Known only from isolated teeth found in North American Cretaceous sediments, Brachyrhizodus provides the collector with a large, easily identifiable, ray tooth. Although common in the Campanian sands of New Jersey, the teeth of this genus are less common in North Carolina and were deemed rare by Welton & Farish (1993: 153) in Texas. Cappetta (1987: 171, 2006) places the genus in the family Myliobatidae BONAPARTE 1838, however the author, seeing rhinopterid-like characteristics as well, prefers the utilization of the superfamily designation. Cappetta (1987: 172) suggests that the western Atlantic fossil record extends into the Maastrichtian; using Cappetta & Case (1975), Case, (1979), Welton & Farish (1993), Hartstein et al (1999) and Schwimmer,(1986: 115), I could find no reported evidence for this statement (although it may exist) -- it appears to be limited to the Campanian. Case & Cappetta (1997) did not include this genus in the Kemp Clay, (Maastrichtian) of Texas.
Teeth of Brachyrhizodus wichitaensis ROMER 1942, originally described from Texas (pages 221-22), are reminiscent of modern rhinopterids (and in many cases, myliobatids). The teeth are hexagonal and vary in width (probably due to position) to form a crushing tooth plate. (Collected teeth lack the labio-lingually orientated wear-grooves associated with a 'purely' grinding dentition.) Unlike the myliobatids, Brachyrhizodus teeth do not have facial "wrinkles" or a strong mortise-tenon odontology to provide interlocking between rows. These teeth also lack roots that extend beyond the lingual face of the crowns. The teeth are polyaulacorhizous in design, but the non-lateral root lobes can vary significantly in width. Although modern rhinopterids and myliobatids do not reflect this characteristic, it is present in Miocene mobulids. Lastly, exposures that produce Brachyrhizodus teeth do not seem to yield caudal spines, which might be expected with myliobatids although they do yield dermal denticles that appear to be associated with that species. With these caveats in mind, Brachyrhizodus has not been placed in a specific family by the author.
When found, teeth of this genus are generally well worn, yielding few finer details. In the early 90's, when collecting the 'Ned's Slump' lens at Big Brook (Mt Laurel Formation), the author found a singular specimen which provided excellent odontological detail and is illustrated in figure . In an unworn condition the crown is high and when viewed occlusally, hexagonal. The labial face extends beyond the roots and has broad enameloid ridges at the base of the crown. The lingual face aligns with the roots and has narrow enameloid ridges at its base. The labio-lateral faces are concave. Viewed basally, the lateral lobes are triangular and the nutrient grooves may or may not contain scattered foramina. As noted earlier, these teeth may be laterally elongated to various extents with two or more lobes (figure provides a few examples). Based on personal experience, it is easy to confuse bilobed Brachyrhizodus teeth with those of Pseudohypolophus when the teeth are worn.
An alternate perspective
Manning (2006:223-30) argues that Thurmond's (1972) Hypolophus? mcnultyi should be placed in Brachyrhizodus, as a longer-lived (Late Albian-Maastrichtian) primitive sister taxon to B. wichitaensis. In large samples, a few B. mcnultyi teeth (Manning pers. com. 2007) bear a narrow third root between the two usual ones. Many B. wichitaensis teeth bear two roots, so polyaulacorhize roots don't distinguish the two species. Also, the crown shape is basically similar, and both have the derived orthodont crown histology. Manning (p 228) further argues that "Rhombodus levis" teeth actually represent lateral teeth of B. wichitaensis. His hypothesis (pg 229) is that B. mcnultyi represents a primitive myliobatid (sensu Compagno 1999).
Manning’s mcnultyi observations are worthy of consideration. There are certainly characteristics of the two that make them difficult to differentiate. A tooth attributed by Cappetta (1987: 141, fig 119k-m) to Pseudohypolophus (as a rajiform) has what appears to be a third lobe separating dual nutrient pores. A very similar tooth-design with a two lobes was used by Welton & Farish (1993: 153 fig 1) to depict B. wichitaensis. Manning’s Rhombodus levis determinations hold promise as well; unlike the rather homogeneous mcnultyi tooth-design, B. wichitaensis teeth display positional variation (most importantly nutrient groove orientation) and given sufficient examples, this hypothesis should be confirmable. I continue to position myself more conservatively and view Brachyrhizodus as a primitive myliobatoid.
Cappetta, H., 1987. Chondrichthyes II. Mesozoic and Cenozoic Elasmobranchii. In: Handbook of Paleoichthyologie, vol. 3b, Gustav Fischer Verleg, Stuttgart, 193 pp.
Cappetta, H., 2006.. Elasmobranchii post-Triadici (index generum et specierum). In: Riegraf, W. (Ed) Fossilium Catalogus I:Animalia 142. Leiden, Backhuys Publish, 472pp.
Cappetta, H. & Case, G., 1975. Contribution ŕ l'étude des sélaciens du groupe Monmouth (Campanien - Maestrichtian) du New Jersey. Palaeontographica Abteilung A, 151:1-46.
Case, G., 1979. Cretaceous Selachians from the Peedee Formation (Late Maestrichtian) of Duplin County, North Carolina, Brimleyana, Vol 2, pp 77-89.
Case, G. and Cappetta, H.. 1997. A new selachian fauna from the late Maastrichtian of Texas. Münchener Geowissenschaften Abhandungen 34:131-189.
Hartstein, E., Decina, L. and Keil, R., 1999. A Late Cretaceous (Severn Formation) Vertebrate Assemblage from Bowie, Maryland. The Mosasaur, 6:17-23.
Manning, E., 2006. Late Campanian vertebrate fauna of the Frankstown site, Prentiss County, Mississippi; systematics, paleoecology, taphonomy, sequence stratigraphy. Unpub. PhD dissertation, Tulane Univ., New Orleans, xvii+419 p., 16 pls.
Robb, A., 1989. The Upper Cretaceous (Campanian, Black Creek Formation) Fossil Fish Fauna of Phoebus Landing, Bladen County, North Carolina, The Mosasaur, Vol 4, pp 75-92.
Romer, A., 1942. Notes on certain American Paleozoic fishes. Amer. Journal of Science; 204(3):216-228.
Schwimmer, D., 1986. Late Cretaceous fossils from the Blufftown Formation (Campanian) in western Georgia. The Mosasaur, III:109-119.
Welton, B. and Farish, R., 1993. The Collector's Guide to Fossil Sharks and Rays from the Cretaceous of Texas. Before Time, Texas. 204 pp.