- Chonetes (Eochonetes) advena Reed, 1917
- Thaerodonta aspera Wang, 1949
Synonymous with Thaerodonta Wang, 1949
1917 Chonetes (Eochonetes) Reed
1928 Sowerbyella Jones
1949 Thaerodonta Wang
1965 Thaerodonta Howe
1965 Eoplectodonta Williams
1965 Eochonetes Williams
1967 Thaerodonta Havliček
1972 Thaerodonta Howe
1974 Thaerodonta Amsden
1974 Eochonetes Amsden
1977 Eoplectodonta Mitchell
1981 Thaerodonta Rõõmusoks
1989 Eochonetes Cocks & Rong
Ordovician (Caradoc – Ashgill)
Eochonetes is an extinct genus of stationary epifaunal suspension feeders
Identification in Hand Sample
- Eochonetes vs. Sowerbyella: Eochonetes is larger, has strongly developed accessory teeth, delthyrial thickening, high lateral ridges as well as dorsal denticles with corresponding ventral fossettes
- Eochonetes vs. Plectodonta: Eochonetes lacks large tubercles on internal surface and dentition is different, Plectodonta has ventral denticles with corresponding dorsal fossettes–the opposite of Eochonetes
- Small in size
- Semicircular outline
- Hinge line is greatest width
- Dorsal denticles; ventral fossettes
- Ventral canals perforating the hinge line (rarely seen)
- Remarks: Cocks and Rong (1989) listed Thaerodonta as a junior synonym of Sowerbyella (Eochonetes) Reed, 1917, on the basis of both genera having hinge denticles in the dorsal valve and corresponding hinge fossettes in the ventral valve. Previously Eochonetes was distinguished from other sowerbyellids in its ventral hinge line being perforated by oblique canals. Cocks and Rong regarded these canals as insignificant taxonomic characters at the generic or subgeneric levels because they have been found only in about half of the Eochonetes populations. The presence of hinge denticulation (dorsal denticles and ventral fossettes) was regarded by Howe (1972) as a diagnostic feature of Thaerodonta, although, denticulations do not occur in all members of the genus contrary to Howe’s (1972, p. 443) statemeent that “regardless of age, shape, size, variation, or species,… each pedicle valve of Thaerodonta bears fossettes and each brachial valve displays the corresponding denticles.” Other internal features commonly used as diagnostic characters in the sowerbyellids, such as the side septa of the dorsal valve, also show considerable variations (Howe, 1965; Macomber, 1970; Mitchell, 1977; cocks and Rong, 1989). Despite wide range of variations in a number of diagnostic charactrs, Thaerodonta can be distinguished easily from Sowerbyella and Eochonetes when alrge populations are examined. Following Potter and Boucot (1992) and Jin et al. (1997), the genus Thaerodonta is retained as an independent genus herein.
Treatise on Invertebrate Paleontology, Part H, Vol. 2 (2000):
- Similar to Sowerbyella but with denticles on lateral parts of dorsal valve hinge line only , opposing small sockets on ventral valve hinge line; canals in ventral valve hinge line variably developed, usually absent.
Cocks & Rong, 1989:
- Diagnosis: Like Sowerbyella but with denticles on the brachial (dorsal) valve hinge line and opposing small sockets on the pedicle (ventral) valve hinge line. Canals in pedicle (ventral) valve hinge line variably developed, usually absent
- Discussion:Close examination of topotype Eochonetes advena reveals the presence of denticles on the brachial (dorsal) valve hinge line (particularly laterally) and corresponding fossettes on the pedicle (ventral) valve hinge line. This is the only point of difference between Thaerodonta and Sowerbyella, and thus Eochonetes is placed ere as a subgenus of Sowerbyella. Various plectambonitaceans, e.g. Chonetoidea radiatula, have perforated hinge lines in the same way as Eochonetes and we follow Jones (1928) in not considering this feature of generic or subgeneric importance, particularly since over half of the specimens in Eochonetes populations lack these canals.
- Since the original erection of Thaerodonta by Wang (1949) many authors have discussed the true generic characters and how the genus may best be separated from Eoplectodonta, Sowerbyella, Viruella, and other closely relocated genera (e.g. Spjeldnaes 1957, Howe 1965, Williams 1965, Havliček 1967, Cocks 1970, Macomber 1970, Howe 1972, Amsden 1974, Mitchell 1977, and Rõõmusoks 1981). Of all these papers, by far the best is Howe (1972) since he not only fingers the hinge lines of the various genera very well, but also discussed their various characters. We follow Wang and Howe in accepting the nature of the denticles on the hinge line as the fundamental difference between Thaerodonta and Eoplectodonta – in the former the protuberances are all on the brachial (dorsal) valve, whilst in Eoplectodonta the denticles are on the pedicle valve and the pits (fossettes) on the brachial (dorsal) valve. In some species and specimens of Thaerodonta the denticles are confined to the lateral parts of the hinge line, in contrast to Eoplectodonta in which some species only have denticles in the center, near the teeth.
- Again following Howe (1972), apart from the presence of denticles we can find no consistent differences between Thaerodonta and Sowerbyella and thus we treat Thaerodonta as a subgenus of Sowerbyella here. Various authors have found differences between Thaerodonta and Sowerbyella in their side septa; however, there is so much variation in Sowerbyella that we cannot use these features to separate the two subgenera.
