Chomatodus linearis (Agassiz, 1838)

Ginter, Michał & Sun, Yuanlin, 2007, Chondrichthyan remains from the Lower Carboniferous of Muhua, southern China, Acta Palaeontologica Polonica 52 (4), pp. 705-727 : 714-716

publication ID	https://doi.org/ 10.5281/zenodo.13741212
persistent identifier	https://treatment.plazi.org/id/954A87EC-2C10-336C-FCDA-F8D87F847B88
treatment provided by	Felipe
scientific name	Chomatodus linearis (Agassiz, 1838)
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Fig. 6B View Fig .

Material.—One tooth.

Description.—The tooth crown is in a form of a mesio−distally elongated bar (mesio−distal dimension 5 mm, average labio−lingual dimension 0.8 mm) with rounded ends, slightly upturned at one end. It is composed of a low central ridge, surrounded by concentric growth layers. Some traces of tubular dentine can be observed in the worn part of the ridge (e.g., Fig. 6B View Fig 2 View Fig right). The base is shorter and narrower than the crown, more than 1 mm deep, almost vertical, and perforated with numerous narrow canals.

Remarks.—The teeth of this type, albeit much larger, have already been described in the mid−19 th century by Agassiz (1833 –43) and Newberry and Worthen (1866). The specimens from the Carboniferous Limestone in the vicinity of Bristol, originally illustrated as Psammodus linearis by Agassiz (vol. 3: pl. 12: 5–13), but subsequently translated to the genus Chomatodus in the text volume ( Agassiz 1833 –43, vol. 3: 108) differ, in addition to the size, by the better differentiated and higher central ridge, less conspicuous concentric growth lines (compare Fig. 10G View Fig ), and the lack of an upturned end.

Newberry and Worthen (1866) followed Agassiz in calling such teeth Chomatodus , but they introduced eight new species based on the material from the Mississippian and Coal Measures of Iowa and Illinois. We consider the differences between these specimens and the type material insufficient for creating new taxa, with an exception, perhaps, for Chomatodus cultellus and Ch. pusillus with a very sharp crest in the place of the central ridge ( Newberry and Worthen 1866: pl. 3: 13 and 14, respectively). The latter authors, probably correctly, placed Chomatodus within the Petalodontidae based on “crown having homologous parts of Petalodus ” ( Newberry and Worthen 1866: 34), apparently meaning the concentric growth lines around the central part of the crown. Woodward (1889) went further, attributing Chomatodus linearis partly to Petalodus itself and partly to Helodus . However, such identification is untenable in the light of modern understanding of the latter two genera.

Despite the obvious differences between the Chinese tooth and the type material we consider them as conspecific. The smaller size and different proportions of the crown parts suggest that the tooth under description comes from a very young individual. The British and American specimens were collected as macrofossils and do not cover all the size range within the species; they probably represent only adult forms.

Chomatodus davisi ( Woodward, 1889) comb. nov. Fig. 9D View Fig .

1875 Chomatodus insignis (Leidy sp.); St. John and Worthen 1875: pl. 10a: 5.

1889 Petalodus davisii sp. nov.; Woodward 1889: 46.

Material.—Two fairly complete teeth and two fragments.

Description.—The two more complete teeth are extremely elongated mesio−distally (2.5 mm) and flattened labio−lingually. They have blade−like crowns, labially convex and lingually concave. The crown of the first tooth ( Fig. 9D View Fig ) is of virtually equal height throughout. The crown of the second, higher tooth ( Fig. 9E View Fig ) is asymmetrical: it slopes down gently distally (?) and on the higher, mesial (?) end it is rounded in lingual view and ends rather abruptly. The outer layer of the crown seems to be partly diagenetically modified and partly removed, especially at the crown−base interface in the higher tooth. Therefore, almost no trace of ornamentation can be observed, save for the lingual face of the distal part of the larger tooth ( Fig. 9E View Fig 3 View Fig , right) where a few vague vertical ripples have remained. It is difficult to say what kind of a mineral tissue builds the crown, but it is possible that at least the highest part of the blade in the larger tooth contains tubular dentine.

The vascularisation of the base is composed of interchanging vertical grooves and canals. The aboral side is flat and narrow, slightly arched, and devoid of foramina. The canals and grooves on the labial side are situated in a horizontal furrow, in which the lingual part of the base could fit. Owing to that feature, the crowns of teeth in a file were packed closely one after another, forming a pavement of blades.

Remarks.—The highest of the teeth described herein is similar to a form referred to by Woodward (1889) as Petalodus davisi ( Fig. 10C View Fig ; Viséan of Armagh, Northern Ireland). The asymmetrical shape of the crown, the vertical ripples on the upper blade, and the canal−groove interchange in the base are the main common features. However, “ P. ” davisi from Armagh displays a partly broken, but still clear growth line, separating the crown from the base, in the manner of Chomatodus or other petalodontiforms. Unfortunately, that region is damaged in the larger Chinese tooth and nothing like a growth line can be observed in the smaller tooth. Some help comes from a tiny, elongated fragment, which presents an evident horizontal ridge between the base and the crown. If that fragment really belongs to the same species as the other teeth that probably means that the latter lost that ridge during fossilisation. The general shape of the teeth described herein more resemble that of Chomatodus than that of Petalodus sensu stricto, and that is why we tentatively propose a new combination, Chomatodus davisi , for Woodward’s (1889) taxon.

Yet another species, Chomatodus incrassatus (St. John and Worthen 1875, pl. 10: 18) possesses an asymmetrical crown, very similar to that of the larger tooth from Muhua, and the characteristic form of the base with a labial furrow. However, there are apparently two growth lines on the lingual and three on the labial sides of Ch. incrassatus and the teeth are much smaller than Ch. davisi , so it is impossible to decide at the moment whether these two are conspecific. If so, Ch. davisi would be a junior synonym.

The long and low tooth from the Viséan St. Louis Limestone of St. Louis, Missouri, referred to as “ Chomatodus insignis (Leidy sp.)” by St. John and Worthen (1875, see especially pl. 10a: 5b) resembles the first tooth of the described above ( Fig. 9D View Fig ). Unfortunately, we had no opportunity to see Leidy’s original ( Palaeobates insignis Leidy, 1857 ) and evaluate St. John and Worthen’s identification; Woodward (1889: 48; see also Hay 1902: 280) considered it doubtful. Thus, we have decided to use, at least temporarily, the name Ch. davisi for the specimens from Muhua.