LETHAlA SEMINAR

!Lethaia, Vol. 1 I , p. 144, Oslo, 197804 I5

Confirmation of the reconstruction of Aptychopsis BRIAN HOLLAND, SVEN STRIDSBERG AND JAN BERGSTR6M

Aptychopsis Barrande was recently described from the upper Middle Silurian at dstra Odarslov. southern Sweden (Berg- strom in Laufeld et al. 1975). The triangular third plate was not found at that time, and it was suggested that Aptychopsis belonged to cephalopods as jaw structures in accordance with the model of Lehmann (1972).

Recently a number of additional specimens have been col- lected at the same locality. These include specimens with the two lateral plates in association as well as specimens with all three plates conjoined. In addition, there are three specimens (collected by Louis Liljedahl and Brian Holland) lying in the opening of the living chamber of a particular type of orthoconic cephalopod shell. In all three cases the triangular notch is concealed just inside the aperture. This consistency in the orientation is definite proof that the aptychopsid belongs to the cephalopod, as suggested by Turek (1978). The calculated fit in size between apertural and aptychopsid outline together with the existence of a third plate which would restrict folding ability proves that Turek (1978) is right in regarding the aptychopsids as opercula rather than as lower jaws. If the triangular plate is dorsal, which seems most likely, the orientation in the three speci- mens shows that the aptychopsid was canied dorsally, as indicated in Turek’s reconstruction. It is therefore a pos- sibility that the two lateral plates were formed by two tentacles like the hood of Nautilus. while the triangular plate may have been formed by the dorsal body surface adjacent to the tentacles.

This raises the question of the nature of ammonoid aptychi and anaptychi. Dorsal exterior structures cannot be trans- formed into ventral jaws. If the jaw interpretation for ammonoids is at all correct, aptychus-like plates must represent two quite different structures. In our opinion, it is possible that some ammonoid aptychi, like Laevaptychus of Physodoceras. in fact represent dorsal structures homologous with aptychopsids (cf. Schindewolf 1958, PI. l : la, b), while other structures with similar growth lines may be ventral jaws [Lehmann 1972 and references therein). It is fully possible that there is a direct phylogenetic connection between aptychopsid opercula and ammonoid opercular aptychi.

The new observations indicate also that the Lower Palaeozoic presumed crustaceans Peltocaris and Discinocaris of similar morphology, inclusive of the additional plate in the notch, may be opercula of orthoconic cephalopods. We do not agree with Turek (1978) that Discinocaris may be a crustacean. Aptychopsis and Peltocaris compare with aptychi and may conveniently be termed paraptychi. while Discinocaris is similar to anaptychi and may be called a paranaptychus.

References Laufeld, S.. Bergstrom, J. & Warren, P. T. 1975: The

boundary between the Silurian Cyrtograptus and Colonus

Fig. 1 . 0 A and B. Two specimens of orthocone cephalopods with Aptychopsis opercula in the aperture of the living chamber. Dorsal views. Note chamber walls and living chamber in A. LM LO 5268 and LO 5269. 0 C. Aptychopsis operculum with all three plates in position. Exterior view. LM LO 5270. All figures natural size.

Shales in SkPne, southern Sweden. Geol. Foren. Stockholm Forh. 97, 207-222.

Lehmann. U. 1972: Aptychen als Kieferelemente der Ammo- niten. Palaont. 2 .46 , 34-48.

Schindewolf, 0. 0. 1958: Ober Aptychen (Ammonoidea). Palaeontographica A:l 11, 1-46.

Turek, V. 1978: Biological and stratigraphical significance of the Silurian nautiloid Aptychopsis. Lethaia 1 1 . 127-138.

Brian Holland, Sven Stridsberg and Jan Bergstrom, Depart- ment of Geology, Solvegatan I3# S-22362 Lund. Sweden; 2nd December, 1977.