
  6. large in size

Lepospondyls

  1. a heterogeneous assemblage of groups with perhaps several origins from among various labyrinthodonts
  2. simple (non-labyrinthine) teeth
  3. no palate fangs
  4. vertebral centra composed of one element
  5. no otic notch
  6. small in size

By contrast,
the large reptile tree introduces a non-traditional topology in which lepospondyls have a single origin. Below (Figs. 1-5) the distribution of several traits are presented graphically.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

Open palate distribution
Basal tetrapods have a solid palate (Fig. 1) in which the pterygoid is broad and leaves no space around the medial cultriform process. Other taxa have narrow pterygoids and large open spaces surrounding the cultriform process. Still others are midway between the two extremes. Traditional topologies attempt to put all open palate taxa into a single clade. Here the open palate evolved three times by convergence.

Figure 2. Size distribution among basal tetrapods in the LRT topology

Figure 2. Size distribution among basal tetrapods in the LRT topology

The length of basal tetrapods
falls below 60 cm in Eucritta and more derived taxa. It also falls below 60 cm in Ostelepis, at the origin of Tetrapoda and Paratetrapoda. Phlegethontia has a small skull, but is otherwise like an eel, and so does not fall below the 60 cm threshold.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT.

Single piece centra
appear in frogs + salamanders, microsaurs and Phlegethontia, by convergence. Intercentra appear in all other taxa.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

Palate fangs
appear in all basal paratetrapods and tetrapods except Phlegethontia, Spathicephalus and Gerrothorax. Exceptionally, Seymouria also had palate fangs.

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

The otic notch
is widespread among basal tetrapods. Those without an otic notch include

  1. One specimen of Phlegethontia that loses posterior skull bones
  2. Six flat-skulled temnospondyls in which the tabular contacts the squamosal. Some of these, like Greererpeton, have figure data that lack an otic notch, but photos that have one.
  3. Salamanders and frogs that greatly reduce posterior skull bones.
  4. All microsaurs more derived than Microbrachis

Let me know
if I overlooked or misrepresented any pertinent data. This weekend I should be able to look at and respond to the many dozen comments that have accumulated over the last few weeks.

 

Basal Tetrapods, slightly revised

Figure 1. Click to enlarge. With the addition of Panderichthys and Anthracosaurus the position of Koilops and Deltaherpeton have shifted to the base of the Temnospondyli.

Figure 1. Click to enlarge. With the addition of Panderichthys and Anthracosaurus the position of Koilops and Deltaherpeton have shifted to the base of the Temnospondyli. Some of that shifting is due to rescoring.

After earlier identifying
phylogenetic miniaturization at the bases of several major clades in the large reptile tree (LRT, 969 taxa), I wondered if similar size-related patterns appear in basal tetrapods.

  1. Osteolepis is smaller than Eusthenopteron. Has anyone removed the scales from the fore fins of Osteolepis to see what the bones inside look like?
  2. Pholidogaster is much larger than Osteolepis, but Colosteus and Phlegethontia are successively smaller with smaller limbs.
  3. Ventastaga and Pederpes are successively smaller than Ichthyostega.
  4. Koilops is much smaller than Ventastaga and Pederpes
  5. Eucritta is much smaller than Proterogyrinus, both in overall size and in relative torso length. Eucritta nests at the base of the Seymouriamorpha + Crown Tetrapoda.
Figure 2. Basal tetrapod skulls in dorsal view.

Figure 2. Basal tetrapod skulls in dorsal view. Tetrapoda arise with flattened skulls. Paratetrapoda retain skulls with a circular cross section. 

 

A word about competing phylogenetic hypotheses…

…from Coates et al. 2002:
re: basal tetrapods: “Debates about phylogenetic hypotheses concerning these basal nodes are often intense, and conflicts arise over differing taxon and character sets, scores, and coding methods (see Coates et al. 2000; Laurin et al.2000).

And that comes eight yeas before
the advent of ReptileEvolution.com and this blog. So, readers, don’t trust one or another analysis (even this one) before giving them a test on your own or waiting for all the fallout to… fall out. At present, they are competing analyses.

At present
there are broad swathes of agreement in many published trees. The disagreements will ultimately iron themselves out. That some workers object to seeing new solutions to problems they feel they have solved already is just part of the process.

References
Coates MI, Ruta M and Milner AR 2000. Early tetrapod evolution. Trends Ecol. Evol. 15: 327–328.
Coates MI and Ruta M 2001 2002. Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Laurin, M., Girondot, M., and de Ricqlès, A. 2000. Early tetrapod evolution. Trends Ecol. Evol. 15: 118–123.