“It is a remarkable fact that all ammonites with lappets (but not necessarily those with rostra) are smaller than those with a simple aperture which most closely resemble them in other respects”. W. J. Arkell’s succinct summary (1957) of the facts almost says it all, but he was shy of interpreting them, though ideas about sexual dimorphism in ammonites were widespread at the time. The notion of dimorphism in ammonites has a long history but only a brief outline follows. But first, two useful terms may be introduced: microconch (m) for smaller forms with lappets; and macroconch (M) for larger forms with simple apertures. The terms were introduced in this sense by J. Callomon 1962, and fig. 1 illustrates the difference.
As long ago as 1840 a Frenchman, M.H.D. de Blainville, proposed the idea that ammonites were bisexual. He offered no examples, but it was a safe assertion since ammonites are cephalopod molluscs and all known cephalopods are bisexual. His suggestion was that, by analogy with the living Nautilus, there would be differences in the inflation of the body chamber, with the egg—bearing female being the larger. But not all cephalopods are dimorphic: Argonauts have minute males parasitic on the much larger female, while the sexes of Sepia spp. are morphologically indistinguishable.
Seven years later A. d’Orbigny (1847) offered an example of dimorphism in the Middle Jurassic ammonite Reineckia, and based his distinction on slight differences in inflation of the shell, following de Blainville by analogy with Nautilus. The idea of sexual dimorphism was certainly in the air but the real criterion for distinguishing males from females still eluded palaeontologists.
W. Waagen in 1869, while classifying the Oppeliidae, noticed that two evolving lineages showed similar, but distinct, forms at the same stratigraphical level throughout the Middle and Upper Jurassic. Nevertheless, he rejected the notion that the two parallel lineages were males and females of just one evolving lineage.
P. Reynes in 1879 was easily convinced that most species of ammonites have two distinct forms, differing by sizes and ornament, whenever there was sufficient material, and asked “To what to attribute this difference…….. if not to sex?”
Then, in 1886, Quenstedt, after noting two examples of small forms with lappets occuring with larger forms with simple apertures, seemed unable to commit himself to the idea that these were males and females of the same species. It is a common experience in science for a worker to have the answer to a problem in his hand and not to recognise it. Analogy with Nautilus again got in the way.
The model of Nautilus was a mistake leading palaeontologists to compare only macroconchs, large and without lappets, for size differences, while regarding smaller microconchs, with tappets, as irrelevant. Lappets were accepted as significant but no one had any idea what they signified. It continued to be believed that the sexes would be distinguished as small differences of inflation in the body chambers of ammonites. Of course there were many objections as authors cited the contrary conditions in Argonauta with its minute males, and Nautilus and Octopus vulgaris in which the males are slightly larger, as is also the case with humans. So the inductive argument by analogy was not helping. But the analogy with Nautilus would not go away and served only as a block to understanding the real significance of the microconchs with lappets.
Relentlessly, the ‘fog of research’ continued. S.S. Buckman (1887—1907) monographed the ammonites of the Inferior Oolite and amply illustrated the presence of two forms: microconchs with lappets and macroconchs with simple apertures, but doubted that he was looking at males and females, and therefore made nothing of it.
Then in 1913, Rollier went back to the Oppellids and noted in the Upper Jurassic, small forms with lappets associated with larger forms with simple apertures, but saw the different forms only as useful for classification.
By now we are well into the 20th century, and some seventy years on from de Blairiville’s claim that ammonites were almost certainly bisexual. Nevertheless, L.F. Spath 1928, while considering the ammonites Distichoceras and Horioceras, wrote “Rollier…...even held that they were merely the female and male of the same species, but there is little concrete evidence in favour of this view”. The tone was still sceptical, and I wonder what he had in mind that could possibly count as “concrete evidence”.
A marvelous piece of field research by Brinkmann (1929) resulted in a vast collection of Kosmoceratid ammonites collected cm—by—cm through the lower part of the Oxford Clay near Peterborough, which showed two evolving lineages. On the one hand there were large evolving forms with entire apertures, matched by another series in parallel but with lappets, each paired form was collected at the same level. Yet he saw them as two different evolving lineages of genera. Each lineage evolved independently, but the pairing was constant. But he was only interested in measuring sizes in each lineage.
Even in the Treatise on Palaeontology, Arkell (1957 pp. L87 & L90) gave only a brief discussion of dimorphism, which played no part in the classification of the ammonites.
But then came a decisive argument from J. Callomon (1962) who brought together the greater part of the field of study, producing convincing examples of dimorphic pairs, each pair collected from one distinct horizon. For purposes of objectivity, the terms macroconch (M) and microconch (m) were introduced to designate matching dimorphic pairs from the same stratigraphical level, without committing oneself to the question of whether the two forms were males and females of the same species.
Acceptance is growing, especially among younger palaeontologists, and examples accumulate at increasing rate. It is an interesting historical study of the growth of an idea backed by evidence which no one could see the significance of, because workers were using the Nautilus as a model; while the case with ammonites was not subtle at all, the differences were obvious once the model of Nautilus was abandoned, and the significance of lappets appreciated. Of course we do not know in the same way that we know about the sexes in Nautilus which can be demonstrated, and the shell inflation measured. But we do have an increasing body of evidence in support of dimorphism, and a growing conviction that the theoretical explanation, that the two forms are males and females, is correct.
Nevertheless most ammonites cannot be matched as dimorphic pairs, and this may be due to collection failure, or to the possibility that dimorphic differentiation took place a number of times when it was an advantage to the ammonites. Another possibility may be that ammonites were basically protandrous hermaphrodites, being male in the young stage and egg-bearing females at a later stage, in the same animal. Then, some stopped growing after the male stage and died, and later evolved lappets on reaching maturity.
It is all speculation but fascinating stuff for discussion, and this article is shamelessly filched from Callomon’s 1962 paper, which was published in the Leicester Literary and Philosophical Society, which suggests that he had trouble with referees who were not convinced. Off prints are rare, but, if you can get hold of a copy, I am sure you will be convinced, even if there is no ‘concrete evidence.
Arkell, W.J. et al., 1957. Cephalopoda: Ammonoidea. Treatise on invertebrate palaeontology (edit. R.C. Moore) part L.
Callomon, J. 1962. Sexual Dimorphism in Jurassic Ammonites. The Bennet Lectures, delivered in the Department of Geology in the University of Leicester, May 31, 1962, and published in the Leicester Literary and Philosophical Society. His extensive bibliography covers most of the key works on ammonite dimorphism.