The skull of Yunnanosaurus huangi Young, 1942 (Dinosauria: Prosauropoda) from the Lower Lufeng Formation (Lower Jurassic) of Yunnan, China

Abstract

The Lower Lufeng Formation (Lower Jurassic: ?Hettangian-?Sinemurian) of Yunnan Province, China, has yielded an important and diverse fauna of terrestrial vertebrates that is dominated by early sauropodomorph dinosaurs (prosauropods and basal sauropods). Nevertheless, few of these animals have been studied in detail, undermining their potential significance in understanding sauropodomorph phylogeny, palaeobiology, and palaeoecology. Here, we present a detailed re-description of the cranial osteology of Yunnanosaurus huangi Young, 1942 and propose an emended diagnosis for this taxon on the basis of numerous autapomorphic characters (including an expanded internarial bar, unusual midline cranial bosses, and the possession of elongate maxillary tooth crowns lacking marginal serrations). Incorporation of these novel anatomical data into existing phylogenetic analyses of sauropodomorph interrelationships substantially affects the resolution, length, and topologies of the trees recovered. Although the phylogenetic position of Yunnanosaurus remains labile, these new analyses undermine previous suggestions that the former was the sister taxon of the southern African prosauropod Massospondylus . Several features of the skull of Yunnanosaurus (small external nares, cranial bosses, tooth crown morphology, and the lack of maxillary foramina) indicate that the palaeobiology of Yunnanosaurus may have been rather different from that of other prosauropods and basal sauropods, although more detailed functional studies and better material are needed to confirm this suggestion.

Cranium and Vertebral Column of Xingxiulong chengi (Dinosauria: Sauropodomorpha) from the Early Jurassic of China

“…The ventral process is caudally bowed. It tapers ventrally and contacts the rostral surface of the dorsal process of the jugal for a moderate length, similar to the condition in Lufengosaurus ; in contrast, the postorbital of Yunnanosaurus has a simple tongue‐and‐groove contact with the jugal (Barrett et al, ). The rostrodorsal process is slender; it inserts into the two rami of the caudal process of the frontal.…”

“…3). This flange was described as "medial lamina" by Barrett et al (2005) and is also present in Lufengosaurus (Barrett et al, 2005), Coloradisaurus (Apaldetti et al, 2014), Massospondylus (BP/1/5241; Chapelle and Choiniere, 2018), Adeopapposaurus (PVSJ610; Martínez, 2009), andPlateosaurus (Prieto-Márquez andNorell, 2011), but absent in Yunnanosaurus (Barrett et al, 2007) and Jingshanosaurus (Zhang and Yang, 1995); however, the lamina in Xingxiulong seems to be longitudinally shorter than those in other nonsauropodan sauropodomorphs. The shaft ventral to the lamina is constricted, whereupon it expands again toward its ventral termination.…”

Root causes of phylogenetic incongruence observed within basal sauropodomorph interrelationships

“…() and that of Pol et al . (), years during which new discoveries allowed more precise scoring for some of the taxa (Barrett et al ., ; Fedak & Galton, ; Galton, ; Langer, França & Stefan, ; Lü et al ., ; Pol & Powell, ; Yates, ; Barrett, ; Bandyopadhyay et al ., ); however, the analyses of Yates et al . () and Pol et al .…”

“…Sometimes, matrices are directly picked from anterior works (Apaldetti et al ., ; Godefroit et al ., ) without reviewing the characters and specimens concerned. Some studies even use different and competing analyses to assess the phylogenetic relationships of a taxon (Barrett et al ., ; Upchurch et al ., ; Sertich & Loewen, ; Yates et al ., ; Apaldetti et al ., ). This only accentuates the problem and increases the volume of data to be compared in order to try to solve the problem.…”

“…A), who used a new matrix developed in the framework of the description of Leonerasaurus taquetrensis Pol et al ., ; however, the arrangement of taxa within sauropodomorphs was different in both analyses. In these circumstances, the position of basal sauropodomorphs is not clear yet, and even led some authors to use these concurrent analyses to try to resolve the phylogenetic relationships of some taxa (Barrett et al ., ; Upchurch et al ., ; Sertich & Loewen, ; Yates, Bonnan & Neveling, ; Apaldetti et al ., ). Because these three cladistic analyses (Upchurch et al ., ; Yates et al ., ; Pol et al ., ) are among the most recent and complete, and did not reach consensual results, we decided to compare them using comparative cladistics (Sereno, ).…”