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.

Fin. 3. Resorption on the inner walls of the gastropod Conus.

margins of the restricted aperture during growth. Continuous growth lines are clearly visible Apertural rim growth beyond the sinuses of the aperture of some oncocerids (Fig. 1). Accordingly, the shape of A question about shell building among the aperture is the result of regulated growth, not oncocerids is: Did resorption take place in the of successive resorption. Furthermore, in some older septa, as in some gastropods (Fig. 3), or on taxa, especially among the Trimeroceratidae, the the apertural margin? If partial resorption of a shell is notably thicker along the growth line just septum did take place, it would be thinner in a proximal to the deepest part of a sinus (Fig. 4). mature animal than it would be when newly An increased thickness is also clearly visible on formed. Because of recrystallization, possible Gomphoceras surgens figured by Barrande 1877, thinning of septa cannot be measured on fossil PI. 515:15-16, where Fig. 16 shows-both the material. For comparison, however, I counted inside and the outside of the shell (Fig. 5 herein). the septa in juvenile Nautilus specimens in order The thicker area probably functioned as a rein- to identify corresponding septa in different indi- forcement to the apertural part of the shell. viduals. The result of my measurements is that Because of the narrow aperture system of the the last-formed septum in a juvenile Nautilus is shell the reinforcement had to be located behind no thicker than the corresponding one in a the actual opening to leave space for the tenta- mature individual. Hence there is no measurable cles and their activities. The inserting lobes in later resorption. A more complicated matter is the aperture are notably thin, even compared whether or not resorption took place along the with the shell behind the reinforcement. LETHAIA 14 (1981) Apertural constrictions in oncocerids 273

of the reinforcement. In the latter case the specimen might as well have had a contracted aperture (aperture having the smallest diameter of body chamber). A true constricted aperture can be determined only in specimens in which the shell around the body chamber is still preserved (Fig. 5). When the sinuses were formed, the aperture became smaller through shell accretion laterally and in some cases also dorsally. Lobes thus grew in from the sides, making the opening smaller (Fig. 6). From Fig. 7 it is evident that two or more specimens of one and the same species may well Fig. 5. Gomphoceras surgens (Barrande) PI. 515:15-16 (Bar- show different apertures. Barrande (1877, PI. 47; rande 1877). The shell thickening__ iust behind the aperture is text page) described how his Phragmoceras cal- clearly shown on these drawings. It is notable that the exterior /istoma (Barran&) might have either three or of the shell does not show any constriction at all. four pairs of lobes. He also mentioned the typic- al asymmetry on the third and fourth pair of From a taxonomic point of view it is most lobes, which is very common among some important to decide whether what looks like a genera in the family Hemiphragmoceratidae constricted aperture (smallest diameter of body (Fig. 7). This asymmetry of sinuses might have chamber closely behind the aperture) in a stein- been caused by the position of the appendages, kern is a true constricted aperture or only a cast as these prevented shell secretion by the mantle

Fig. 6. Six successive growth stages of the same specimen, a hemiphragmoceratid. Tfiese stages have been identified by blackening out all subsequently formed parts of the shell, following the growth lines. RM Ma 56802. 274 St9en Stridvherg LETHAIA 14 11981J

Fig. 7. Four specimen5 of the same species (sofar undescribed), a member of Hemiphragmoceratidae. All X 0.7. 0 A. (RM Mo 56796) with 68sinuses. 0 B. (RM Mo 5681 I) with 7 sinuses. 0 C. (RM Mo 56802) with 6 sinuses. 0 D. (RM Mo 56790) with 5 sinuses. The drawings correspond to the photographs. The hyponomic sinus is very poorly preserved.

