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Apertural Constrictions in Some Oncocerid Cephalopods

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Apertural Constrictions in Some Oncocerid Cephalopods Apertural constrictions in some oncocerid cephalopods SVEN STRIDSBERG Stridsberg, Sven 1981 12 15: Apertural constrictions in some oncocerid cephalopods. Lethaia, Vol. 14, IETHAIA pp. 269-276. Oslo. ISSN 0024-1 164. In some oncocerid cephalopods the shape of the aperture, siphuncle and the general outline of the shell have long served as generic characters. The aperture is mostly elaborated into a certain number of sinuses which take their final shape only in the adult. Therefore, knowledge of the relative age of the animal is required. The last chamber may serve as an indicator of age. A last chamber smaller than the second last indicates a mature specimen. This is because continued growth would have caused the death of the animal as the buoyancy turned negative. Moreover, it is of great importance to study the growth lines along the peristome to observe whether growth has ceased or not. Growth variations have been compared with growth stages. Furthermore, a constricted or contracted aperture can only be determined on specimens with the shell still preserved. Functional parallels are drawn between the Aprychopsis operculum and the restricted aperture. 0 Cephalopoda, Oncorerida. aperture, ontogeny, growth lines, functional morphologv, Aptychopsis. Silurian,Gotland. Sven Stridsberg, Geologkka Institutionen, Siilvegatan 13, S-223 62 Lund. Sweden: 16th January. 1981. Two families within the oncocerid cephalopods, whether a certain specimen is adult or not. The namely the Hemiphragmoceratidae and the problem is to decide which character or charac- Trimeroceratidae, have a very complex aper- ters we can use to determine adulthood. As there ture, consisting of a varying number of sinuses. are no soft parts to examine, the characters must These sinuses have played an important role in be found in the growth pattern of the shell. establishing new genera. Because of the number Stenzel (1964: K87) gives four indicators for of sinuses, different genera like Tri-, Tetra-, determination of maturity in living Nautilus: ‘( 1) Penta-, Hem-, and Octameroceras have been thickening of the aperture margin, (2)decrease in described. the volume of the last chamber, (3) greater However, on dealing with many specimens in thickness of the last septum, and (4) widening of the above-mentioned families, it became evident the black border on the inside of the shell that some irregularities occur in the sinus pat- aperture.’ terns. Whether this is due to normal variation in However, on fossil material some of these a population of adult individuals or whether indicators cannot be used. Indicator (1) will be growth stages are also involved has not been discussed under ‘Apertural rim growth’. Under discussed previously. Further ontogenetic stu- the heading ‘Septal growth and chamber size’ dies are therefore required before it is possible to indicator (2) will be discussed. Point (3) is most decide whether a certain specimen is mature or unreliable because of recrystallization phe- not. During the final growth of the shell the nomena (Reyment 1958: 146-147). Point (41, aperture gets smaller, as can be seen on the finally, will of course not be found on fossil growth lines around the aperture on well pre- material. served specimens (Fig. 1). This means that two Collins et al. (1978) have also studied maturity conspecific specimens of different ontogenetic in Nuutilus. Only one of the characters that they age do not look the same; the younger one has a use can be applied to fossil material, namely wider aperture. This intraspecific variation of deepening of the ocular sinuses. They state that aperture configuration might have caused the ‘approximation and increased thickening of the establishment of new species with the current final one or two septa are reliable indicators of systematic approach. near, rather than full, maturity of the shell’. Maturity determination Septal growth and chamber size In order to ensure that new species or genera During its growth the nautiloid produces a new are described on a solid basis we must know septum at regular intervals. Whether this is 210 Siren Stridshrug LETHAIA 14 (1981) Fig. I. 0 A. Growth lines around the aperture of a trimeroceratid. SGU Type 1213. The same specimen as in Fig. 4.0 B. Growth lines on an inserting lobe on what might be the remains of a hemiphragmoceratid. SGU Type 1214. Both specimens are silicified and were found at Mollbos. Gotland strictly dependent on time or not has been The chamber building described above is re- discussed (Kahn & Pompea 1978), but it is peated until the cephalopod has reached its adult probably a more physical impulse, such as stage. Theoretically, we would expect that every buoyancy, that controls growth. As the soft new chamber provided space enough to allow parts increase in volume