Rhynchosauroides hyperbates: what a handprint!

Rhynchosauroides hyperbates (Late Triassic). Note the perfect impression of tiny scales here, along with toe drag marks and the lineup of the metatarsals with 2 and 3 longer than 4, very rare among reptiles.

Figure 1. Rhynchosauroides hyperbates manus (Late Triassic). Note the perfect impression of tiny scales here, along with toe drag marks and the lineup of the metacarpals with 2 and 3 longer than 4, very rare among reptiles. Like Rotodactylus digit 1 barely impresses. Unlike Rotodactylus, digit 5 impresses laterally here. Unlike the holotype, here the palm is impressed.

I found this image of a pristine Late Triassic manus ichnite online here and wanted to know which Late Triassic genus or family it might belong to. I found out it was referred to Rhynchosauroides hyperbates by Roy Schlische (1996). Here is his abstract.

“Previously known only from the holotype from the lower Passaic Fm. (E. Norian), segments of a superb long trackway of R. hyperbates have been recovered from a roadcut for the Schuylkill Expressway through the Lockatong Fm. (L. Carnian) of the Newark basin. Walking and swimming trackways as well as resting belly impressions are present, all with remarkably detailed skin impressions. Composite drawing of the manus and pes using the methodology of Baird confirm his original interpretations of the skeletal reconstruction. Cladistic analysis of the reconstruction shows that the trackmaker had feet of the primitive diapsid pattern. Consideration of the range of diapsids known from the Triassic suggests that a sphenodontid was the most likely trackmaker, as Baird observed.” 

Schlische, referred me to two students now working with it, but they did not respond.  Triassic expert Paul Olsen referred me to Olsen and Flynn 1989. Another paper on this trackway is on its way to publication. I understand this image once graced the cover of Science magazine in 1989. Yes, it’s that good!

Noteworthy in the image above, 
m1.1 apparently was kept within the fleshy body of the other metatarsals according to the ichnite. Who knew?? But it makes a certain sense considering the length of mc2.

A little about the ichnogenus Rhynchosauroides
There are several ichnospecies associated with the ichnogenus Rhynchosauroides, which is found worldwide. From all I can gather, it is a wastebasket ichnogenus most commonly employed for lepidosauromorph traces typically defined by a relatively longer digits laterally.

Figure 2. The trackway of Rhynchosauroides hyperbates as originally compared to a trackmaker (below) and here compared to a relatively smaller living lizard in dorsal view.

Figure 2. The trackway of Rhynchosauroides hyperbates as originally compared to a trackmaker (below) and here compared to a relatively smaller living lizard in dorsal view.

The holotype of Rhychosauroides is a much smaller specimen. Referred specimens by Baird do not have the elongate metatarsal 2, but have a more standard asymmetrical metatarsus.

Figure 3. The holotype of Rhynchosauroides,  ANS 15210 (upper left), is a much smaller specimen. Referred specimens by Baird (larger images above) do not have a short mt4. Rather they have a more standard asymmetrical metatarsus. Plus, these are all digitigrade impressions, unlike figure 1. So, based on these differences the unknown image in Figure 2 does not appear to be similar to these. But then, there were many more tracks found around Rhynchosauroides hyperbates and some of them may more closely resemble these.

Type specimen:
ANS 15210, is the ichnogenus holotype (Fig. 3) and the type locality is Smith Clark’s quarry (tracks), Milford, level B, which is in a Norian terrestrial shale/sandstone in the Passaic Formation of New Jersey.

Figure 2. Rhynchosauroides tyrolicus (Avanzini and Renesto 2002) along with their paper models of the digitigrade manus and pes, their reconstruction of the Macrocnemus-like trackmaker (gray), and a posterior view of a walking lizard (blue). I have rearranged the limbs of their trackmaker to match the tracks and match the lepidosaur in blue.

Figure 4. Click to enlarge. Rhynchosauroides tyrolicus (Avanzini and Renesto 2002) along with their paper models of the digitigrade manus and pes, their reconstruction of the Macrocnemus-like trackmaker (gray), and their posterior view of a walking lizard (blue). Note the left pes is still not planted while the left manus is planted. I have rearranged the limbs of their trackmaker to match the tracks and match the lepidosaur in blue. Note this is a distinctly different ichnospecies.

Rhynchosauroides tyrolicus,
another species of Rhynchosauroides from the Triassic of Europe, was closely matched to Macrocnemus by Avanzini and Renesto 2002. I think they did a great job, employing a cut paper pes model to discover a better match in 3D to the digitigrade traces. This simple yet effective technique was also used in Peters 2000a, b. Avanzini and Renesto 2002 provided a reconstruction Fig. 2 in gray) that matched the footprints, but it was an odd configuration requiring the left pes to overtake the left manus before the left manus could lift off the substrate. An alternative is offered (Fig. 2 in black) that more closely matches the movement of an actual lepidosaur images (Fig. 2 in blue) published by Avanzini and Renesto 2002 showing the left manus and right pes in contact with the substrate at one point. This would also match the trackway. It’s the same step cycle. Just the timing is shifting a little. Then again, in figure 2, the pose is virtually the same as figured by Avanzini and Renesto.

Digitigrady
A digitigrade Macrocnemus phylogenetically precedes a digitigrade Cosesaurus (matched to digitigrade Rotodactylus traces) at the base of the other fenestrasaurs, including pterosaurs, now shown to have produced digitigrade traces in the Late Triassic.

The ichnogenus Rhynchosauroides
A wide variety of lepidosaur-ish traces have been assigned to the wastebasket Rhynchosauroides. Now that Macrocnemus nests with the tritosaur lepidosaurs, this ichnotaxon also extends to the tritosaurs.

References
Avanzini M and Renesto S 2002. A review of Rhynchosauroides tyrolicus Abel, 1926 ichnospecies (Middle Triassic: Anisian-Ladinian) and some inferences on Rhynchosauroides trackmaker. Rivista Italiana di Paleontologia e Stratigrafia 108(1):51-66.
Baird D 1957. Triassic reptile footprint faunules from Milford, New Jersey. Bulletin of the Museum of Comparative Zoology117(5):449-520.
Olsen PE and Flynn JJ 1989. Field Guide to the Vertebrate Paleontology of Late Triassic Age Rocks in the Southwestern Newark Basin (Newark Supergroup, New Jersey and Pennsylvania. The Mosasaur 4:1-43, Delaware Valley Paleontological Society.
Peters, D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41.
Peters, D 2000b. A redescription of four prolacertiform genera and implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 293-336.
Schlische RW 1996. Uniquely preserved trackway of the reptile ichnotaxon Rhynchosauroides hyperbates BAIRD from the Late Triassic of Arcola, Pennsylvania, associated forms, and significance to Carnian-Norian extinction online abstract

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