CLYPEODONTA

Norman, David B., 2015, On the history, osteology, and systematic position of the Wealden (Hastings group) dinosaur Hypselospinus fittoni (Iguanodontia: Styracosterna), Zoological Journal of the Linnean Society 173 (1), pp. 92-189 : 170-172

INFRAORDER CLYPEODONTA ‘SHIELD- TOOTHED’ NEORNITHISCHIANS ( NORMAN, 2014; FIGS 50 , 52 ).

Phylogenetic definition (node-based)

Hypsilophodon foxii , Edmontosaurus regalis , their most recent common ancestor, and all of its descendants.

A consideration of the known range of more basal neornithischian taxa is beyond the scope of this analysis so a node-based definition acts as a general phylogenetic ‘place-holder’. Until the proximate sister taxa to clypeodontan ornithopods have been identified reliably, a stem-based definition cannot be proposed.

Characters (with their numbers in parentheses so that they can be cross-referenced to Appendix 1) that

are supported under ACCTRAN and DELTRAN optimizations are unremarked. Where only one optimization identifies a character this is recognized in parentheses.

Character-based (Linnaean) diagnosis

1. Antorbital fenestra small, subcircular with large fossa (10)

2. Broad quadrate embayment shape (29) (ACCTRAN)

3. Frontals broad and roof orbits (34) (ACCTRAN)

4. Wear facets continuous across adjacent crowns (55)

5. Dentary enamel asymmetrically distributed (57)

6. Marginal denticles tongue-shaped (58) (ACCTRAN)

7. Tooth roots longitudinally grooved (59) (ACCTRAN)

8. Dentary crowns broad and shield-shaped (60) (ACCTRAN)

9. Dentary crown develops thickened, inrolled oblique shelves (62)

10. Dentary primary ridge prominent (63) (ACCTRAN)

11. Dentary crown prominent ridge with subsidiary ridges on either side (64) (ACCTRAN)

12. Dentary crowns broader in lingual view than opposing maxillary crowns (65)

13. Alveolar trough grooves reflect the shape of successional crowns (66)

14. Maxillary crowns bear multiple labial ridges (68) (ACCTRAN)

15. Manus digit III with three phalanges (87) (ACCTRAN)

16. Postacetabular process tapers posteriorly (91) (ACCTRAN)

17. Preacetabular pubic process rod-shaped (93) (ACCTRAN)

18. Ischial shaft expanded laterally at distal end (97) (ACCTRAN)

19. Obturator process positioned midshaft (98) (ACCTRAN)

20. Femoral extensor groove broadly open (102) (ACCTRAN)

Commentary

This deceptively substantial list reflects the fact that this derived subgroup of ornithopods is being compared with the basal ornithischian condition represented by Lesothosaurus . The most important features within this listing highlight the form of the dentition: shield-shaped crowns with unevenly distributed enamel; crown margins fringed by tongue-shaped denticles; the development of discrete enamel ridge patterns on the lingual side of dentary crowns and the labial sides of maxillary crowns; and the differentiation in the form of the teeth seen in the maxillary and dentary dentitions. All of these characters combine to distinguish the clypeodont condition from that seen in more basal neornithischians.

DIVISION HYPSILOPHODONTIA ( COOPER, 1985;

FIGS 50 , 52 )

Included taxa in this analysis are: Hypsilophodon foxii , Zalmoxes robustus , and Tenontosaurus tilletti . However, this clade contains additional closely similar taxa: Zalmoxes shqiperorum , Mochlodon sp. , Rhabdodon sp., Muttaburrasaurus langdoni , Kangnasaurus coetzeei , and the ‘Antarctic ornithopod’ (A. C. Milner & P. M. Barrett, unpubl. data).

Phylogenetic definition (node-based)

Hypsilophodon foxii , Tenontosaurus tilletti , their most recent common ancestor, and all of its descendants.

A node-based definition of Hypsilophodontia has been employed here until more detailed consideration has been made of a wider range of proximate taxa.

