Cylindracanthus ( Leidy 1856a )

Ciobanu, Rodica & Trif, Nicolae, 2016, Cylindracanthus (Leidy, 1856) In The Collections Of The Natural History Museum, Sibiu, Acta Palaeontologica Romaniae 12 (1), pp. 25-31 : 26-28

Genus Cylindracanthus ( Leidy 1856a)

Cylindracanthus sp.

( Fig. 3 a-h)

Type Species Cylindracanthus rectus (Agassiz 1843) ; type species according to Schultz (1987) and Parris et al. (2001); note that Parris et al. (2007) acknowledges Cylindracanthus ornatus ( Leidy 1856a) as type of this genus.

Material: 1 isolated fossil (inv. no: 34317) from the Breckner Collection, Natural History Museum of Sibiu.

Origin: Eocene limestone from Turnu Roşu (Porceşti)

Diagnosis:

The below diagnosis is based on Cylindracanthus rectus (Agassiz 1843) ; in the original study, this fossil is described (under the name Coelorhynchus rectus ) as having a circular section and a straight, very slightly conical rod-like morphology. There is an inner channel extending almost throughout the entire length of the element. In the amended diagnosis, Arambourg (1952) emphasizes the presence of deeply fluted longitudinal grooves with 40 to 55 ridges, as main characteristic feature of the taxon. He also states that no obvious bilateral symmetry or tooth base development is present.

Parris et al. (2001) amended the description of this taxon again, and showed that the bilateral symmetry is, in fact, present. This is demonstrated by the shape and disposition of the internal canal(s), and by the paired grooves that might represent teeth bases. The number of ridges between the two grooves/tooth alveoli varies considerably. Arambourg (1952, p.272) and Weems (1999, p.61) both considered C. rectus as a typical member of the genus for the Eocene, while the range of the genus itself extends from the Cretaceous to the Eocene.

At least ten more species of the genus Cylindracanthus are described in the literature (e.g., Schultz, 1987), but their status seems more or less uncertain.

Remarks:

One can easily note that the descriptions of the different species attributed to Cylindracanthus are more or less sketchy. In general, it is stated that these remains are cylindrical, slightly conical, with external ridges running along the entire length of the element. The total number of ridges seems to be, for the moment, the only characteristic that differentiates between the species. However, beside this reliable character, two opinions of different authorities, together with their specific nuances, must be noted:

- one important note to consider is in Leriche's descriptions (1905, 1906) of C. rectus . Leriche makes a few notes relying on complete specimens. Like others before him, he remarks the cylindrical transversal section for the most part of the length, but, at the wider end of the element (i.e., the part that is supposed to join with the rest of the skull), he notes the existence of an important sulcus, a groove (sillon - in French) that deepens and widens rapidly towards the end. This groove, located on the presumed upper (dorsal) side, has a narrower equivalent on the inferior (ventral) side.

- a more "mainstream" description belongs to Fowler (1911, p.141) who writes about the genus: “...spine very long, slender, external face longitudinally ridged and grooved, each ridge corresponding to a wedge-shaped plate which forms small sector of spine. Central cavity relatively small, sometimes in part simple, but usually divided by median partition. Division plane passing through middle of partition, thus allowing spine to be readily split into two symmetrical halves”. Fowler (1911, p.141, 142) described his ornatus specimens, and for the acus species he used the description from Cope (1870, p.294). The differences between species are minor, linked to size, the central canal, and the number of ridges. In regard to size, Cope (1870, p.294) indicates that C. acus is smaller than C. rectus Agassiz and C. ornatus Leidy. It is important to note that Fowler recognizes the bilateral symmetry.

Description:

The specimen we report here is obviously only a fragment, most likely originating from the anterior part of the rostrum. The length of the fragment is of 25.7 mm; at its wider end, it has a diameter of 8.5 mm, while on the smaller end, this reduces to only 6.05 mm ( Fig. 3 a-b). The cross-section is circular and shows a central canal ( Fig. 3 c-d). The external surface seems to be a thick, deeply fluted enamel-like layer that shows 32 ridges. We observed a feature also described by Leidy (1856a, p.12), i.e., that some ridge pairs unite into single ones ( Fig. 3 ef). Also, it is important to note the presence of two deeper grooves, slightly wider than the rest, which could represent the insertion place of blunted teeth as discussed by Parris et al. (2001). The two grooves are bilateral and symmetric, separated by only one pair of ridges. No actual teeth have been observed, but insertion places (the alveoli) appear to present ( Fig. 3 g-h).

Figures

Fig. 3 The fossil specimen referred to Cylindracanthus sp. (inv. no: 34317): a lateral view of the specimen, and b, interpretative drawing; c natural transverse section of the specimen, and d, interpretative drawing; e details of joining longitudinal ridges, and f, interpretative drawing; g details of the alveoli, with h interpretative drawing.