Sunday, April 10, 2011

Manidens condorensis



Introducing Manidens condorensis, a new Heterodontosaur from the Middle Jurassic of Patagonia...

Fig 1.- My reconstruction of Manidens condorensis.

Some critters have a mysterious appeal, which is difficult to explain. I always felt some fondness for a meter long creature named Heterodontosaurus from the Early Jurassic of South Africa. If you ignore my now lost childhood productions, it is among the first dinosaurs I’ve drawn. So I was naturally very thrilled to learn that yet another relative of Heterodontosaurus, the Patagonian Manidens condorensis has just been unearthed from the Cañadón Asfalto Formation of Middle Jurassic age. Diego Pol, Olivier W.M. Rauhut and Marcos Becerra are describing the new species in a paper about to be published in Naturwissenshaften.

Fig 2.- Old (2007) pencil sketch of Heterodontosaurus tucki in quadrupedal pose, one of the first dinosaur I’ve drawn during my adult life. Yes, I know, it was not very good, just for reference...

Heterodontosaurus and Manidens belong to a small group of ornithischian (bird-hipped) dinosaurs called heterodontosauridae. In the world dominated by giants, these critters were comparatively very tiny. The largest representative, Lycorhinus angustidens, also from the Early Jurassic of South Africa and mainly known from dentaries, had an estimated length of only 2 meters, while Fruitadens haagarorum from the Upper Jurassic of Colorado, with an estimated length of 65-75 cm has been dubbed the smallest known ornithischian at the time of its discovery.

Heterodontosaurs are also characterized by their highly differentiated teeth, thus the meaning of their name (“different-toothed lizards”). They have a pair of well-developed tusk-like canines on the lower jaw that are accommodated by a gap (diastema) in the upper jaw when the mouth is closed. They also have highly derived cheek teeth that are, in the most derived forms, columnar in shape with almost no space in between, reminiscent of the mammalian molars. The toothless tip of the upper jaw may also have supported a beak. All in their teeth cry for a preferential vegetarian diet. What did they look like? Before Tianyulong (see below), representations of heterodontosaurs were based on the only complete skeleton known for the group, i.e the Heterodontosaurus tucki specimen SAFM K1332 housed in the South African Museum (Santa Luca, 1980). They were lightly built and had a very long tail. The arms were quite long for a dinosaur with hands having all five digits. The leg proportions tibia/femur show adaptation for speed, indicating they were probably agile bipedal runners.

Fig 3.- Cast of the Heterodontosaurus tucki specimen SAFM K1332. By Flickr user kyknood uploaded on Wikipedia by user Funkmonk under a Creative Common License.

Heterodontosaurs are a rare find and little was known of their affinities until quite recently. This is due not only to the scarcity of their remains but also to the large gaps in the fossil record of early ornithischians (there were simply not many other critters to compare to). Before Y2K, only three species were known with high degree of confidence, Heterodontosaurus tucki, Abrictosaurus consors and Lycorhinus angustidens, all from the Early Jurassic of Southern Africa, with in addition some dubious taxa, Lanasaurus, Geranosaurus both from South Africa and the enigmatic Echinodon becklesii from the Early Cretaceous of England known from a jaw fragment with teeth, which might or might not be an heterodontosaur (Norman & Barrett, 2002). Recent studies all place Echinodon outside the heterodontosaur clade.

Fig 4.- Illustration of Heterodontosaurus tucki, adult and juveniles indicating a probable omnivorous diet, made for the press release following the publication of Butler et al., 2008b.

However things changed quickly and quite dramatically in the last few years. All the previous indubitable heterodontosaurs were from the Early Jurassic of South Africa. Discoveries from the new millennium have considerably extended the temporal and geographical range for the group, covering the Late Triassic of South America (Báez & Marsicano, 2001), the Early Cretaceous of China (Zheng et al., 2009) and the Upper Jurassic of North America (Butler et al., 2010).

As for their systematic affinities, scientists in the past have variously placed them as an early but highly specialized sister taxon to the marginocephalians (ceratopsians and pachycephalosaurs) or as the most basal members of ornithopods. Recent phylogenetic analyses indicate that they actually represent an early offshoot of basal ornithischians (Butler et al., 2008a) and this seems to be comfortably confirmed by subsequent studies (Butler et al., 2010, Zheng et al., 2009, Pol et al., 2011). The presence of the canine-like teeth in a juvenile specimen of Heterodontosaurus also questioned the previous assertions that these tusks were a mark of sexual dimorphism and primarily used for sexual display (Butler et al., 2008a). Not so sure, what to do with the tusk less specimen of Abrictosaurus, now... but it is also true that there is a need to revise the South African materials (What a perfect paleontology PhD subject, any taker?) Finally, heterodontosaurs experienced a drastic change of look after the discovery of a remarkably well-preserved specimen from the Liaoning province of China (Zheng et al., 2009). Indeed, the fossil of Tianyulong confuciusi has clear impressions of filamentous integuments covering the body like a fur. They may be analogous to the proto-feathers found in theropods and the quills found on the basal ceratopsian Psittacosaurus. Well, no more scaly and skinny heterodontosaur, time for fuzzy Mesozoic furry balls…

Fig 5.- Complete view of Manidens condorensis.

