Ngwevu intloko

For our first "prosauropod" entry since Ledumahadi mafube back in September 2018, we return to the Upper Elliot Formation of South Africa. This time around, it's massospondylid Ngwevu intloko. Or, is it just a distorted, young, or otherwise odd individual of the Upper Elliot classic Massospondylus carinatus?

Genus and species: Ngwevu intloko; both the genus and species names are derived from Xhosa, with the genus name meaning "grey" and the species name meaning "head", giving "grey head". This is a reference to a nickname for the type specimen, which features a grey skull (Chapelle et al. 2019). Per the authors, the name is supposed to be pronounced "Ng-g'where-voo in-tloh-koh".
Citation: Chapelle, K. E. J., P. M. Barrett, J. Botha, and J. N. Choiniere. 2019. Ngwevu intloko: a new early sauropodomorph dinosaur from the Lower Jurassic Elliot Formation of South Africa and comments on cranial ontogeny in Massospondylus carinatus. PeerJ 7:e7240. doi:10.7717/peerj.7240.
Stratigraphy and geography: Fossils of N. intloko are definitely known only from the Tevrede (1077) Farm in the Fouriesberg District of Free State Province, South Africa. Stratigraphically, the type specimen is from the uppermost Upper Elliot Formation, deposited sometime in the Hettangian–Sinemurian time frame of the Early Jurassic. The site has also yielded fossils of another, undetermined sauropodomorph (Chapelle et al. 2019).
Holotype: The type specimen of N. intloko is BP/1/4779, a skull and most of an articulated skeleton (the major piece missing is the tail), reposited at the Evolutionary Studies Institute of the University of the Witwatersrand, in Johannesburg, South Africa. Preservation of much of the skeleton is not as good as the skull. A second specimen, SAM-PK-K1314, is reportedly similar but more crushed, so it was not referred to N. intloko at this time (Chapelle et al. 2019).

The skull of BP/1/4779 is definitely the star of the N. intloko show. It is, as the name suggests, a sort of medium grey in color, with a fairly blunt wedge shape in dorsal view. The portion of the skull in front of the eyes is short and high, and there is a slight nasal arch. (The overall effect is as if the animal slept with its nose pressed in a corner). There is a definite tilt to the teeth and quadrate, indicating some post-mortem deformation, and in general cranial features look slightly swept forward and up. The bones are described as somewhat more robust than those of contemporaneous Massospondylus carinatus.

Figure 1 in Chapelle et al. (2019), showing the type specimen of Ngwevu intloko (scale is given as 10 mm, but that is incorrect given the other figures; 50 mm seems more likely). Even if you don't buy it being a different species from Massospondylus carinatus, it's still an interesting skull. CC-BY-4.0.

N. intloko is based on one of those specimens that had been laying low under an established name (M. carinatus in this case; see Gow et al. 1990 and Sues et al. 2004), and therein lies the rub: is it really something different taxonomically, or it is actually just Massospondylus (distorted, juvenile, sexual dimorph, etc.)? Obviously Chapelle et al. have opted for the first position, but it's important to understand their reasoning, so let's have a look.

Last year about this time I covered a similar "prosauropod", Yizhousaurus sunae. If you hop over there, you'll notice that the skull of Y. sunae has a similarly wedge-shaped skull in dorsal view. In fact, Adam Yates left a comment to that post about a similar Massospondylus skull nicknamed the "goblin". Feel free to correct me, Adam, but would that happen to be BP/1/4779? [note, 2019/08/11: nope, see first comment below.] Adam contended that the unusual features of Y. sunae and the "goblin" were more likely taphonomic than taxonomic. Chapelle et al. described the skull of BP/1/4779 as not crushed dorso-ventrally and showing only slight anterodorsal distortion. I agree that the skull does not appear to have been crushed in a brittle way. It does appear to be skewed anterodorsally. Chapelle et al. cited the quadrate's displacement from vertical by 12.5 degrees. Presumably this also has somewhat exaggerated the bluntness of the snout. In the absence of complete nasal bones, I can certainly see how compressional forces moving primarily posterior to anterior could have smooshed the snout more than what is readily apparent, but the shortness of the facial bones also suggests that there isn't enough bone to make a more elongated adult Massospondylus profile.

Figure 3 from Chapelle et al. (2019) shows the various individual bones (scale is 10 mm). CC-BY-4.0.

Could it then be a partially grown Massospondylus, with a corresponding short face? With a skull length of 133.87 mm (5.27 in), BP/1/4779 slots in neatly between skulls identified as adult Massospondylus (BP/1/5241, 187.04 mm or 7.36 in) and juvenile (BP/1/4376, 96.12 mm or 3.78 in), so that checks out. However, histological analysis indicated that BP/1/4779 was at least 10 years old when it died, and more or less adult (Chapelle et al. 2019). How about a sexual dimorph? Little evidence exists to support or refute the possibility, although it would be odd that with so many specimens of Massospondylus, they would all be one sex except for this one. Chapelle et al. (2019) noted that most of the differences they found between B/1/4779 and Massospondylus could be related to feeding, which would also be odd for a large dimorphic herbivorous vertebrate, but would be reasonable for different taxa.

Another possibility, which unfortunately is almost impossible to assess, is that BP/1/4779 is a Massospondylus with a genetic mutation. I bring this up because I've had several pet rats over the years. A common variant is the "dumbo", in which a pharyngeal arch mutation leads to a rat with ears set low on the sides of the head, a relatively large and broad skull, and a relatively small lower jaw, among other difference. They look very distinct, and the mutation doesn't appear to affect the quality of the rat's life. Doubtless Massospondylus carinatus (and every other species for that matter) had non-lethal mutations that produced visually distinct skeletal anatomy. The problem is, how could we hope to identify such mutations in the fossil record? (And, of course, we could also get into individual variation, geographic variation, variation over time, etc., but let's not get too far into it or we'll end up with every mid-sized "prosauropod" in Plateosaurus.)

Chapelle et al (2019) ran a phylogenetic analysis and found N. intloko to be most closely related to the Chinese genus Lufengosaurus within the Massospondylidae and distinct from Massospondylus carinatus. Interestingly, second Massospondylus species M. kaalae was also separate from M. carinatus in this analysis.

I think it's fair to say that BP/1/4779 is not your average M. carinatus. Even if some of the features are exaggerated by deformation, the significantly smaller size at an old age indicates it was up to something different, whether as a different species, an old but small and rather gnarled M. carinatus, or something else. Chapelle et al. (2019) suggested it was more of a generalist than M. carinatus, based on the blunter jaw profile, and that the shorter, more robust skull could generate a stronger bite.

References

Chapelle, K. E. J., P. M. Barrett, J. Botha, and J. N. Choiniere. 2019. Ngwevu intloko: a new early sauropodomorph dinosaur from the Lower Jurassic Elliot Formation of South Africa and comments on cranial ontogeny in Massospondylus carinatus. PeerJ 7:e7240. doi:10.7717/peerj.7240.

Gow, C. E., J. W. Kitching, and M. A. Raath. 1990. Skulls of the prosauropod dinosaur Massospondylus carinatus Owen in the collections of the Bernard Price Institute for Palaeontological Research. Palaeontologia Africana 27:45–58.

Sues, H-D, R. R. Reisz, S. Hinic, and M. A. Raath. 2004. On the skull of Massospondylus carinatus Owen, 1854 (Dinosauria: Sauropodomorpha) from the Elliot and Clarens formations (Lower Jurassic) of South Africa. Annals of Carnegie Museum 73:239–257.