Enigmatic Triassic Taxa - Doswellia kaltenbachi

As I stated in an earlier post the newest issue of the Journal of Vertebrate Paleontology contains three, count them three, important new Triassic papers. In fact Jeff Martz at Paleo Errata has already proclaimed it possibly the best issue of JVP ever (for Triassiphiles). The first Triassic paper (Dilkes and Sues, 2009) is one that I have been awaiting for a long time and received a preview of (poster presentation) at the SVP meeting in Cleveland last year. The original Doswellia description published back in 1980 is well done; however, plagued by poor figures and lacks a clear phylogenetic analysis. Thus, a revision has been long necessary, especially given recent revisions within Archosauromorpha and that North American archosauriformes are rare. Furthermore, Doswellia is from the Newark Supergroup, a unit with a poor (but much improving) body fossil record.

The age of the Taylorville Basin sequence is somewhat ambiguous. As noted by Dilkes and Sues (2009) the deposit is Carnian in age but other authors have argued that the age ranges from early to late Carnian. Recent revisions of the Late Triassic timescale (e.g., Furin et al, 2006) would place the Falling Creek Formation solidly in the latest Carnian with the possibility that it may also be partly earliest Norian in age.

The skull of Doswellia is one of the more interesting features of the taxon. The skull possesses the euryapsid condition with only supratemporal fenestra being present. Unfortunately the anterior portion of the skull is missing so the presence of an antorbital fenestra cannot be ascertained. An elongated referred dentary suggests the presence of an elongated rostrum (Weems, 1980). The skull is broad and shallow and Doswellia possesses teeth on the pterygoid. The skull reconstructions in Weems (1980) can be difficult to interpret (but see the nice reconstruction from Palaeos below). Dilkes and Sues (2009) provide drawings of the actual specimen in dorsal, ventral and posterior views; however, I as left wishing that a lateral drawing was provided as well, including a close-up lateral view of the braincase.

Another unique feature of Doswellia is faceted transverse processes of the dorsal vertebrae for reception of the capitulum of the dorsal ribs. Furthermore, the dorsal ribs are strongly angled (~90˚) ventrally suggesting a deep body. The reconstruction below based on Weems (1980) is also from Palaeos.

Doswellia possesses a dorsal carapace of numerous rows of interdigitating osteoderms. Possession of these osteoderms has suggested to some past authors a close relationship with aetosaurs (e.g., Bonaparte, 1982). Articulated portions include a nuchal collar (osteoderms right behind the skull) and 5-6 rows of more posterior armor. These rows consist of at least eight columns of square plates with a pitted ornamentation of the dorsal surface and a distinct raised eminence. More posterior osteoderms appear to be flexed (possibly around the tail?).

Although superficially similar to the osteoderms of aetosaurs, the dorsal carapace of Doswellia differs from that of aetosaurs in the increased number of columns (8 vs. 4) and the lack of distinct lateral osteoderms. In fact, the dorsal carapace of Doswellia is more similar to known aetosaurian ventral carapaces, although aetosaur ventral osteoderms do not possess raised eminences. The reconstruction below is from Wikipedia.

The ilium is also distinct in that it is extremely expanded dorsally and laterally. Such an expanded ilium is only found in the archosauriforms Vancleavea campi and drepanosaurs (Parker and Barton, 2008), although in Vancleavea the iliac blade is less expanded anteroposteriorly and is not deflected as strongly laterally.

The femur is a characteristic bone (although poorly preserved according to Dilkes and Sues [2009]) in that is is more derived than basal archosauriforms in lacking a distinct intertrochanteric fossa, a ventral ridge system, and possessing a distinct head. The drawing of the femur in both Weems (1980) and Dilkes and Sues (2009) is similar to that of Vancleavea campi (Parker and Barton, 2008) and differs from that of Erythrosuchus, Proterosuchus, and basal archosauromorphs.

The phylogenetic analysis includes 85 characters and 15 taxa including other enigmatic taxa such as Turfanosuchus and Yonghesuchus. Dilkes and Sues (2009) claim to be the first analysis to include these latter taxa (and should have been) although the phylogenetic analysis of Parker and Barton (2008) also includes Doswellia and Turfanosuchus. However, the Dilkes and Sues paper was in press at the time our paper was published. Furthermore, the initial submission by Parker and Barton did not include either Turfanosuchus or Doswellia in the phylogenetic analysis because I was aware of the forthcoming work by Dilkes; however, the non-inclusion of Doswellia and Turfanosuchus was criticized by a reviewer of the paper and thus these taxa were included in the final submission.