during growth, the body gas in an amount representing additional floating chamber has to grow larger. The increasing capacity large enough to counteract the greater weight due to progressive growth of both the soft weight imposed by a new septum, enlargening of parts and the shell forces the animal to build a the body chamber and enlarged soft parts. new septum to maintain buoyancy. Before the However, after the very last septum has been new septum has been completed, the increasing built, the space behind it will, accordingly. not weight from the construction must be balanced be required as compensation for a following by liquid pumped out of the shell. After complet- septum, and therefore this last chamber can be ing the new chamber, which to begin with is distinctly smaller than the next-to-last. Up to the filled with liquid to withstand the pressure last chamber the space between successive septa (Denton & Gilpin-Brown 1973:251), the animal has increased regularly. The fact that the last can continue to increase the body chamber by chamber in mature or almost mature Na~tilusis adding shell material in the apertural region. This smaller than the next-to-last is well known (Col- procedure can continue as long as the increasing lins etal. 1978), but thus far no explanation of weight can be counterbalanced by the liquid in the mechanism behind has been given. the new chamber. The small last chamber allows the apertural The assumption that Nuuti1u.s uses the liquid rim to grow with a mass equivalent only to that in the chambers only for enabling vertical move- which can be compensated by the space in the ment has been opposed by Collins et al. (1980). last chamber. After this phase the animal cannot They concluded that the camera1 fluid in adult build additional septa, for there is no possibility specimens functions only as ballast. to compensate for the added weight of this wall. The question arises as to what mechanism is The pattern of steadily increasing chamber involved in vertical movement. Is this accom- volumes up to the last chamber was not always plished by swimming alone? maintained by cephalopods with orthoconic lon- LETHAIA 14 (19811 Aperturul constrictions in oncocerids 27 1 Fig, 2. 0 A. An adult Nnirti/us pompilius (Linnaeus) with a notably diminished last chamber. B. A juvenile specimen of the same species with a normally increased last chamber. giconic shells. After the formation of a certain The study of recent Nautilus has clearly number of chambers with normal increase in shown that the last chamber is small in adult volume, chambers with distinctly increased specimens (Fig. 2A) and that this is not the case volume may appear, the volume then decreasing in juvenile animals (Fig. ?B). Like the described to ‘normal’ again in successive chambers. This oncocerids, Nairtilirs does not have cameral or phenomenon may be explained by the fact that siphonal deposits. those shells had cameral and/or siphonal de- On studying the fossil material at my disposal I posits, which may have had the function of have found that a great majority of the speci- balancing the cephalopod so that it could main- mens of Hemiphragmoceratidae do have a small tain a horizontal orientation during growth. De- last chamber (Fig. 4). As the animal could not position of CaC03 is supposed to have begun grow any larger after having formed a small during the juvenile stage (Flower 1939). and in chamber, because of the buoyancy conditions order to compensate for the weight resulting discussed above, it must be concluded that these from the above-mentioned shell growth plus the specimens are adult. The only alternative ex- deposits in previously formed chambers or in the planation is that the individual had stopped siphon, the volume of the new chamber had to growing permanently because of some internal increase distinctly. After the formation of some physical disturbance, as the buoyancy gained chambers large enough to balance and compen- from the small chamber is consumed by the sate CaCOl growth and deposition at various necessary growth of the body chamber in the places in the shell, smaller chambers could apertural area. Thus the growth of another sep- appear again, indicating a temporarily inhibited tum would have ended with the grounding of the deposition. individual. As the apertural rim growth is thus Thus the pattern of cameral size may be used the final growth event, there might be a small to identify periods of formation of cameral and variation among almost fully grown specimens of siphonal deposits. the same species with a small last chamber. 272 Sven Stridsberg LETHAIA 14 (1981) Fig. 4. The reinforcement on the inner ventral side of a trirneroceratid. The specimen is silicified and was found at Mollbos, Gotland. The same specimen as in Fig. IA. SGU Type 1213. In the lower part of the picture the remains of the last two septa can be seen. The last chamber was notably smaller than the next-to-last.
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