INTERNAL STRUCTURAL FEATURES OF THE SHELL OF MIDDLE CALLOVIAN AMMONITES V.V. Drushchits, L.A. Doguzhayeva, and T.A. Lominadzc Moscow University: Institute of Paleobiology, Georgian Academy of Sciences, Tbilisi ABSTRACT: The scanning electron microscope was used to study the structure of the protoconch wall, the nepionic ridge, the first and subsequent whorls, the septa, the septal necks and their accompanying elements ( cuffs and annular deposits) in Middle Callovian ammonites belonging to the families Cardioceratidae (Ca doceras, Pseudocadoceras) and Kosmoceratidae (Kosmoceras, Sigaloceras). The members of the two families have been found to have features in common and differences. Information is given concerning the size of the protoconch, the caecum and the fixator, and concerning ontogenetic alteration in the height of the whorls, the diameter of the siphuncle and the number of septa. .. .. .. The preliminary results of a study of the protoconch and phragmocone in Middle Callovian mem­ bers of the genera Cadoceras, Pseudocadoceras and Kosmoceras have been examined in a recently published paper ( Drushchits, Doguzhayeva and Lominadze, 1976). These studies were subsequently continued; the genus Sigaloceras was additionally investigated. We were able to examine some !:!hells with a scanning electron microscope (SE M). Al l the specimens investigated were collected in the main from Middle Callovian clays on the left bank of the Oka River at Yelat'ma village (Rya­ zan Province). Organic structures - fixator ( prosiphon), caecum and siphuncle - have been pre­ served in many of the ammonites. Cadoceras and Pseudocadoceras are of the family Cardioceratidae; Kosmoceras and Sigaloceras of the family Kosmoceratidae. These genera and families have long attracted attention. Their ex­ ternal characters have been described in some detail and the alteration of the suture line in the course of ontogeny has been investigated (Bodylevskiy, 1926; Shevyrev, 1960; Schindewolf, 1965). Nevertheless, the systematic position of the families to which these genera belong remains open to dispute. Arkell (Arkell, Kummel and Wright, 1957) places the families Cardioceratidae and Kos­ moceratidae in the superfamily Stephanoceratoidea. In Principles of Paleontology (1958) they were placed in different superfamilies: the family Kosmoceratidae (along withthe families Parkinsoniidae, Reineckeiidae and Morphoceratidae, which have nothing in common with the Kosmoceratidae and are now accorded to the superfamily Perisphinctoidea) in an eponymous superfamily, while Arkell's proposal was accepted for the Cardioceratidae. Shevyrev ( 1960) , who was convinced that the scientific classification of ammonites had to be phylogenetic, relied on ontogenetic studies and demonstrated the taxonomic importance of the suture line at different systematic levels. According to his data and to our own observations, very early inception of the second inner lateral lobe Jl is to be observed in the ontogenetic development of mem­ bers of the superfamily Stephanoceratoidea (including the family Kosmoceratidae) and the suture line is complicated by inner lateral lobes. Thus, for example, the final formula of the line is as follows in the genus Sigaloceras: 1 while in Kosmoceras the finalsuture line has one lobe more: ((VU2U1U2)V1) (U2U U�:I'I11 U'It D.i3:111 D, (V1V) 1 U1111211 Like kosmoceratids, members of the family Cardioceratidae are typified by very early inception of the second inner lateral lobe I', but in contrast to most of the Stephanoceratoidea, it is by the umbilical lobes that the suture line is complicated in members of this family. When we investigated the suture line in the ontogeny of cardioceratids (subfamily Cadoceratinae, genera Cadoceras, Pseudocadoceras, Longaeviceras and Quenstedtoceras), we found that development of the line was 1 2 1 completely identical until a stage corresponding to the formula: ( V1V1) ( U2U1U2) U U U3,U4U\11 D. It is only in the final stages that the differences between the genera emerge. Translated from Osobennosti vnutrennego stroeniya rakoviny srednckelloveyskikh ammonitov, Paleont. Zhur., 1977, no. 3, p. 16-29. Paleont. Jour. 1977, no. 3 271 ISSN0031-0301/77/0300-0271 :; 07.50/0 © 1978 Scripta Publishin g Co. PAL.EONTOL.OGICAL JOURNAL Complication of the suture line by the umbilical lobes is also to be seen in members of the family Macrocephalitidae( Schindewolf, 1965; Lominadze, 1967). It is on this basis that Shevyrev( 1960) quite correctly combines the families Macrocephalitidae and Cardioceratidae in the superfamily Macrocephalitoidea. Schindewolf ( 1965) differs in considering thatneither the Macrocephalitidae nor the Cardiocera­ tidae can be removed from the superfamily Stephanoceratoidea, and that there is no need to combine them in the new superfamily Macrocephalitoidea, as proposed by Shevyrev. According to Schinde­ wolf, development of the suture line is monotypic in kosmoceratids, macrocephalitids and cardio­ ceratids. The authors undertook the present investigations in view of the differences