The analysis by Dilkes and Sues (2009) recovers Doswellia as the sister taxon of Proterochampsidae within Archosauriformes. Two interesting findings of this study are the recovery of Erythrosuchus as more derived than Euparkeria (traditionally the archetypal derived non-archosaurian archosauriform) and the recovery of phytosaurs (Parasuchia) as the sister taxon of aetosaurs and more derived than Gracilisuchus and Qianosuchus. The latter is a “rauisuchid” whereas the former has always been recovered as more derived than phytosaurs in all analyses in which it has been included.

Dilkes and Sues (2009) provide detailed discussion attempting to explain a more derived Erythrosuchus; however, I am still somewhat dubious based on the less derived morphology of the femur in Erythrosuchus. Furthermore, Dilkes and Sues code (and discuss) Erythrosuchus as possessing osteoderms; however, Gower (2003) has argued that osteoderms were not present in this taxon.

Another erroneous coding is the proposed synapomorphy of a ventral carapace in phytosaurs and aetosaurs. Although phytosaurs possessed gular (throat armor) they lack the elaborate ventral armor carapace found in aetosaurs. Changing this coding could possibly drop phytosaurs to a more basal position within Archosauria in this analysis (although I have not rerun it yet personally).

I have never felt comfortable with the basal placement of Turfanosuchus within Archosauriformes, despite it also falling out there in my own analysis (Parker and Barton, 2008). This is because of the morphology of the ankle, especially the presence of a hemispherical calcaneal condyle, which is generally a suchian character (Sereno, 1991). In fact, the calcaneum of Turfanosuchus is so similar to that of suchians it is hard for me believe that this morphology was derived twice in archosauriform phylogeny, but this will be up to future analyses to determine.

I did find the Systematic Paleontology section to be somewhat ambiguous as it is an unranked list, yet does not include Archosauriformes (the most inclusive named clade that includes the clade of Doswellia + Proterochamsidae). Furthermore, it also includes the taxonomic name Doswellidae, which is not diagnosed or established anywhere in the paper including the phylogenetic analysis. Thus, Doswellidae remains a Linnaean taxon (Family) which is not supported by a phylogenetic analysis.

Nonetheless, my criticisms are minor and somewhat "nit-picky" because overall I feel that Dilkes and Sues (2009) have provided a very good redescription of the material and a solid foundation for future workers to include this important taxon in phylogenetic analyses. Furthermore, they corroborate Long and Murry’s (1995) referral of osteoderms from the Dockum Group of Texas to Doswellia, demonstrating a North American distribution for this taxon and suggesting that it may have some biostratigraphic significance. In this case, the base of the Dockum Group (which possesses non-phytosaurid phytosaurs, the aetosaur Lucasuchus, rhynchosaurs, and Doswellia) may be latest Carnian in age and thus probably older than the base of the Chinle Formation.

REFERENCES

Bonaparte, J.F. 1982. Classification of the Thecodontia. Geobios Mémoire Spécial 6:99-112.

Dilkes, D., and H.-D. Sues. 2009. Redescription and phylogenetic relationships of Doswellia kaltenbachi (Diapsida: Archosauriformes) from the Upper Triassic of Virginia. Journal of Vertebrate Paleontology 29:58-79.

Furin, S., Preto, N., Rigo, M., Roghi, G., Gianolla, P., Crowley, J.L., and S.A. Bowring. 2006. High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs. Geology 34:1009-1012. doi: 10.1130/G22967A.1

Gower, D.J. 2003. Osteology of the early archosaurian reptile Erythrosuchus africanus, Broom. Annals of the South African Museum 110:1-84.

Long, R.A., and P.A. Murry. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin 4:1-254.

Parker, W.G., and B.J. Barton. 2008. New information on the Upper Triassic archosauriform Vancleavea campi based on new material from the Chinle Formation of Arizona. Palaeontologia Electronica Vol. 11, Issue 3; 14A: 20p;
http://palaeo-electronica.org/2008_3/158/index.html

Sereno, P.C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Society of Vertebrate Paleontology Memoir 2:1-53.

Weems, R.E. 1980. An unusual newly discovered archosaur from the Upper Triassic of Virginia, U.S.A. Transactions of the American Philosophical Society 70:1-53.