Systematic Paleontology |
(Key) |
General.--Aptychi present a particular problem to systematists. They are distinctive enough to be useful in stratigraphy, particularly where they are locally abundant enough to constitute "aptychus beds" (Campbell, 1946; Trauth, 1930). It is useful to be able to distinguish the forms by name, and indeed, many names were applied to these fossils by earlier workers before their nature was understood. However, the variability resulting from preservation and compaction has led to the erection of more species than was perhaps warranted (Turek, 1978).
As cephalopod conchs were discovered with associated aptychi, conflicts were discovered with aptychus names often having priority over those well-known for the conch. Article 23 of the Code of Zoological Nomenclature (ICZN, 1985) would, in strict interpretation, have the earlier established name become that of the taxon, although section (b) of that article indicates that if this were to cause instability or confusion, an author can refer the case to the Commission for an individual ruling. In either case, a name is suppressed.
There is a complex hierarchy of ammonite taxonomy, based entirely on characters exhibited by the conchs. Aptychi do not possess sufficient morphological variation to permit diagnosis at the generic or specific levels. Thus in some cases, single "species" of aptychi have been found to belong to two or more genera of ammonites as distinguished by conchs.
A simple solution to this problem was proposed by Arkell (1954; 1957a), to wit, the suppression of all names based solely on the aptychus of an ammonite. This proposal does assure the stability of ammonite nomenclature, at the expense of abandoning names useful for identifying aptychi as discrete entities separate from the remainder of the animal, as is often the case.
A more sweeping suggestion was made by Moore and Sylvester-Bradley (1957b) that a separate, parallel system of nomenclature be established for "parataxa;" names based on aptychi, individual conodonts, and isolated holothurian elements. In particular these names would compete with whole-animal names for the purposes of homonymy but not for priority. This proposal was fully supported by Arkell (1957b) as an extension of his original intent.
This parataxon proposal has provoked much debate, such that nearly thirty years later the question of parataxa has again been "put aside...for further in-depth study and future consideration." (ICZN, 1985; p.xii). Until this question is resolved, the assignment of specimens to specific taxa is necessarily a cautious endeavour. Arkell (1957a) suggests using Trauth's (1927, 1928, 1930, 1931, 1935, 1936) system of nomenclature as form-genera only, while others simply refer to aptychi or anaptychi in general terms if association with specific cephalopods cannot be proved (Lehmann, 1971, 1981; Harper, 1989). Trauth's genera, however, are in many cases junior synonyms of older taxa. In this work, the taxa described for Devonian specimens will be considered appropriate, reserving "anaptychus" as a morphological term only. The "genus" Anaptychus Oppel, 1856, is an erroneous citation, as Oppel used the word merely as a morphological term describing the aptychus seen in Ammonites planorbis (Citation of Oppel, 1856, in Moore and Sylvester-Bradley, 1957b). Anaptychus Stimpson, 1860, (Crustacea), and Anaptychus Schlumberger, 1868, (Cephalopoda), are junior subjective synonyms of Sidetes Giebel, 1847.
Class CEPHALOPODA LEACH, 1817
Genus SIDETES GIEBEL, 1847
Type species.--Sidetes striatus GIEBEL, 1849
Anaptychus SCHLUMBERGER, 1868; non anaptychus OPPEL, 1856, morphological term; non Anaptychus STIMPSON, 1860, Crustacea.
Pholadocaris WOODWARD, 1882
Cardiocaris WOODWARD, 1882
?Ellipsocaris WOODWARD, 1882
Lisgocaris CLARKE, 1882
Spathiocaris CLARKE, 1882
Idiotheca GIRTY, 1909
Palanaptychus TRAUTH, 1927
Neoanaptychus NAGAO, 1931
Diagnosis.--Semielliptic carbonaceous structure, weakly convex. Ornamentation of fine concentric lines about a medial anterior (by definition) rostrum, parallel to posterolateral margin, intersecting anterior margin at nearly right angles. Species S. striatus characterized by straight anterior margin; width twice the length.
Description.--Structure semielliptic, length 22mm, weakly convex. Broad, width twice length. Anterior margin straight. Posterior margin smoothly curved. Ornamentation concentric with posterior margin, perpendicular with anterior margin, finely spaced at about 15/cm. Composition carbonaceous, probably conchiolinous, with no evidence of calcareous component.
Locality of type species.--Unknown, "from hard sandstone banks near Salzburg" (Giebel, 1849). Age is Late Cretaceous (Senonian).
Type.--Location unknown.
Remarks.--Five species of Sidetes, described below, are recognized from the Ohio Shale. All are preserved as carbonaceous films, compressed and flattened to varying degrees. None appear to be accompanied by a calcitic or aragonitic component. All have a roughly semielliptic outline, and bear concentric ornamentation which terminates at the anterior margin in a manner unlike that of brachiopods or bivalves.
In addition, the specimens described by Cooper (1932) are here reexamined, as they were collected from the time-equivalent Woodford Shale of Oklahoma, a unit similar in character and depositional setting to the Cleveland Shale (Cardott and Lambert, 1985). The Woodford specimens are preserved in a different manner than the Ohio forms, however. They do not exhibit the extreme flattening common with the Ohio specimens, but are to varying degrees three-dimensional, with a significant mass of matrix material preserved within the interior of the structure, almost as a "steinkern." In general, they seem to have been preserved within calcareous or phosphatic concretions, a common alternative mode of preservation for (Upper-) Paleozoic aptychi (Mapes, 1987). All are similar to the Ohio specimens, with the addition of one species, S. gouldi, not recognised in the Ohio material.
For comparison, we also include two Virginia specimens of S. emersoni, Clarke's (1882) type species of Spathiocaris, originally described from New York.
In his discussion of Spathiocaris lata, Ruedemann described transverse frontal grooves reaching halfway to the anterolateral angles. These grooves caused him to, "recall those of the aptychus of some ammonites" (Ruedemann, 1916). Such grooves are not seen in any specimen in this study. It is possible that they may represent in some manner a reflexed portion of the anterior margin analogous to the short inner lamella seen in anaptychus-type ammonite jaws from the Mesozoic (Lehmann, 1979; Kanie, 1982; Tanabe, 1983). Such structures may well have been obliterated in highly compressed material as is common in the Ohio Shale. Exterior molds, of course, would not reveal the inner lamellae. Finally, the Woodford Shale (Cooper, 1932) material has not been prepared to reveal the interiors of the convexly folded specimens, so such structures may well be preserved within the matrix which remains.
The species are differentiated on the basis of the ratio of breadth to length, the angle of the anterior margin, the general outline of the structure in its extended, flattened form, and, to a lesser degree, the nature of ornamentation. To this end, the following key is provided as an aid in species identification:
A Key to the species of aptychi from the Ohio Shale:
1a. Width greater than length. > 2
1b. Width equal to or less than length. > 3
2a. Width twice length; ornamentation finely spaced, 16/cm. S. newberryi
2b. Width about 3/2 length; ornamentation coarse, about 8/cm. S. gouldi
3a. Width half to 4/5 length. > 4
3b. Width about equal to length. > 5
4a. Anterior margin acutely convex; outline elliptical. S. ulrichi
4b. Anterior margin straight or broadly concave; outline triangular. S. chagrinensis
4c. Anterior margin acutely concave, notched. S. emersoni
5a. Outline sub-pentagonal, lateral margins nearly parallel, meeting anterior margin at distinct angle. S. lutheri
5b. Outline semielliptical, greatest width near anterior margin, narrowing posteriorly. Anterior margin broadly concave. S. lata