Читать книгу Hadrosaurs - David A. Eberth - Страница 58

1. Dentary dentition with small, diamond-shaped crowns integrated into a closely packed dental magazine.

2. Dentary and maxillary crowns bear a single median primary ridge (carina) on their labial or lingual surfaces, respectively.

3. Surangular foramen absent.

4. A shallow embayment in the jugal wing of the quadrate to accommodate a disc-shaped quadratojugal that completely occludes the quadrate (paraquadratic) foramen.

5. Modification of the acromion process to form a pendant promontory orientated along the long-axis of the scapular blade.

6. Carpus represented by no more than two small discoidal elements.

7. Phalanges of digit I of the manus absent.

8. Metacarpal I absent.

9. Ilium has a region of the dorsal margin, posterodorsal to the ischiadic peduncle, that forms a discrete everted lip (pendule, see below) that overhangs the ischiadic peduncle of the ilium.

10. Postacetabular ramus of the ilium forms a laterally flattened bar-like structure.

11. Pedal ungual phalanges are strongly dorsoventrally compressed, short proximodistally, and very broad with a rounded anterior margin; distinct claw grooves on the lateral margins are absent, and are therefore truly hoof-shaped when compared to those seen in more basal taxa.

Note here that the term “pendule” is suggested for the everted and downturned tongue-shaped structure that projects lateroventrally from the dorsal margin of the ilium. This word is proposed for two reasons. First, it obviates the need for repeated complex descriptive phraseology to describe the feature. Second, this structure has been mistakenly referred to as an “antitrochanter” (e.g., Lull and Wright, 1942). The true antitrochanter is an articular facet located on the posterior margin of the acetabulum and is not topographically similar or anatomically homologous to the development of the dorsal margin of the ilium. Similarly, the term “supra-acetabular crest” has been proposed more recently for this structure (Prieto-Márquez, 2011); unfortunately this latter usage creates a homonym with the anatomically distinct supra-acetabular crest (a specialized feature derived from the supra-acetabular buttress of tetrapods [as per Romer, 1956]) that forms a prominent ridge located on the acetabular margin of the ilium in derived archosaurs (e.g., Charig, 1972).

Comments The sister taxon to this clade, Probactrosaurus gobiensis, displays important anatomical differences that distinguish this and all non-hadrosauromorphan taxa from hadrosauromorphans: dentary crowns are comparatively small, and almost diamond-shaped, but they retain an asymmetrical aspect when viewed lingually, and bear accessory ridges running parallel to the distally offset primary ridge. In addition, a surangular foramen is present, and the quadrate has a semicircular embayment in the jugal wing rather than the shallow embayment seen in Tethyshadros (Dalla Vecchia, 2009) and more derived hadrosauromorphans. The acromion process of the scapula curves forward (rather than forming an overhanging promontory-like structure that lies parallel to the main axis of the scapular blade as in hadrosauromorphans).

In the forelimb, the radius, ulna, and metacarpals are elongate and slender in Probactrosaurus and hadrosauromorphans, which is suggestive of a general trend toward gracility in this part of the skeleton. However, Probactrosaurus retains a small, spike-like pollex, which implies the presence of an at least partially competent and ossified carpus (Norman, 2002). The ilium of Probactrosaurus has a modestly everted dorsal margin (Norman, 2002), but there is no evidence of either strong eversion or development of a tongue-shaped pendule, as described in Tethyshadros. The unguals of the pes of Probactrosaurus are comparatively elongate and truncated at their tips (Norman, 2002).

Bactrosaurus and Telmatosaurus retain a consistent topology in many different analyses (Prieto-Márquez, 2010, 2011; Wang et al., 2010; McDonald, 2012b; Wu and Godefroit, 2012), as, respectively, successive outgroup taxa to the well-established node-based clade (Euhadrosauria) represented in this analysis by the sister taxa Parasaurolophus and Saurolophus.