Cylindracanthus ornatus Leidy, 1856

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 : 150-152

Cylindracanthus ornatus Leidy, 1856

Fig. 56 A–C

Cylindracanthus ornatus Leidy, 1856: 302 .

Coelorhynchus ornatus – Cope 1871: 6 , pl. 10, fig. 7.

Cylindracanthus rectus – Leriche 1942 : pl. 8, fig. 3b.

Cylindracanthus rectus – Casier 1966: p. 174 , pl. 22, figs 1–5.

Material examined

UNITED STATES OF AMERICA – Alabama • 3 isolated teeth; Claiborne Group; ALMNH PV1992.28.3.2, MMNS VP-8947, SC 2012.47.153.

Description

Taxon known only by its rostrum. Rostrum elongated, cylindrical, tapering anteriorly to blunt point; circular cross section measuring up to 20 mm in rostral diameter. Outer surface with strong sub-rounded to flat parallel ridges of varying width along length. Ridges on dorsal surface occasionally intersect or coalesce near anterior end. Two parallel ventral grooves each contain single row of acrodont teeth and/ or alveoli; ventral grooves wider than all others on rostrum, do not extend to rostrum tip. One-to-two hollow concavities visible in cross section.

Remarks

Three species of Cylindracanthus have been reported from Cretaceous and Eocene deposits in Alabama (see Thurmond & Jones 1981), including C. acus Cope, 1870 , C. ornatus Leidy, 1856 , and C. rectus (Agassiz, 1843) . Leriche (1942) argued that C. acus and C. ornatus were junior synonyms of C. rectus , giving this latter taxon a range that extended from the Late Cretaceous to the Eocene. Parris et al. (2001), however, provided convincing evidence that C. ornatus was a distinct taxon and could be separated from C. rectus and C. acus by the presence of two distinct parallel ventral grooves, each containing a single row of acrodont teeth. Parris et al. (2001) also noted that the range of C. ornatus extended from the Late Cretaceous into the Eocene, whereas C. acus and C. rectus were confined to the Eocene. Although Parris et al. (2001) suggested that all three species should be viewed as valid, they stated that a more extensive review of the genus was necessary. Regarding the Eocene C. acus and C. rectus, Thurmond & Jones (1981) suggested the two could be differentiated by rostral diameter ( C. acus up to 7.0 mm; C. rectus up to 20 mm) and number of ridges around the rostral circumference ( C. acus up to 19; C. rectus up to 46). Although Thurmond & Jones (1981) mentioned these differences could be related to ontogeny, they concluded that the two taxa were distinct due to the absence of specimens of an intermediate size.

We examined Cylindracanthus specimens that were derived from several Cretaceous and Eocene deposits in Alabama, which allowed us to formulate several conclusions regarding the taxonomy of the species in this genus. We view C. acus as a junior synonym of C. rectus because our sample exhibits a clear size gradient between small and large specimens. Our sample also indicates that the rostral diameter and number of ridges around the circumference are not taxonomically useful characteristics, but instead are related to ontogeny. Furthermore, because the rostra decrease in diameter and ridges often intersect and combine anteriorly, counting the number of ridges on rostrum fragments is problematic because, in cross-section, the distal end of a complete rostrum has fewer ridges than does the proximal end. Of the specimens from Claiborne Group formations, rostra were broadly divided into three groups: 1) those with tooth-bearing ventral grooves; 2) those without tooth-bearing grooves, and; 3) those where the presence of tooth grooves could not be determined due to specimen preservation (i.e., incomplete rostral diameter, matrix-filled grooves). We found that specimens from both groups 1 and 2 can have nearly identical rostral circumferences, suggesting that the presence or absence of teeth is not related to ontogeny. This lends support to the conclusions of Parris et al. (2001) that C. ornatus (those with teeth) is a distinct species from C. rectus (those without teeth) and that these species were coeval during the Eocene. It could be argued that the presence or absence of tooth grooves reflects sexual dimorphism, but all of the Cretaceous specimens we examined have this distinct characteristic. This observation leads us to conclude that the presence or absence of tooth-bearing grooves is a taxonomically viable characteristic that can be used to distinguish the two species. This also indicates that only C. ornatus had a range extending back to the Late Cretaceous. Many specimens, including those figured by Maisch et al. (2016: 8, fig. 2, 12–14), are not well enough preserved to confidently identify them to species (i.e., specimens ablated, grooves obscured by matrix), and we herein identify them only as Cylindracanthus sp.

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the contact of the Tallahatta and Lisbon formations and the basal Lisbon Formation at site ACov-11. Lower Lutetian, zones NP14 and NP15.

Figures

Fig. 56. Cylindracanthus Leidy, 1856, rostra. A–C. Cylindracanthus ornatus Leidy, 1856, SC2012.47.153, basal Lisbon Formation. A. Ventral view. B. Cross–section. C. Close–up of denticle rows. — D–G. Cylindracanthus rectus (Agassiz, 1843). D–E. MSC 38378, “upper” Lisbon Formation. D. Cross–section. E. Lateral view. F–G. WSU 5019, basal Lisbon Formation. F. Cross–section. G. Lateral view. Scale bars: A–C =2 mm; D–G= 5 mm.