What is the common change in coiling in the late Cretaceous didym?

In the Late Cretaceous Didymoceras, the Ushaped part is separated from a helicospirally coiled preadult shell, whose coiling axis forms an angle of 60–90° to that of the terminal demi-whorl (e.g. Kennedy et al. 2000).

What is the effect of dimorphism on ammonoid populations?

In many cases, the mix of intraspecific variability within both sexes, evolutionary changes, phenotypic plasticity (Wilmsen and Mosavinia 2011) and dimorphism blurs the patterns of disparity in ammonoid populations to such extent that the various phenomena can hardly be distinguished (Kennedy and Wright 1979; Reyment 1988; Kassab and Hamama 1991).

What are the common modifications that have been documented from ammonoids?

Among the mature modifications known from nautilids listed above, the majority has also been documented from ammonoids, except the shell growth band, the septal thickness, the white venter, and the reduction of the cameral liquid.

What is the common type of lappet in the middle Jurassic?

Another example has been discussed by Keupp and Riedel (2010): in the Middle Jurassic microconch Ebrayiceras, the lateral lappets are very large (half the size of the last whorl) and nearly fused with smaller ventrolateral lappets, thus forming oval ventrolateral openings.

What is the effect of the size of the males on the reproductive cost?

The energetic cost of reproduction (the energy that is available for the ovaries and thus eggs) can be significantly reduced when the size of the males, and thus their energy intake, is reduced.

How many septa are more tightly arranged than the preceding ones?

More than twenty septa might be more tightly arranged than the preceding ones (e.g., Pernoceras crebriseptum in Korn and Titus 2006), documenting a prolonged reduction of the growth rate near the termination of growth (Fig. 7.1).

Open AccessBook Chapter10.1007/978-94-017-9630-9_7

Mature Modifications and Sexual Dimorphism

Christian Klug, +4 more

- 01 Jan 2015

- pp 253-320

TL;DR: Most ammonoids display a distinctly different morphology when they are adults or subadults, and depending on the taxon, these mature modifications may comprise changes in coiling, changes in ornamentation, and conspicuous changes of the terminal aperture.

Abstract: Most ammonoids display a distinctly different morphology when they are adults or subadults. Depending on the taxon, these mature modifications may comprise changes in coiling, changes in ornamentation, and conspicuous changes of the terminal aperture. These mature modifications permit, at least when a combination of which occurs, to identify adult specimens. Co-occurrence of adult forms with differences in size, ornament strength, and aperture shape led already half a century ago to the conclusion that many ammonoids display dimorphism. The smaller forms are usually dubbed microconchs and the larger ones macroconchs. The degrees of differences between the antidimorphs vary and are more or less characteristic for certain clades. Such dimorphism is also known from Recent cephalopods. According to actualistic comparisons and considerations concerning reproductive biology of Recent cephalopods, it is commonly assumed that the microconchs represented the males and macroconchs the females. This hypothesis still needs verification from soft-tissue preservation.

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Most frequently asked questions

1. What are the contributions mentioned in the paper "Mature modifications and sexual dimorphism" ?

Klug et al. this paper discussed the relationship between mature modifications and sexual dimorphism in ammonoid shells.

2. What is the conspicuous adult modification in the Cretaceous?

The possibly most conspicuous adult modification in the Cretaceous among the regularly coiled ammonites is that of the Albian acanthoceratoidean Mortoniceras, which forms a long ventral spine, which may be directed ventrally ( M. equidistans) or even curved posteriorly ( M. rostratum), forming almost a complete loop (e.g., Cooper and Kennedy 1979).

3. What did he argue that caused the change in whorl expansion rate?

He argued that this change in whorl expansion rate was linked to a change in body chamber length, which, in turn, caused a change in the syn vivo orientation of the shell.

4. What is the plausible line of reasoning?

The most plausible line of reasoning appears to be that of Lehmann (1981), who inferred that the macroconchs were the females because the maturation of eggs takes longer than that of spermatophores, implying a longer lifespan and thus a larger adult size.

Fig. 7.3 Black layer and black band in Mesozoic ammonoids (a–d from Klug et al. 2007; e–i from Klug et al. 2004). a Phylloceras heterophyllum, SMNS 26462, tenuicostatum Zone, Ohmden, Germany; dm 87 cm. Note the jaws and the black band. b Harpoceras falciferum, falciferum-bifrons Zone, Holzmaden, dm 24 cm. c Psiloceras planorbis, PIMUZ 6519; planorbis Zone, Hettangian, Blue Anchor, Somerset, UK, dm 45 mm. Note the black band and black layer. d Lytoceras ceratophagum, SMNS 26465; falciferum Zone, Toarcian, Ohmden, Germany; dm 41 cm. e, f Paraceratites atavus, lateral and dorsal views, SMNS 24503, atavus Zone, Neckarrems, col. M. Warth; dm 61 mm. g–i, Ceratites spinosus, lateral and dorsal views, SMNS 25255–33, spinosus Zone, Heckfeld; dm 102 mm. Images: a, b Staatliches Museum für Naturkunde Stuttgart. d Urweltmuseum Hauff. c T. Galfetti. e–i W. Gerber

Fig. 7.6 Triassic mature modifications. a, b Halorites macer, PIMUZ 31068, Norian, Bihati?, Timor, Indonesia, dm 103 mm. Note the change in whorl section. c, d Orestites cf. frechi, PIMUZ 31069, Norian, Timor, Indonesia, dm 75 mm. Elliptical coiling and constricted aperture. e, f Lobites ellipticus, PIMUZ SQL43399, Norian, Timor, Indonesia, dm 62 mm. g Arcestes sp., PIMUZ 31067, Norian, Timor, Indonesia, dm 152 mm. h Halorites cf. phaonis, PIMUZ 31066, Norian, Timor, Indonesia, dm 124 mm. i, j Ceratites cf. compressus, MHI 688732, Anisian?, Garnberg, Germany, dm 75 mm. k, l Protrachyceras archelaus, from Mojsisovics (1882), Ladinian, Agordo, Italy, dm 132 mm

Table 7.1 Sexual dimorphism in Devonian to Triassic ammonoids

Fig. 7.12 Modified peristomes of adult microconchs of the Jurassic and Early Cretaceous. Note the rather uniform lappets in the haploceratoids, the rather straight lappets in the perisphinctoids, and the disparity in the lappets among the stephanoceratoids. Image sources: V. Schlamp ( Elatmites, Paralingulaticeras, Parataxioceras lothari), J.-S. David ( Cadomoceras cadomense; Cymbites laevigatus; Ebrayiceras pseudoanceps; Hildoceras lusitanicum; Morrisiceras schwandorfense; Oecotraustes bomfordi); D. Bert ( Kosmoceras phaeinum); H. Chatelier ( Saynoceras); Quenstedt (1885: Amaltheus margaritatus; Leioceras opalinum); R. Roth ( Creniceras crenatum; Sutneria platynota); P. Branger ( Indosphinctes; Normannites orbignyi); Dietl (2013: Grossouvria). Djanelidzé (1922: Spiticeras kiliani, modified or reduced); Atrops and Reboulet (1995: Neolissoceras grasianum, modified or reduced); Ernst and Klug (2011: Oecoptychius refractus)

Table 7.2 Mature modifications in Devonian and Carboniferous ammonoids

Fig. 7.8 Adult specimens of the antidimorphic couple Oecoptychius refractus (Reinecke 1818) ( top left; microconch) and Phlycticeras pustulatum (Reinecke 118) ( bottom, macroconch). Herznach-Member, Callovian, Swiss Jura Mountains. Note the morphologic and size difference

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