The holotype of Manidens condorensis is a partial associated skeleton including most of the skull, the anterior part of the vertebral column, pelvic girdles, left scapula and coracoids. Referred materials include a set of teeth. The phylogenetic analysis conducted by Pol et al., indicates that it is more derived than Tianyulong and Fruitadens but slightly less than the South African genera. The teeth of Manidens are also quite interesting. Whereas the more derived members of the heterodontosaurs such as Heterodontosaurus show clear adaptation for an herbivorous diet (closely packed teeth, high tooth crowns, and extensive wear facets on the maxillary and dentary teeth), the basal members lack those, pointing to a more omnivorous diet. But Manidens is in an intermediate stage with high tooth crowns but no wear facets. Basal members of heterodontosaurs were probably ecological generalists that fed on miscellaneous diet including plants but also small animals such as invertebrates.
Manidens was extremely small for a dinosaur, Pol et al., give it a size comparable to Fruitadens, but I believe it might have been a bit smaller if I trust the scales indicated in the figures of their respective articles. The examination of the holotype skeleton of Manidens indicates that it is from an old adult, despite its small size and comparatively large eye socket, while the material from Fruitadens are based on juveniles.

Fig 6.- Comparison of the skulls (only known parts are represented) of different heterodontosaurs, drawn to scale. a) Fruitadens haagarorum [modified from Butler et al., 2010], b) Tianyulong confuciusi [modified from Zheng et al., 2010], c) Abrictosaurus consors [modified from...(forgot the ref)] note the lack of tusks, this was interpreted as belonging to a female, d) Manidens condorensis [modified from Pol et al., 2010], e) Heterodontosaurus tucki [modified from Butler et al, 2008b]

There are still a lot of mysteries surrounding these critters due to the large gap in their fossil record. For instance, the most derived members from South Africa all date from the Early Jurassic, while the most primitive and less specialized member, Tianyulong, is from the Early Cretaceous. Did the more derived strictly herbivorous members went extinct out-competed by new groups of hungry ornithischians, while the more primitive unspecialized and opportunistic members survived because they could do a meal from almost anything? Who knows? It was also originally thought that the group originated in the Gondwana where the most primitive and earliest ornithischian, Pisanosaurus mercki, has been found but with the discovery of Tianyulong from China and Fruitadens from North America, all the cards are reshuffled. Looking forward to the next big heterodontosaurian discovery…

Fig 7.- Phylogeny of heterodontosaurs [adapted from Pol et al., 2011].

References:
Báez A.M., Marsicano C.A. 2001. “A heterodontosaurid ornithischian dinosaur from the Upper Triassic of Patagonia”. Ameghiniana 38:271–279.

Butler, R. J., Upchurch, P. & Norman, D. B. 2008a “The phylogeny of the ornithischian dinosaurs”. J. Syst. Palaeontol. 6, 1–40.

Butler R.J., Porro L.B., Norman D.B. 2008b. “A juvenile skull of the heterodontosaurid dinosaur Heterodontosaurus tucki from the ‘Stormberg’ of southern Africa”. J Vert Paleont 28:702–711.

Butler R.J., Galton P.M., Porro L.B., Chiappe L.M., Henderson D.M., Erickson G.M. 2010. “Lower limits of ornithischian dinosaur body size inferred from a new Upper Jurassic heterodontosaurid from North America”. Proc R Soc B 277:375–381.

Norman, D. B. & Barrett, P. M. 2002. “Ornithischian dinosaurs from the Lower Cretaceous (Berriasian) of England”. Spec. Papers Palaeontol. 68, 161–189.

Pol, D.; Rauhut, O.W.M.; and Becerra, M. 2011. "A Middle Jurassic heterodontosaurid dinosaur from Patagonia and the evolution of heterodontosaurids". Naturwissenschaften in press.

Santa Luca, A. P. 1980. “The postcranial skeleton of Heterodontosaurus tucki (Reptilia, Ornithischia) from the Stormberg of South Africa”. Ann. South Afr. Mus. 79, 159–211.

Thulborn, R. A. 1970. “The systematic position of the Triassic ornithischian dinosaur Lycorhinus angustidens”. Zoo. J. Linn. Soc. 49, 235–245.

Zheng, X.-T., You, H.-L., Xu, X. & Dong, Z.-M. 2009. “An Early Cretaceous heterodontosaurid dinosaur with integumentary structures”. Nature 458, 333–336.

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