Burnhamia daviesi ( Woodward, 1889 )

Ebersole, Jun A., Cicimurri, David J. & Stringer, Gary L., 2019, Taxonomy and biostratigraphy of the elasmobranchs and bony fishes (Chondrichthyes and Osteichthyes) of the lower-to-middle Eocene (Ypresian to Bartonian) Claiborne Group in Alabama, USA, including an analysis of otoliths, European Journal of Taxonomy 585, pp. 1-274 : 144-146

Burnhamia daviesi ( Woodward, 1889)

Fig. 53

Rhinoptera daviesi Woodward, 1889: 126 , pl. 3, fig. 6.

Burnhamia daviesi – Cappetta 1976: 564 .

Material examined

UNITED STATES OF AMERICA – Alabama • 12 isolated teeth; Claiborne Group ; MSC 38821, MSC 35789, MSC 38823 (4 specimens), MSC 38826, MSC 38878 (5 specimens) .

Description

Median teeth very wide and relatively narrow. Lateral teeth two-to-three times wider than long. In oral view, all specimens with six-sided crown with sharp lateral angles. Occlusal surface concave to varying degrees. Concave surface completely surrounded by a conspicuous rim; rim bears fine network of reticulated ridges. Labial crown face vertical; crown foot often developed into a sharp projection. Lingual face with rounded transverse ridge at the crown foot; has a narrow suprajacent furrow. Often, a second less well-developed transverse ridge is near the crown apex. All crown faces ornamented with fine reticulated ridging and coarser vertical wrinkling. Ornamentation is conspicuous on median teeth; less so on distal lateral teeth. Root polyaulocorhize; relatively few lamellae are separated by wide nutritive grooves and may be one-half or equal to the crown height. Root lamellae may extend slightly past the lingual crown foot.

Remarks

Illustrations of the holotype provided by Woodward (1889), Hovestadt & Hovestadt-Euler (2013), and Underwood et al. (2017) showed that the dentition of Burnhamia daviesi was similar to that of Rhinoptera , consisting of a median row of very wide teeth that is flanked by multiple rows of lateral teeth that decrease in size towards the commissure. Gross tooth morphology and limited crown wear has led to interpretations that Burnhamia was a pelagic planktivore related to extant devil rays, and recent phylogenetic analyses placed Burnhamia securely within the Mobulidae ( Zhelezko & Kozlov 1999; Cappetta 2012; Underwood et al. 2017).

Within the Tallahatta Formation, the concave occlusal surface on median and lateral teeth easily distinguish Burnhamia from any of the other coeval Myliobatidae , including Rhinoptera , Aetomylaeus , Myliobatis and Pseudaetobatus . This singular feature also serves to separate Burnhamia from teeth of Leidybatis , Aetobatus , Myliobatis , Aetomylaeus and Rhinoptera that occur in the Lisbon Formation, as well as teeth of the latter three taxa from the Gosport Sand. However, Burnhamia could be confused with Eoplinthicus yazooensis , a mobulid taxon that also inhabited the Gosport Sand paleoenvironment (see below). Both genera are exceedingly rare within the sample of several thousand Myliobatidae teeth we examined from the Gosport Sand, but Burnhamia can be distinguished by an occlusal surface that is equal to, or only slightly smaller in area than, the crown base, whereas the occlusal surface of E. yazooensis is significantly smaller in area than the crown base. Additionally, the labial crown foot is developed into a sharp horizontal or basally directed projection, but the lower half of the labial face of E. yazooensis is broadly rounded. In profile, the lateral angles of Burnhamia are much more sharply defined than on E. yazooensis . Also, the occlusal surface of Burnhamia teeth bears a reticulated network of ridges, whereas surfaces of E. yazooensis exhibit a series of fine labiolingually oriented striations. The crown of Burnhamia has a sharp six-sided outline in occlusal view, whereas the labial margin on E. yazooensis is less sharply defined.

Some teeth within the Tallahatta Formation and Gosport Sand samples exhibit a particularly concave occlusal surface surrounded by a very sharp ridge, a condition that is similar to specimens that Underwood et al. (2017) identified as possible male teeth within the species. The morphologies of the Burnhamia teeth from the Tallahatta Formation and Gosport Sand overlap, and we therefore consider them to be conspecific. The single specimen from the Lisbon Formation is ablated, but it also conforms to teeth recovered from the other two formations.

It has been postulated that Burnhamia diverged into two lineages, one where the teeth become progressively smaller and less ornamented over time (i.e., Burnhamia fetahi ), and a B. daviesi lineage that continued until the middle Eocene, giving rise to Eoplinthicus ( Cappetta 1985; Noubhani & Cappetta 1992; Cappetta & Stringer 2002). However, within our sample, which spans from the Ypresian to middle Bartonian, the B. daviesi teeth remain morphologically constant over that time and have stratigraphic overlap with Eoplinthicus in the Gosport Sand.

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the lower Tallahatta Formation at site ADl-1, the basal Lisbon Formation at site ACov-11, the basal Gosport Sand at site ACl-4, and the Gosport Sand at site ACl-15. Upper Ypresian to middle Bartonian, zones NP14 to NP17.

Figures

Fig. 53. Burnhamia daviesi (Woodward, 1889), teeth. A–E. MSC 38878.2, median tooth, lower Tallahatta Formation. A. Oral view. B. Basal view. C. Labial view. D. Lingual view. E. Profile view. F–J. MSC 38878.1, median tooth, lower Tallahatta Formation. F. Oral view. G. Basal view. H. Labial view. I. Lingual view. J. Profile view. K–O. MSC 38826, median tooth, basal Lisbon Formation. K. Oral view. L. Basal view. M. Labial view. N. Lingual view. O. Profile view. P–T. MSC 38878.3, lateral tooth, lower Tallahatta Formation. P. Oral view. Q. Basal view. R. Labial view. S. Lingual view. T. Profile view. U–Y. MSC 38823.4, lateral tooth, Gosport Sand. U. Oral view. V. Basal view. W. Labial view. X. Lingual view. Y. Profile view. Z–DD. MSC 38823.3, median tooth, Gosport Sand. Z. Oral view. AA. Basal view. BB. Labial view. CC. Lingual view. DD. Profile view. EE–II. MSC 38823.2, lateral tooth, Gosport Sand. EE. Oral view. FF. Basal view. GG. Labial view. HH. Lingual view. II. Profile view. JJ–NN. MSC 38823.1, lateral tooth, Gosport Sand. JJ. Oral view. KK. Basal view. LL. Labial view. MM. Lingual view. NN. Profile view. OO–SS. MSC 38878.4, lateral tooth, lower Tallahatta Formation. OO. Oral view. PP. Basal view. QQ. Labial view. RR. Lingual view. SS. Profile view. TT– XX. MSC 38821, lateral tooth, basal Gosport Sand. TT. Oral view. UU. Basal view. VV. Labial view. WW. Lingual view. XX. Profile view. YY–AC. MSC 35789, lateral tooth, lower Tallahatta Formation. YY. Oral view. ZZ. Basal view. AB. Labial view. AC. Lingual view. Labial at top in oral and basal views. Labial at top in oral and basal views. Scale bars = 2 mm.