because of their activities. Notable in all speci- Conradoceras pseudoconradi, but based his mens is that the asymmetric pair, or pairs, seem conclusions on the illustrations in Barrande to indicate that the left sinus is situated more 1865. If he had been given the opportunity to ventrally than the right. (Orientation: facing the study the type specimen (Fig. 8 herein) he would aperture and turning dorsal side up.) have noticed the beginning of two lobes, not just It is also obvious that the limited development one, growing from one of the sides (the aperture of sinuses on the ventral side must mean a is destroyed on the other side). Thus the indi- reduction in the moving facilities of the soft parts vidual had three pairs of sinuses and died when and might perhaps indicate a change of life in a growth stage similar to that in Fig. 6C. habits. Space reduction for the soft parts is also a Nevertheless, Foerste was right in his conclu- stage that each individual has suffered during sion that this specimen is not an immature growth from juvenile to adult (Fig. 6). Tetrameroceras. Its aperture is very similar to In individual development, the shell aperture that of Ortamerella (Teichert & Sweet must have passed all stages from wide open to 1961:611), but the latter genus has three pairs of final closure (Fig. 6). During consecutive phases lateral lobes. Conradoceras, however, has only the shell might have recalled the morphology of two pairs of lateral lobes but is not congeneric other less complicated taxa. Immature speci- with Hexameroceras Hyatt 1884, the lobes of mens are very rare, but if we were to sludy the which are mainly dorsal. It is very likely that the peristome of the type specimen of Conradhceras type specimen of Conradoceras pseudoconradi pseudoconradi Foerste 1926 (illustrated in Bar- as mature would have become a shape much the rande 1865, PI. 49:4-7), we would find such a same as that of the specimen in Fig. 7C. Furth- specimen. In his description Foerste states that ermore, Foerste mentioned in his description of Conradoceras has two scallops but ‘is not an Conradoceras (Foerste 1926:361) that specimen immature form of Tetrameroceras Hyatt 1884, 2166A in the collection at Harvard University though species of the latter genus must have evidently belongs to the same genus but had passed through a similar stage’. As far as I know, ‘three scallops along the antero-lateral margin of Foerste did not see the type specimen of his the dorsal lobe of the aperture’. According to LETHAIA 14 (1981 Apertural constrictions in oncocerids 275

fishes, which should have affected the mode of life of the cephalopods. A restricted aperture is one way to solve the protection problem. As restriction only occurred among the adult individuals and supposing this modification did act as a means of protection, it might be assumed that the juvenile, unprotected individuals were adapted to an environment different from that of the adults. Another group of Silurian cephalopods (orthoconic) developed an aptychopsid operculum (Turek 1978; Holland et al. 1978), a device later widespread and further developed in the ammo- nites. An Aptychopsis made up of calcite (Turek 1978) must have had the primary function of protection of the soft parts, which indicates that protection was essential for some groups of cephalopods during this period. So there is good reason to assume that the restriction of the aperture among the oncocerids played a role for the protection of these cephalopods against Fig. 8. Photograph of the type specimen of Conradoceras pseudoconrudi Foerste 1926 (illustrated with a drawing on PI. powerful predators. 49: +7 in Barrande 1865). On the left hand side of the aperture (to the right in the picture) the beginning of two lobes are Ackn~,M,lc,dRement.s. - Thi\ study was carried out at the visible. The dorsal one was not mentioned by Foerste when he Department of Historical Geology and Palaeontology, Uni- made the description of Conradoceras. Specimen at Narodni versity of Lund and I am indebted to the former head of the Muzeum. Prague, L9150. Department, Gerhard Regnell, and the present temporary head of the Department, Anita Lofgren, who both critically read the manuscript. The non-silicified fossils used belong to Nafur- historisku Riksmusiet (RM) in Stockholm and Narodni Mu- zeum in Prague, and for access to this material I thank Valdar Foerste’s description, Conradoceras had only Jaanusson and Vojtech Turek respectively. The material de- two scallops, but as the generic description was rives from a special effort under the designation Silicified based on only one specimen of Conradocerus Fossils, organized within Project Ecostratigraphy by Lennart Jeppsson. The originals are deposited in the collections of pseudoconradi he could not possibly evaluate Sveriges Geologiska Undersdkning (SGU) in Uppsala. Harry the variation within the genus. The specimen Mutvei (RM) made very valuable comments on the manu- 2166A might represent a stage between the type script, and further improvements were made by Jan specimen of Conradoceras and a fully mature Bergstrom (SGU) and Lennart Jeppsson (Department of His- torical Geology and Palaeontology in Lund). To all these specimen. persons 1 give my sincere thanks. The necessary result of shell studies of the Hemiphragmoceratidae shows that the number of sinuses and their size alone are not reliable characters for the discrimination of species.

Form and function The advantage of a restricted aperture like that References of the oncocerid cephalopods is not obvious. Barrande, J. 1865: Systemc tilrtrien du centre de la Boheme. One explanation might be that aperture closure Premiere Purtie: Recherches pa1eontologique.s 2. Ciphalo- promoted protection of the soft parts. No form podes. PIS. 1-107. Praha. of restriction is known in any earlier cephalo- Barrande. J. 1877: Systemr silurirn du centre de lu Beheme. pods except some of the tarphycerids and the Premiere partie: Recherches pal8onrologique.s -7. Closse des ellesmerocerids. However, during the Silurian a Mollusques, Ordre des Cephulopodes. PIS. 461-544. Praha. Collins, D., Westermann. G. E. G. & Ward, P. 1978: The new group of predators reached an increasing mature Nautilus: its shell and buoyancy. Geological Sociery importance in the marine faunas, namely the of America. Abstructs with programs 10, 382. 276 Sven Stridsberg LETHAIA 14 11981)