of opinion concerning the systematic position of the genera and families under consideration, based mainly on suture line ontogeny, and also because of the lack of information on the internal structure of their shells. The scanning electron microscope was used to investigate the microstructure of the protoconch wall, the nepionic ridge and the subsequent whorls, the septa and septal necks and their accom­ panying cuffs and annular deposits. Two protoconch diameters1 were measured in median section: the greater diameter passing through the proseptum (0 ), and the lesser diameter perpendicular to it (D 3), the diameter of the shell of the ammonitella(Dam), the diameters of the corresponding whorls (011 02 etc. ) , the diameter of the caecum measured along the spiral (C 1), and a second diameter measured perpendicular to the first (C3), the length of the band of thefi.xator (the term is proposed as a replacement for "prosiphon", Drushchits, 1976), the number of septa to each whorl (W 1, W2 etc.) and the distance between them, the height of the whorls of the spiral measured every half-whorl (H0• 5, H1, H1• 5 etc. ), the diameter of the siphuncle from the point at which the ° caecum gives way to the siphuncle( S0) and after every 180 (S0• 5, S11 S1• 5 etc. ), the ratio of siph­ uncle diameter to whorl height( Soflfo, S/H1 etc.), the ratio of the height of each successive whorl ° to the preceding one (H/H0, H1• JHo. 5 etc. ) calculated at intervals of half a whorl (180 ) for the corresponding heights. The angle of the nepionic line was determined as the angle between two straight lines constructed from the center of the protoconch to the proseptum and from the center to the nepionic .line located in front of the nepionic ridge. (The nepionic line at the end of the nep­ ionic ridge on the shell defines the position of the aperturalmargin of the ammonitella. A line that arose from the nepionic ridge on the cast was previously mistaken by us for the nepionic line. ) Ammonitella ( Drushchits and Khiami, 1970) is the name applied to the first or juvenile (Ivanov, 1971, 1975) stage in the postembryonic development period of ammonites. It had a shell consisting of the protoconch, the first whorl as far as the nepionic line and, probably, one or two septa. This stage lasted throughout the planktonic mode of life of the juvenile ammonite, i. e. from the time of hatching from the egg to the end of· the formation of the nepionic ridge. After its construction had been completed and the ammonite had transferred to the mode of life peculiar to the species( nek­ tonic, nektonic-benthonic or benthonic) a new period began, the adolescent period, during which the shell wall that was constructed differed in structure from the single prismatic layer in the wall of the protoconch and the first whorl, and there was formation of new septa of lamellar microstruc­ ture and a new type of shell in heteromorphous ammonites( Drushchits, Doguzhayeva and Mikhay­ lova, 1977). Only previously unknown structural characteristics of the shell are described below. Family CARDIOCERATIDAE H. Douville, 1890 Subfamily CADOCERATINAE Hyatt, 1900 Cadoceras Fischer, 1881 Materia 1. £. tschefkini Orb. - six spec. , � modiolareOrb. - one spec. , Cadoceras sp. - one spec. In median section the protoconch is ellipsoidal with a slightly flattened verter(fig. 1, c). In C. tschefkini the greater diameter is 0. 48-0. 53 (on average 0. 51), the lesser diameter 0.38-0. 39 (On average 0. 38); the difference between the two diameters is 0. 10-0. 15 (on average 0. 12) ; in Cadoceras sp. the greater diameter is 0.52, the lesser diameter 0. 35; the figures for£. modiolare are respectively 0. 52 and 0. 38. 1 In all specimens measured the diameter of the ammonitella ranged between 0. 83 and 0. 88 (on average 0. 86). The diameter of the first whorl was approximately one (between 1. 00 and 1. 16), that of the second approximately two( 1. 87-2. 10), of the thirdapproxi­ mately four( 3. 43-4.12) and of the fourthapproximately eight ( 7. 60-8. 05). _ 1All linear dimensions are given in mm. 272 V.V. DRUSHCHITS, L.A. DOGUZHAYEVA, AND T.A. LOMINADZE FIGURE 1. Diagrammatic sections through the shell in the genus Cado­ �: a - caecum aLtd commencement of siphuncle, b - junction between pro­ septa and protoconch wall, c- median section through protoconch and 1st whorl, d- nepionic ridge and nepionic line, e- structure of septal neck in 2nd whorl; all drawings, except lc, made from SEM photographs. !OMS- inner organic membrane of siphuncle, F - flange, me - membrane, PCH- initial chamber (protoconch), NR- nepionic ridge, NL- nacreous layer, NepL- nepionic line, PL- prismatic layer, PSN- prochoanitic septal neck, S, S1, � ••••- speta, SN- siphuncle, FX- fixator, c- caecum. The wall of the protoconch and the first whorl is of prismatic microstructure. One specimen was examined by SEM. Two prismatic microlayers are distinguishable in the adoral part of the protoconch wall. One microlayer dies out around the proseptum, while the other be­ comes thicker(pl. IT, illus. 1d, e; fig.