Tatisaurus oehleri, Simmons, 1965

Norman, David B., Butler, Richard J. & Maidment, Susannah C. R., 2007, Reconsidering the status and affinities of the ornithischian dinosaur Tatisaurus oehleri Simmons, 1965, Zoological Journal of the Linnean Society 150 (4), pp. 865-874 : 866-867

TATISAURUS OEHLERI

Tatisaurus oehleri Simmons, 1965 , was established on the basis of a left dentary (FMNH CUP 2088) collected from the ‘Dark Red Beds’ of the Lower Lufeng Formation, in the vicinity of Ta Ti village, Lufeng County, Yunnan Province, China. Simmons (1965) assigned Tatisaurus to the ornithischian ornithopod family Hypsilophodontidae (which was then regarded as a primitive and rather generalized group of small-bodied ornithischians that were ancestral to the later and more clearly defined ornithischian groups of the Late Jurassic and Cretaceous); in his comparative discussion Simmons hinted that Tatisaurus shared some anatomical features (presumed to be ‘primitive’) with armoured or ankylosaurian ornithischians. In a general review of the Ornithischia, Steel (1969) assigned Tatisaurus to the basal ornithischian family Heterodontosauridae on the basis of its stratigraphical age and possession of a similarly robust dentary. Thulborn (1971, 1972) followed Simmons’ original interpretation by including Tatisaurus within a similarly broadly conceived bipedal and cursorial Hypsilophodontidae ; this referral was supported (although considered questionable) by Galton (1972) in his review of ornithopod evolution. Colbert (1981) reaffirmed its position as a hypsilophodontid ornithischian, but noted that it was poorly preserved and consequently difficult to analyse systematically. Attridge, Crompton & Jenkins (1985) resuscitated the idea that Tatisaurus was a heterodontosaur in a tabulation of early Jurassic taxa. Later authors have suggested alternative placements, but these have been exclusively within the dermally armoured ornithischian clade Thyreophora: Coombs, Weishampel & Witmer (1990) and Norman, Witmer & Weishampel (2004) considered Tatisaurus to be a basal thyreophoran; Dong (1990) proposed that Tatisaurus was a primitive stegosaur, referable to the family Huayangosaurinae ; and Lucas (1996) synonymised Tatisaurus with the basal thyreophoran Scelidosaurus as Scelidosaurus oehleri .

Referral of Tatisaurus to the clade Stegosauria ( Dong, 1990) has important implications for early thyreophoran evolution. The earliest reported stegosaur specimens are two isolated shafts of limb bones (probably femora; BRSMG Cb3869, Cb3870) described by Galton (2005). They were collected from the Westbury Formation (Late Triassic: Rhaetian) of England, but are extremely poorly preserved. Both bones lack articular ends and much of the outer bone surface is missing. The shafts are straight in lateral view, as seen in both sauropod saurischians and stegosaurs, but any trace of the fourth trochanter (assuming that these shafts represent femora) has been completely eroded. Galton proposed that the fourth trochanter was a low ridge (as in stegosaurs) rather than being more prominent (as seen in sauropods); self-evidently this character cannot be confirmed and although probably dinosaurian we cannot identify these elements beyond Reptilia indet. (see also Butler, Porro & Heckert, 2006).

Given the indeterminate nature of the Westbury Formation material, the earliest and most basal member of the clade Stegosauria currently recognized is Middle Jurassic, Huayangosaurus taibaii , from the Lower Shaximiao Formation (?Bajocian, Chen et al., 1982;?Bathonian–Callovian, Dong & Tang, 1984) of Sichuan Province, China ( Dong, Tang & Zhou, 1982; Sereno & Dong, 1992; Maidment, Wei & Norman, 2006). It is an approximate contemporary of a more derived European form, Lexovisaurus durobrivensis , known from the Lower Oxford Clay (middle Callovian) of England, and the Marnes à Belemnopsis latesulcata Formation (early Callovian) of France ( Galton, 1985, 1990).

The thyreophoran Scelidosaurus harrisonii Owen, 1861 from the Lower Lias (Sinemurian) of England has been variously interpreted as a primitive stegosaur ( Romer, 1956), an ornithopod ( Thulborn, 1977), a basal thyreophoran ( Sereno, 1986), or as the most basal ankylosaur ( Norman, 1984; Carpenter, 2001). Recent cladistic analyses consistently suggest that Scelidosaurus is a basal thyreophoran and the sister taxon to Ankylosauria + Stegosauria ( Sereno, 1999; Norman et al., 2004; Maidment, Wei & Norman, 2006; Butler et al. 2006). Nath, Yadagiri & Moitra (2002) briefly described material, which they referred to Ankylosauria, from the Kota Formation (Lower Jurassic) of India; however, they did not specify the synapomorphies that justified this referral and Ayyasami & Yadagiri (personal communication – cited in Wilson & Mohabey, 2006) indicate that these specimens represent a crocodylomorph. Pending a more detailed description of this material, we consider it as referable to Thyreophora incertae sedis. The earliest unambiguous members of the clade Ankylosauria are Middle Jurassic: Sarcolestes leedsi from the Lower Oxford Clay (middle Callovian) of England ( Lydekker, 1893) and Tianchiasaurus nedegoaperferima from the Toutunhe Formation (Bathonian–Callovian) of China ( Dong, 1993).

The earliest definite stegosaurs and ankylosaurs are either Bajocian or Bathonian in age, suggesting that the minimum date by which the two clades must have diverged is the earliest Bajocian. If Tatisaurus were referable to Stegosauria (as suggested by Dong, 1990), this would indicate that stegosaurs and ankylosaurs were present by the Early Jurassic (Sinemurian) and implies significant (minimum 25 million years) ghost lineages for both clades.

Lucas (1996) posited a radical reinterpretation of Tatisaurus ; he synonymized it with the basal thyreophoran Scelidosaurus and created the new binomen S. oehleri ( Simmons, 1965) – although this taxonomic reassignment has not gained wide acceptance (e.g. Irmis, 2004; Norman et al., 2004). This taxonomic reassignment allowed Lucas to propose an early Sinemurian date for the Lower Lufeng Formation because Scelidosaurus remains are reliably dated as early Sinemurian in England ( Newman, 1968; Ensom, 1987, 1989). Padian (1989) had also described isolated dermal scutes from the Kayenta Formation (Glen Canyon Group) of Arizona, USA, which seemed comparable with those seen in the genus Scelidosaurus (see also Tykoski, 2005). This taxonomic adjustment, linked to its wide geographical distribution, led Lucas (1996: 84) to propose Scelidosaurus as an international index taxon that merited the establishment of a biostratigraphic ‘ Scelidosaurus biochron’ for the early Sinemurian stage of the Lower Jurassic Period. The proposed synonymy of Tatisaurus and Scelidosaurus (if valid) has significant implications for Lower Jurassic terrestrial stratigraphic correlation.

The taxonomic status of T. oehleri is reassessed below, as are the various phylogenetic positions to which it has been assigned, their implications for thyreophoran evolution, and the dating of the Lower Lufeng Formation of China.

Institutional abbreviations: BRSMG, Bristol City Museum and Art Gallery, Bristol, UK; FMNH CUP, Field Museum of Natural History (Catholic University of Peking collection), Chicago, IL, USA; OUM, Oxford University Museum of Natural History, Oxford, UK.