- Some authors, e.g. Wang (1949) and Howe (1972), have argued that there are strongly developed accessory teeth and delthryial thickening in Thaerodonta which distinguish it from Eochonetes, but we can find no substantial differences between them. Therefore Eochonetes is now established as a senior synonym of Thaerodonta.
- General Description: Shell moderately large – 0.5-0.7 inches in width; semicircular shell outline; straight hinge line; cardinal extremities acute; concavo-convex; ornamentation unequally costellate, sometimes lamellose; shell pseudopunctate, fibrous, pseudopunctae large, arranged in crowded radial rows
- Ventral Valve Description: Long, plane, orthocline interarea; crural fossettes developed for part or whole length of hinge, crural fossettes deep; pseudodeltidium small, convex, apical; teeth usually small; thick dental plates advancing to form lateral ridge of muscle field; accessory teeth well developed; delthyrial cavity divided by a strong horizontal thickening between base of dental plates and above median septum; Muscle field:Bilobed anteriorly; median septum sharp; extending anteriorly about a third of the length, then bifurcated anterolaterally; adductor scars small, oval, situated beneath delthyrial thickening; adductor tracks large, triangular; diductor scars broad, straight, expanded anteriorly; adjustor scars prominent, elongate; ovarian areas tuberculate
- Dorsal Valve Description: Short hypercline interarea; cardinal denticles projected, occupying part or whole length of the hinge – sometimes irregular; cardinal process short, highly elevated; chilidial plates strong, attached to brachial processes by shell callus and separated from cardinal process by deep grooves; brachial processes – widely divergent, sharply pointed; floor of notothyrium thickened by shell swelling;Muscle field: Separated medianly by 2 high bladelike ridges; adductor scars subequal in size, elongate – inner pair straight, outer pair rounded laterally; separated from each other by a low ridge; ovarian areas tuberculate
- Sowerbyella vs. Thaerodonta:Those abundant in Richmond of North America have been previously assigned to Sowerbyella due to the semicircular outline, bifurcated median septum in ventral valve of the two genera make their similarity most striking. Thaerodonta is usually larger than Sowerbyella and there are several important differences internally: denticulation including dorsal denticles and ventral fossettes along with developed accessory teeth. Delthryial thickening and high lateral ridges in dorsal muscle area are also distinguishing character
- Thaerodonta vs. Plectodonta: Differs from Plectodonta in lack of large tubercles on internal surface and more importantly in condition of denticulation. Denticulation in Plectodonta is opposite of Thaerodonta with ventral denticles and corresponding dorsal fossettes.
- Discussion: Interesting morphological feauture of Thaerodonta is the thickening in the delthyrial cavity unlike Sowerbyella, thickening of Thaerodonta is a large and slightly concave plate on the floor of the delthyrium. It is supported ventrally by the median septum, thus forming two conical cavities; the importance of this thickening is uncertain, if we interpret the conical cavities as being the location of the adductor muscles the thickening may have some protecting purpose for the muscles, as the cardinal process is exceedingly strong and highly elevated ventrally;No functional foramen is known in any species of Thaerodonta; adductor scars in the dorsal valve can be seen clearly in every shell studied, but contrary to Kozlowski’s statement on Plectodonta, they are on the outside of the submedian ridges
- General Description: Shell concavo-convex; transversely semielliptical; widest along hinge line; twice (or more than twice) as wide as long; cardinal angles rectangular
- Ventral Valve Description: Pedicle valve gently and quite uniformly convex; beak small, acute, scarcely incurved, not prominent or swollen, but rising above hinge line; hinge-area triangular, narrow, with several (4-5) oblique tubules on each side of beak running inwards through substance of shell from upper edge of hinge-area; interior of valve with pair of subtriangular pointed muscle scars, in contrast posteriorly but strongly diverging at about 120* anteriorly, extending about one-third the length of valve; each scar obscurely and unequally divided lengthwise by weak median ridge, and separated posteriorly by stronger shorter ridge; at the anterior points of the muscle-scars there arise short wide vascular trunks, each of which divides into two divergent smaller trunks, splitting up into numerous smaller vessels which run forwards subparallel to the margin; the rest of the interior is covered with elongated radial lines of small pustules, most developed near the margins
- Dorsal Valve Description: Brachial valve with broad widely divergent subtriangular low crural plates (ending in fine points), fused at base so as to form transverse cardinal plate, deeply and widely cleft in middle; cardinal process very small, of three (or four) very short thin lobes; muscle scars large, very faintly impressed; pair of thick septa slightly divergent, extending forwards for more than half the length of valve from base of cardinal process, dying out anteriorly and apparently dividing the continguous large oval faint muscle-scars into outer and inner portions; rest of the interior is covered with radial lines of elongated pustules; surface of shell is ornamented with fine close thread-like radiating lines of which every fourth or fifth one is slightly stronger than the others
- Remarks: Peculiar tubules through the substance of the shell in the cardinal region are characteristic of the group of shells which includes Chonetes, Chonostrophia, Chonopectus, Chonetina, and Chonetella, and show in casts of this species as small rods, but they do not seem to correspond with external spines, as there are no traces of the latter in any of the Girvan specimens It is doubtful if the so-called septa or ridges in the brachial valve are homologous with the true “brachial ridges” of Chonetes proper, and the muscle scars of the pedicle valve are more like those of Plectambonites. It seems probable that at least a subgenus should be set up for its reception, and the name Eochonetes may be suggested.