Collins. D., Ward, P. & Westermann, G. E. G. 1980: Function Kahn, P. & Pompea, S. 1978: Nautiloid growth rhythms and of camera1 water in Nautilus. Paleobiology 6, 168-172. dynamical evolution of the Earth-Moon system. Nature 275, Denton. E. J. & Gilpin-Brown. J. B. 1973: Flotation mechan- 606-61 1. isms in modern and fossil cephalopods. Advances in Marine Reyment, R. A. 1958: Some factors in the distribution of fossil Biology I/. 197-268. cephalopods. Stockholm Cvntribution in Geology I. Acta Foerste. A. F. 1926: Actinosiphonate. trochocerid, and other Universitatis Srockholmiensis. cephalopods. Denison Uniwrsit?. Bulletin. Journal of the Stenzel. H. B. 1964: Living Nautilus. In Moore, R. (ed): Scientfk Lahorutories 21, 285-384, pls. 32-53. Trentise on Incerrebrnre Paleontology K,KSY-K93. Flower, R. H. 1939: Study of the Pseudorthoceratidae. Teichert. C. & Sweet, W. C. 1962: Nomenclature notes. Paleont. Americanu -7: 10, 214 pp. Octamerello, new generic name for a Silurian oncocerid Holland. B.. Stridsberg, S. & Bergstrom, J. 1978: Confirma- cephalopod. Journcil ofPuleonrology 36, 61 1-613. tion of the reconstruction of Aptychopsis. Lethaia 11, 144. Turek. V. 1978: Biological and stratigraphical significance of Hyatt, A. 1884: Genera of fossil cephalopods. Boston Socie- the Silurian nautiloid Aptychopsis. Lethaia /I, 127-138. r?. of Nuturd Hi.ytory 22, 277-278.

LETHAIA REVIEWS Lethaia, Vol. 14, p. 276. Oslo, 1981 12 15

A disguised national atlas of Foraminifera

ANDERS MARTINSSON

Lethuia would hardly have accepted the Strutigruphicd Atlas been particularly well investigated there. The pre-Carbonifer- of Fossil Foruminifera within its highly selective review pro- ous faunas, for example, occupy just a little more than a page gramme if there had been the slightest indication in the title, of text, about a third of which is a reference list. In this atlas, blurb or catalogue texts that it was not an international pub- published under wch a general title, there is no picture to lication. On the contrary, the publisher recommends it as ‘a illustrate how pre-Carboniferous foraminifers look. The necessity for oil companies, palaeontological laboratories, and Quaternary has yielded If pages of text, a table and a plate. all areas of world geological investigation, including off-shore The column between these two extremes is more generously exploration in Europe, the Americas. and Asia’ [why not Afri- covered by well-organized team-work, generally profusely ca?]. Possibly a restriction could have been traced in another illustrated with plates of somewhat varying quality and graphs sentence: ‘The volume will have reference value throughout which sometimes display strikingly inferior reproduction of the world, and particular application in Britain and Western lines and lettering 1e.g.. Figs. 6.1 .I, 6.1 .2 and 7.21, not affect- Europe, and in North America.’ It is not until the reader opens ing their information value, however. the book that he discovers that it is a sister publication of A So, with reiterated regret that another publisher has joined Stratigraphical Index of British Osfrucoda, published under a the Springer trend of issuing textbooks or unduly textbook-like correct and honest title by another publisher, the See1 House publications under ostentatious titles without coverage in the Press in Liverpool. Not even the binding or the size are the text, the Atlas is recommended for what it really is, namely a same, such that !he two volumes hitherto published in the British national inventory of Foraminifera with a considerable ‘British Micropalaeontology Society Series’ could have match- international reference value as regards the Carboniferous- ed each other on the shelf - why make the spine 5 mm shorter Neogene sequence. when there is nothing in the body of the book that dictates such an arrangement? Of course a book on the foraminifers of a small area in Europe has an international reference value, as have a great Reference many local investigations, particularly when representing the same high quality as the present one. This international refer- Jenkins, D. G. & Murray, J. W. 1981: StratigraphicalAtlas of ence value, however, depends greatly on which marine se- Fossil Foraminifera. 310 pp. Ellis Honvood Limited, quences are present in Britain (and Ireland) in a facies suitable Chichester (distributors: John Wiley & Sons). ISBN for foraminifers, and which parts of the column happen to have 0-853 12-210-5. hice f25.00.