Sunday, July 14, 2019

Aquilarhinus palimentus

I get to knock another notable fossil park off my list today with a new basal hadrosaur from Big Bend National Park. The NPS doesn't have a park covering the classic Upper Great Plains terrestrial Upper Cretaceous rocks, but it does have Big Bend, one of the best southern North American Upper Cretaceous areas known (to say nothing of its Lower Cretaceous and Cenozoic records). Our visitor today is the arch-snouted trowel-jawed Aquilarhinus palimentus.

But first, a brief note which connects to the history of this blog: "Lori" the Morrison troodontid has been officially described, as Hesperornithoides miessleri (Hartman et al. 2019). There's really no point in my writing anything about it, because the two lead authors (Scott Hartman and Mickey Mortimer) have their own blogs where they are covering it and I certainly couldn't add anything to them, so check them out!

Genus and species: Aquilarhinus palimentus; the genus name is derived from the Latin "aquila", meaning "eagle", and Greek "rhinos", meaning "nose", making "eagle nose" in reference to the arched nasal bones. The species name is from the Latin "pala", meaning "shovel", and "mentus", meaning "chin", making "shovel chin" (Prieto-Márquez et al. 2019). All together, it's something like the "shovel-chinned eagle nose", which also sounds like something you'd hear thrown around in a stereotypical Wild West saloon just before the tables get knocked over, so it's not out of place in west Texas.
Citation: Prieto-Márquez, A., J. R. Wagner, and T. Lehman. 2019. An unusual "shovel-billed" dinosaur with trophic specializations from the early Campanian of Trans-Pecos Texas, and the ancestral hadrosaurian crest. Journal of Systematic Palaeontology. Advance online publication. doi:10.1080/14772019.2019.1625078.
Stratigraphy and geography: A. palimentus was found in the lower shale member of the Aguja Formation. As Prieto-Márquez et al. (2019) note, to date Aguja Formation paleontology is basically known from the upper shale member, so this species is part of something new. Additional reports on the large vertebrates of the lower Aguja are promised in the text, so it should round out more in the coming years. The lower shale member is regarded as early Campanian in age, on the order of 81–80 million years. Lower Campanian terrestrial rocks are not particularly well-sampled in North America, so this also has broader implications. Geograpically, the type specimen was found in southwestern Big Bend National Park, in a mudstone bed also laden with carbonized plant material. The plant material may have held up the small bones and kept them from being washed away (Prieto-Márquez et al. 2019).
Holotype: The type and only known specimen of A. palimentus is Texas Memorial Museum (TMM) 42452-1, a collection of bones found disarticulated but associated over about 4 square meters (about 43 square feet). These include "a sphenoid fragment, both nasals, right maxilla, right jugal, right quadratojugal, partial left and right palatines, partial right dentary, partial first ceratobranchial, partial neural arch of atlas, fragments of two cervical centra, two cervical ribs, partial sacral rib, left carpal, nearly complete left manus, postacetabular process of right ilium, fragment of right ischium, partial astragali, pedal phalanx III-1, and four pedal unguals" (Prieto-Márquez et al. 2019).

TMM 42452-1 was initially excavated back in 1983, with a return visit in 1999. It appeared in coauthor Jonathan Wagner's 2001 thesis as Kritosaurus sp. Other hadrosaur bones were found in the same stratigraphic interval as TMM 42452-1, some not too far from the type locality, but Prieto-Márquez et al. (2019) opted to be cautious and not assign them to A. palimentus. These fossils include a few more partial skull bones (braincase, parietal, frontal, and postorbital) and fragmentary limb and girdle bones (scapula, ilium, pubis, humerus, femur and tibia), of unremarkable hadrosaurian form.

The skull of TMM 42452-1 is on the smallish side for a hadrosaur (estimated at 57 cm long, or 22 in), but interpreted as relatively tall and broad, so boxier than the typical hadrosaur. The relative breadth of the skull may be related to the form of the jaw. Most of the right dentary is present. It is fairly deep, without much of a toothless anterior portion. The symphyseal process, where the two sides of the mandible meet, angles up toward the midline, meaning if you were looking at the mandible from the front, the chin would make a W shape instead of the typical hadrosaurian U shape. (Presumably this would have some effect on the postulated hadrosaurian jaw movements around the symphysis.) The predentary has not been found, but must have had a similar W-shaped cross-section to attach to the dentary. The overall effect would be similar to a garden trowel, with a lengthwise ridge strengthening a scoop shape (Prieto-Márquez et al. 2019).

The nasal are strongly arched, which is a fairly common feature of non-lambeosaur hadrosaurs. There are many subtleties about hadrosaur nasal arching, though, and A. palimentus has its own: unlike the narrow arch of the various species of Gryposaurus, A. palimentus has a nasal arch which is broad at its base in front of the eyes. The arch is most similar to that of Latirhinus uitstlani, which also has a broad-based arch. L. uitstlani, though, has a more robust arch (Prieto-Márquez et al. 2019). It's worth noting that the two species show up as sister taxa outside of the saurolophs and lambeosaurs, so depending on how your genericometer is tuned, you might be tempted to assign both species to the same genus.

A. palimentus comes with two talking points right out of the title of the paper: the "shovel bill" and its implications, and the state of the ancestral hadrosaurian crest. It's a bit disappointing that the press for this species has been all about the "shovel bill", and we don't actually have the premaxillae or the predentary. Instead, the shape of the bill can only be inferred from the dentary. (Bajadasaurus pronuspinax suffers from this as well: for all of the illustrations of pointy punk sauropods, a grand total of one such vertebra has been described, so we don't actually know much about how the spines of the rest of the neck looked.) Nevertheless, as mentioned, the odd shape of the symphysis certainly constrains the form of the predentary, which would have been quite different from your typical hadrosaur. Prieto-Márquez et al. (2019) suggested that A. palimentus was adapted to shoveling or scooping food, similar to gomphotheres, sirenians, hippopotami, pyrotheres, and desmostylians. A lot of these groups of mammals are (or were) aquatic or semiaquatic, and it's possible that A. palimentus was spending its time around water mucking in wet sediment. This would also, incidentally, make A. palimentus by far the most ecologically specialized hadrosaur known (Prieto-Márquez et al. 2019).

Concerning the crest, A. palimentus bears arched nasals, like many other hadrosaurs, and plots out basal to the sauroloph-lambeosaur split. Given the wide distribution of crests in hadrosaurs, it seems reasonable that hadrosaur crests are all modifications on the nasal arch, and that saurolophs (the "flat-headed hadrosaurines" of yore) did not actually start out crestless (Prieto-Márquez et al. 2019).

References

Hartman, S., M. Mortimer, W. R. Wahl, D. R. Lomax, J. Lippincott and D. M. Lovelace. 2019. A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight. PeerJ 7:e7247. doi:10.7717/peerj.7247.

Prieto-Márquez, A., J. R. Wagner, and T. Lehman. 2019. An unusual "shovel-billed" dinosaur with trophic specializations from the early Campanian of Trans-Pecos Texas, and the ancestral hadrosaurian crest. Journal of Systematic Palaeontology. Advance online publication. doi:10.1080/14772019.2019.1625078.

Wagner, J. R. 2001. The hadrosaurian dinosaurs (Ornithischia: Hadrosauria) of Big Bend National Park, Brewster County, Texas, with implications for Late Cretaceous paleozoogeography. Thesis. Texas Tech University, Lubbock, Texas.

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