Detour into Choristodera

This week a paper on unusual prehistoric aquatic reptiles was published. I am speaking, of course, of Brownstein (2022) on choristoderes. (If you thought it was going to be spinosaurs, you must be new around here!)

We have a soft spot for choristoderes here at Equatorial Minnesota Towers, even if they rarely rate so much as an "and also featuring Champsosaurus" credit in sci-pop culture. Brownstein (2022) includes the classic Champ but is more focused on the short(er)-faced choristodere Simoedosaurus, although under a fresh coat of taxonomic paint: the North American species S. dakotensis is moved to the new genus Kosmodraco on the grounds of anatomical differences and differences in time and place from the type species, European S. lemoinei. It also gets a new friend, K. magnicornis. Which said, sure... but the species of Kosmodraco still clade more closely to Simoedosaurus than to anything else, so if your genericometer was so tuned, you could still include them in Simoedosaurus with a clear phylogenetic conscience. (There are a lot of anatomical differences between the two genera. My only quibble with Kosmodraco is that it's an awfully pretty name for a choristodere; it sounds more like an an extravagantly crested pterosaur. Meanwhile, Champsosaurus itself is still secretly a taxonomic booby trap waiting to be sprung. Sooner or later someone is going to do something with the various species that is entirely legal by the rules of taxonomy and yet manages to displease everyone else.)

The type skull of Kosmodraco magnicornis. It'll take a moment to orient yourself: the bitey part is surprisingly short (see the "r. lacrimal"? The eye was behind that). What you're looking at is a modest snout attached to a greatly flaring right "cheek" (which comes with a scalloped fringe). Figure 1 in Brownstein (2022). CC BY 4.0.

Either way, the skull is worth a look. There's garden-variety "weird" and then there's "why is this all practically all post-orbital?" (It certainly wasn't for intellect, most of the posterior of choristodere skulls being a series of struts and bars. Note that the maxillary teeth of Kosmodraco only go back as far as the eyes.) Champsosaurus, as we've already seen, had a long, narrow toothy muzzle, with the rest of the skull being broad and low. Kosmodraco had a skull that was still low but much more wedge-shape in dorsal view, as if someone smooshed or amputated a Champsosaurus-like snout. The business end of a Kosmodraco skull is an interesting analog for the modern alligator gar. It's the back of the skull where things get different, as Brownstein (2022) notes. Kosmodraco has a lot more skull going on behind the eyes, which themselves are elevated on skull like those of an alligator. The posterior margin is also ornamented with a series of knobs, and the skull is quite low (Brownstein 2022).

A comparison of the palatal regions of choristoderes Kosmodraco and Champsosaurus to an alligator and an alligator gar also serves as a comparison of basic facial shapes for the four. Similarities between Kosmodraco and the gar aren't as great if you continue through the rest of the skull. Figure 13 in Brownstein (2022). CC BY 4.0.

I'd like to take the opportunity to note that there's another extinct group of aquatic tetrapods with flat, blunt, broad skulls and eyes relatively far forward on the skull, which has so far avoided comparisons to Simoedosaurus/Kosmodraco: the metoposaurid amphibians of the Triassic. Again, it's not a perfect comparison (metoposaur snouts are blunter), but it would seem to point to the long-term existence of a niche for freshwater predators with certain cranial adaptations.

One of the other points noted by Brownstein is there is more diversity in choristoderes than you might suspect by simply looking at the number of genera. For example, as of this writing Champsosaurus is composed of several species over about 20 million years. North American specimens have tended to be lumped with either the short-faced Simoedosaurus or the long-faced Champsosaurus.

References

Brownstein, C. D. 2022. High morphological disparity in a bizarre Paleocene fauna of predatory freshwater reptiles. BMC Ecology and Evolution 22: article number 34. doi:10.1186/s12862-022-01985-z.

The Grand Pitch Formation

Back in October I posted on a formation I saw in Maine, the Matagamon Sandstone. While going through my photos, I realized I had a number of scenic and interesting shots of another formation, also not widely known: the Grand Pitch Formation.

