Henderson, Robert A., and McKenzie, E. Donald (2002) Idanoceras, a new heteromorph ammonite genus from the late Albian of eastern Australia. Journal of Paleontology, 76 (5). pp. 906-909.

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Abstract

THE LATE Albian marine fossil record from eastern Australia derives from the sedimentary succession of the Great Artesian Basin deposited in a vast epicontinental sea which then covered much of the continent (see Frakes et al., 1987). Ammonites of this age are common but their generic diversity is low. Heteromorph assemblages almost exclusively comprise the taxa Myloceras, Labeceras sensu stricto and Labeceras (Appurdiceras) of the Family Labeceratidae that were widely distributed in higher latitudes of the Southern Hemisphere during Late Albian time (see Aguirre Urreta and Riccardi, 1988; Klinger, 1989). Some 19 endemic species of these genera are recorded from the Great Artesian Basin in the present literature (Etheridge, 1892; Whitehouse, 1926; Reyment, 1964) and there are additional undescribed species (Henderson and McKenzie, unpublished data). The Australian Late Albian epicontinental sea was clearly a site of significant speciation for Labeceras and Myloceras and it has been argued that the Great Artesian Basin represents the evolutionary center for these genera (Henderson, 1990).

Middle Albian assemblages from the Great Artesian Basin include the heteromorph genera and subgenera Anisoceras, Hamites (s.s.), Protanisoceras, and Rossalites (McNamara, 1980; McKenzie, 1998), which are all widely distributed forms. However, a quite different pattern prevails in the Late Albian. Although heteromorph genera of pandemic distribution, such as Scaphites, Anisoceras, Hamites, Idiohamites, and Mariella, are represented on the Australasian borderlands, they are virtually unknown in the Great Artesian Basin (see Henderson, 1990) even though planktic foraminifera (Haig, 1979) and nannoplankton (Shafik, 1985) indicate that unhindered connections to the open ocean prevailed for at least part of this time. Among some 1,500 heteromorph specimens held in formal collections acquired for more that a century that we have examined, including the acquisition of extensive new collections, records of pandemic genera amount to one specimen of Worthoceras, and a cluster of Ptychoceras associated with a solitary Worthoceras specimen on a single hand-sample recently collected by Robert Jones of the Australian Museum. Colonisation of the Great Artesian Basin by pandemic taxa, such as those represented nearby on the northern Australian and New Zealand continental margins, was clearly possible but did not occur in any substantial manner. It is suggested here that Late Albian diversification of the members of the Labeceratidae occupied the niches available to ammonites of heteromorph morphology in the Australian epeiric sea, to the exclusion of pandemics.

Part of that evolutionary diversification is documented here in the form of a new monotypic genus, Idanoceras, a novel representative of the Labeceratidae affiliated to, and perhaps derived from, Labeceras (Labeceras). Idanoceras mimics the shell form of the cosmopolitan genus Hamites and is thought to have adopted a similar life-style as a product of convergent evolution. The Labeceratidae represent the final evolutionary pulse of the Superfamily Ancylocerataceae which were the dominant group of Valanginian–Aptian heteromorph ammonites.

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