Ammonites of the Arctic (Taxon of the Week: Arctocephalitinae)

Ammonites are one of the classic animal groups of the Mesozoic. These coil-shelled cephalopods are guaranteed a mention in almost every popular book alluding to that time period. But what is often glossed over in popular accounts is that ammonites were an extremely speciose group, making them one of the best-studied groups in understanding marine fossil diversity.


Specimens of the arctocephalitine ammonite Arcticoceras harlandi. This species probably reached a diameter of around 10 cm though most preserved specimens are smaller as the large body chamber tends to break apart before preservation. Figures 5 and 6 show a microconch (see below). Figure from Rawson (1982).


Diagram showing the internal septa of a mature whorl from Arcticoceras harlandi. Figure from Poulton (1987).

Ammonite identification is, by all accounts, a tricky beast. Donovan et al. (1980) admitted that "Students tell us in their essays that one of the desirable attributes of a zonal fossil is that it should be easily recognizable. Most ammonites are not". Ammonites as a whole are readily distinguished from other shelled cephalopods by the ridiculously complex sutures separating chambers. However, lineages of ammonites in different periods and times often converged with each other in their morphology and successful identification often requires, in addition to simple morphology, consideration of such matters as geographical provenance and the nature of forms found in contiguous strata. And quite frankly, I'll be buggered if I've got the intellect to distinguish most of them.

That carping aside, the Arctocephalitinae were a subfamily of ammonites restricted to the region of the modern Arctic Ocean during the middle part of the Jurassic. Arctocephalitines are represented by an extensive fossil record found in localities such as Greenland, northern Canada and Siberia which have allowed a reasonable degree of success in tracing their lineages. The Arctocephalitinae are the basal radiation of the family Cardioceratidae, arising from early Sphaeroceratidae during the latter half of the Bajocian epoch; the subfamily Cadoceratinae was derived from within the Arctocephalitinae during the succeeding Bathonian and would itself give rise in turn to the Cardioceratinae (Donovan et al., 1980; ammonite researchers have so far been unimpressed by arguments for strict monophyly as a guiding principle in classification). The cadoceratines would outdo their arctocephalitine forebears by spreading beyond the Boreal region.


Specimen of 'Costacadoceras'; the asterisk indicates the start of the body chamber. This 'genus' includes the microconches of Arctocephalitinae. Microconches were much smaller, morphologically distinct forms of ammonite that were found alongside the usually more abundant and more characteristic larger forms (macroconches). It is now universally accepted that microconches and macroconches represent distinct sexes of a single species (with, by analogy to modern cephalopods, microconches probably being male and macroconches female) but matching a particular microconch with a particular macroconch is often not possible. Figure from Mitta (2005).

During the period of the earliest two genera of Arctocephalitinae, Cranocephalites and its descendant Arctocephalites, the subfamily had the Arctic to itself; no other ammonite families had reached the largely isolated ocean (Navarro et al., 2005). The arctocephalitines were largely laterally compressed with deep angular whorls (discocones). Things changed with the arrival of another family, the similarly discoconic Kosmoceratidae, in the Arctic Basin around the time of the origin of the third main arctocephalitine genus, Arcticoceras. The arrival of the kosmoceratids seems to have provided a competitive impetus to arctocephalitine evolution: the overall disparity in the family decreased and they were pushed out of the discocone niche. Instead, the succeeding cadoceratines were initially cadicones with broad shallow whorls though some cadoceratines returned to a discocone form after leaving the Arctic Basin.

REFERENCES

Donovan, D. T., J. H. Callomon & M. K. Howarth. 1980. Classification of the Jurassic Ammonitina. In: House, M. R., & J. R. Senior (eds) The Ammonoidea pp. 101-155. Academic Press: London & New York.

Mitta, V. V. 2005. Late Bathonian Cardioceratidae (Ammonoidea) from the middle reaches of the Volga River. Paleontological Journal 39 (Suppl. 5): S629-S644.

Navarro, N., P. Naige & D. Marchand. 2005. Faunal invasions as a source of morphological constraints and innovations? The diversification of the early Cardioceratidae (Ammonoidea; Middle Jurassic). Paleobiology 31 (1): 98-116.

Poulton, T. P. 1987. Zonation and correlation of Middle Boreal Bathonian to Lower Callovian (Jurassic) ammonites, Salmon Cache Canyon, Porcupine River, northern Yukon. Geological Survey of Canada Bulletin 358: 1-155.

Rawson, P. F. 1982. New Arctocephalitinae (Ammonoidea) from the Middle Jurassic of Kong Karls Land, Svalbard. Geological Magazine 119: 95-100.