Comes with waterfalls!

The Grand Pitch Formation goes back in the literature to the 1930s, when it was known as the Grand Falls Formation (Ruedemann and Smith 1935). This name, though, was already in use, so the more specific Grand Pitch name was substituted (Neuman 1962). The name refers to the Grand Pitch, a waterfall on the East Branch of the Penobscot supported by more resistant beds of the formation.

Resistant beds like these.

If you've taken a historical geology class in North America, you've probably spent some time with the assembly of eastern North America. Back when I was taking that class, it was a three-stage process marked by the Taconic, Acadian, and Alleghanian (or Appalachian) mountain-building events (orogenies). Well, as you might guess, it's a bit more complicated than that. (Just a bit.) In actual practice, the North American craton, microplates, continental fragments, island arcs, and all and sundry were bumping and jostling and colliding with each other all the time. In the present example, the Grand Pitch Formation was deposited not in North America, but on a Gondwanan terrane known as Ganderia (or Gander) that eventually piled up on the continent after a series of its own adventures (including running into another terrane) (Neuman and Max 1989).

Just like our slice of the Equator in Minnesota, here in Maine you can stand on a former sliver of the tropics.

The Grand Pitch Formation is a heterogeneous unit, including beds of gray, green, and red siltstone and slate, quartzite, and minor amounts of graywacke and tuff (Neuman 1967). Siltstone and slate are charming lithologies but are not noted for resistance to weathering; instead, the falls are supported by quartzite beds. The depositional environment has been interpreted as a continental slope-rise setting (Wellensiek et al. 1990).

Finer-grained beds as seen at the surface: not recommended for load-bearing outcrops.

It's a pretty thick formation, encompassing at least 1,500 m (5,000 ft) (Neuman 1967), but it's not in mint condition, to say the least. The formation has undergone several episodes of deformation, going back to the Ganderia days with a Cambrian–Ordovician event termed the Penobscot Orogeny or Disturbance (Neuman and Max 1989).

Red and gray beds make it easy to see minor faulting here.

The age of the Grand Pitch Formation is not entirely clear. Only one kind of fossil has ever been reported from it, the invertebrate trace fossil Oldhamia, which looks kind of like a fireworks burst or a palm frond and is thought to have been produced by something "mining" beneath microbial mats (Seilacher et al. 2005). Oldhamia was most abundant in the early Cambrian, but is not limited to that time frame, nor does its occasional presence mean the entire Grand Pitch Formation has to be that age, either (Neuman 1962, 1967). Generally the formation is attributed to some interval of the Cambrian.

Going back to deformations and alterations, here we have a patch of the formation scored with glacial striations.

References

Neuman, R. B. 1962. The Grand Pitch Formation: new name for the Grand Falls Formation (Cambrian?) in northeastern Maine. American Journal of Science, series 5, 260:794–797.

Neuman, R. B. 1967. Bedrock geology of the Shin Pond and Stacyville quadrangles, Penobscot County, Maine. U.S. Geological Survey, Washington, D.C. Professional Paper 524-I.

Neuman, R. B., and M. D. Max. 1989. Penobscottian-Grampian-Finnmarkian orogenies as indicators of terrane linkages. Pages 31–45 in R. D. Dallmeyer, editor. Terranes in the circum-Atlantic Paleozoic orogens. Geological Society of America, Boulder, Colorado. Special Paper 230.

Ruedemann, R., and E. S. C. Smith. 1935. The Ordovician in Maine. American Journal of Science, series 5, 30:353–355.

Seilacher, A., L. A. Buatois, and M. G. Mángano. 2005. Trace fossils in the Ediacaran–Cambrian transition: behavioral diversification, ecological turnover and environmental shift. Palaeogeography Palaeoclimatology Palaeoecology 227(4):323–356.

Wellensiek, M. R., B. A. van der Phijm, R. Van der Voo, and R. J. E. Johnson. 1990. Tectonic history of the Lunksoos composite terrane in the Maine Appalachians. Tectonics 9(4):719–734.