Claosaurus agilis Marsh, 1872

Prieto-Márquez, Albert, 2011, Revised diagnoses of Hadrosaurus foulkii Leidy, 1858 (the type genus and species of Hadrosauridae Cope, 1869) and Claosaurus agilis Marsh, 1872 (Dinosauria: Ornithopoda) from the Late Cretaceous of North America, Zootaxa 2765 (1), pp. 61-68 : 62

Claosaurus agilis Marsh, 1872

Fig. 2

Holotype. YPM 1190 , a partial skeleton consisting of a fragment of coronoid process from the left dentary, maxillary fragments with several teeth exposed, partial left postorbital, cervical through caudal vertebral series missing the neural spines, right scapula, partial left and right sternals, left and right humeri, proximal and distal segments of both ulnae and radii, nearly complete right ilium, distal-most end of the right ischium, nearly complete left and right femora, partial tibiae missing their shafts, and partial left and right pedes.

Type locality and horizon. The holotype and only known exemplar of Claosaurus agilis was collected from the Smoky Hill Chalk Member of the Niobrara Chalk Formation (near Smoky Hill River, Logan County, Kansas, USA) ( Carpenter et al. 1995); these beds are Late Coniacian in age according to Everhart and Ewell (2006).

Amended diagnosis. Hadrosauroid dinosaur characterized by the unique combination of the following humeral and iliac characters: deltopectoral crest length less than 48% of humerus length and with wide arcuate laterodistal corner; anteroposterior width of supraacetabular crest being approximately 75 per cent of length of central plate of ilium; and ventral apex of supraacetabular crest positioned anterior to the posteroventral corner of ischial peduncle of ilium..

Remarks. Carpenter et al. (1995) tentatively diagnosed Claosaurus agilis on the basis of the relatively short length of the preacetabular process in YPM 1190. As preserved in this specimen, the process is certainly proximodistally shorter (in relation to the anteroposterior length of the iliac central plate) than in any other known iguanodontian ornithopod (Prieto-Márquez unpublished data). However, the distal-most tip of the preacetabular process in YPM 1190 is reconstructed, as evidenced by the presence of plaster ( Fig. 2b ). This indicates that the preacetabular process is most probably incomplete and, thus, C. agilis cannot be diagnosed based on the possession of a relatively short process.

As shown in Table 1, C. agilis is the only non-hadrosaurid hadrosauroid species that combines such a relatively wide supraacetabular crest with its ventral apex being located anterior to the posteroventral corner of the ischial peduncle of the ilium. All other non-hadrosaurid hadrosauroid taxa that have a supraacetabular crest that is 70 per cent or more of the length of the iliac central plate show a ventral apex of this crest positioned posterior to the ischial peduncle ( Fig. 3 ). The validity of the diagnosis proposed here depends on interpretation of the actual position of the ventral apex of the supraacetabular crest in YPM 1190. In this exemplar, the lateroventral margin of the crest is not completely preserved. However, the preserved lateroventral border indicates that the apex must have been located at least slightly anterior to the level of the ischial peduncle. Other possible interpretations of the position of the apex would place it even more anteriorly. The most conservative estimate is considered here ( Fig. 2a ). An anteriorly-positioned apex of the supraacetabular crest is convergent with Saurolophidae ( Prieto-Márquez 2010) ( Fig. 3 ).

The proposed iliac characters for diagnosing Claosaurus agilis (and Hadrosaurus foulkii ; see below) do not display intraspecific variation such as that due to ontogeny. In those hadrosauroid taxa in which different ontogenetic stages are known preserving appendicular elements (e.g., Bactrosaurus johnsoni , Brachylophosaurus canadensis , Maiasaura peeblesorum , Saurolophus angustirostris , Edmontosaurus spp. , Corythosaurus spp. , Lambeosaurus spp. , or Hypacrosaurus stebingeri ) the anteroposterior breadth of the supraacetabular crest and the position of its ventral apex relative to the posterior tuberosity of the ischial peduncle are maintained throughout ontogeny (Prieto-Márquez in press and unpublished data). More specifically, this is exemplified when comparing the B. canadensis juvenile specimen MOR 1071-7-15 -98-215 with the adults of the same species MOR 794 or MOR MOR 8-2-98-469; the juvenile ZPAL MgD-I 159 with the adult MPC-D 100/706, both referable to S. angustirostris ; the adult H. stebingeri MOR 549 with the neonate ilia from MOR 548; or the juvenile Lambeosaurus sp. specimen AMNH 5340 with the adult L. lambei ROM 1218 . Among basal hadrosauroids, the fact that those characters are maintained throughout ontogeny is documented in B. johnsoni ; for example, in the juvenile AMNH 6577 and the adult or larger subadult IRSNB 95E5/25.

Figures

FIGURE 2. (a) Right ilium of the holotype specimen of Claosaurus agilis (YPM 1190) in lateral view. The horizontal and double-arrowed white lines indicate the breadth of the supraacetabular crest being approximately 75 per cent of the length of the central plate of the ilium. The vertical white arrow indicates the position of the ventral apex of the supraacetabular crest relative to the posteroventral corner (black arrow) of the posterior tuberosity of the ischial peduncle. (b) Line drawing of the right ilium of YPM 1190 showing in gray the areas with plaster reconstruction.

FIGURE 3. Distribution, in a phylogenetic context, of the relative length and profile of the laterodistal corner of the deltopectoral crest of the humerus, and the anteroposterior width of the supraacetabular crest and position of its apex relative to the ischial peduncle of the ilium in Hadrosaurus foulkii and selected major groups of iguanodontian dinosaurs. The cladogram has been simplified from the strict reduced consensus tree presented by Prieto-Márquez (2010). From left to right, specimens used as clade or taxon representatives are as follows. Non-hadrosauroid Iguanodontia: skull of Iguanodon bernissartensis (IRSNB 1731), and right humerus (NHMUK R6616; reversed) and left ilium (NHMUK 11521) of Mantellisaurus atherfieldensis. Nonhadrosaurid Hadrosauroidea: skull of Equijubus normani (IVPP V12534), and right humerus (AMNH 30728; reversed) and right ilium (AMNH 30735; reversed) of Gilmoreosaurus mongoliensis. Hadrosaurus foulkii: line drawing of the historical skull model sculpted by Waterhouse Hawkins, and left humerus and left ilium of ANSP 10005. Saurolophinae: skull of Edmontosaurus annectens (SM R4036) and left humerus of Gryposaurus latidens (AMNH 5465). Lambeosaurinae: skull of Parasaurolophus walkeri (ROM 768) and left humerus of P. cyrtocristatus (FMNH P27393). The right ilium (reversed) representing both saurolophines and lambeosaurines (i.e., Saurolophidae) is that of Brachylophosaurus canadensis (MOR 794). Skulls and ilia are shown in left lateral view; humeri are displayed in anteromedial view.