Character-based (Linnaean) diagnosis

1. Occiput with a trapezoidal outline (1)

2. Premaxilla overlaps the nasal posterodorsally in the midline (9)

3. Lacrimal overlaps the posteroventral margin of the prefrontal (14)

4. Lateral surface of the rostral process of the maxilla modified by a large foramen and/or a boss (16)

5. Jugal forms an anteroposteriorly abbreviated plate that forms a markedly dorsoventrally expanded plate beneath the infratemporal fenestra (18)

6. Jugal−quadrate suture with a trough on the medioventral edge of the jugal (23)

7. Fenestration of the quadratojugal (25) – secondarily lost in Zalmoxes ( Weishampel et al., 2003)

8. Lateral surface of the quadrate shaft bears a sinuous ridge (27)

9. Quadrate (paraquadratic) foramen absent (28)

10. Quadrate (jugal wing) embayment broadly open (29)

11. Postorbital, squamosal process with a vertical indentation (37 − ACCTRAN) not present in Hypsilophodon ( Galton, 1974)

12. Dentary tooth primary ridge very prominent (63 − DELTRAN)

13. Dentary crown dominant primary ridge flanked by variable number of subsidiary ridges (64 − DELTRAN)

14. Maxillary crown covered by an array of subsidiary ridges (68 − DELTRAN)

15. Caudal ossified tendons form a sheath (epaxially and hypaxially) around the distal caudal series (74) uncertain in Zalmoxes spp.

16. Rod-like preacetabular process of the pubis (93 – DELTRAN) laterally compressed in Tenontosaurus , convergent with iguanodontians

Commentary

The hypsilophodontian clade, as defined here, marks a fundamental morphological (and implicitly phylogenetic) division within the Clypeodonta ( Figs 49– 51 ). The most characteristic features of representatives of this clade are to be found in the dentary and maxillary tooth crowns; this is potentially valuable because teeth have a comparatively high preservational potential. The clade, if it proves to be robust when subjected to future systematic analysis, is of considerable evolutionary interest because hypsilophodontians (notably the large-bodied tenontosaurs and Muttaburrasaurus ) exhibit convergent (homoplastic) postcranial morphologies when compared to those seen amongst large-bodied members of the sister-clade Iguanodontia.

Hypsilophodontians form a clade that specifically exclude Th. neglectus and a wide range of more basal neornithischian taxa, e.g. Agilisaurus, Yandusaurus , Jeholosaurus , Hexinlusaurus , Othnielia , Gasparinisaura , Orodromeus , Parksosaurus , Thescelosaurus spp. , Bugenasaura , and others ( Butler et al., 2008). This fundamental change in clade composition necessitates the abandonment of previous phylogenetic definitions of the Iguanodontia (sensu Sereno, 2005) and prompts a repositioning and redefinition of that clade name (as follows).

Figures

Figure 49. The tree used for Sereno’s (1998, 2005) stabilized ornithopod clade nomenclature. Is this a stable tree? Specific points of contention include the status of: Hypsilophodontidae (status disputed, widely regarded as paraphyletic – Butler et al., 2008), Iguanodontia (constituent taxa and topographical position of Tenontosaurus disputed – here), Camptosauridae (status and constituent taxa uncertain – McDonald, 2011), Probactrosaurus (topographical position disputed – Norman, 2002, 2004; Wang et al., 2010; Wu & Godefroit, 2012), Iguanodontidae (family status disputed and inconsistent constituent taxa – here, Norman, 2004; Wu & Godefroit, 2012), Hadrosauriformes (constituent taxa disputed – here, Norman, 2004; McDonald, Barrett & Chapman, 2010a), Hadrosauroidea (topographical position of Ouranosaurus disputed – Norman, 2004; McDonald, Barrett & Chapman, 2010a; Wang et al., 2010; Wu & Godefroit, 2012).

Figure 50. A tree based upon the analysis that generated Figure 48, with particular nodes and stems named (see text for discussion/explanation). Abbreviations: CI, consistency index; RC, rescaled consistency index; RI, retention index.

Figure 51. A tree ‘adjusted’ (using the MacClade 4 tree window editor) by the addition of three operational taxonomic units (Batyrosaurus, Hypselospinus, and Ouranosaurus) that lie immediately basal to the ‘iguanodontoid’ subclade into the stem of that subclade (compare with Fig. 49). Tree length: 316 (consistency index: 0.57; retention index: 0.78).

Figure 52. A phylogeny derived from Figure 49 calibrated, by reference to known stratigraphical ranges of individual taxa plotted, against the geological timescale. Approximate age ranges of known taxa are indicated by black rectangles, and where there are implied stratigraphical ranges these are indicated by connecting tramlines. In instances where implied divergence events have occurred and generated substantial ghost-ranges, these are indicated by cross-hatching.