Taxonomy and palaeobiology of the Bathonian (Middle Jurassic) tulitid ammonite Morrisiceras

Abstract

Taxonomic revision of a number of species belonging to the Middle Bathonian tulitid genus Morrisiceras ( Morrisiceras Buckman – macroconchs and Holzbergia Torrens – microconchs) from the Polish Jura (south-central Poland) and England, shows that the genus is in fact represented by one, albeit morphologically very plastic (especially macroconchs), species Morrisiceras morrisi (Oppel). In the Polish Jura area, all of the previously described morphotypes of the macroconch, along with its microconch Holzbergia, are characterized by their identical stratigraphic ranges, both being confined to the Middle Bathonian Morrisi Zone. Much smaller microconchs (males) may have attained maturity earlier than their macroconchs (females). Morphologically very plastic macroconchs seem to have attained full maturity at various shell diameters. Such phenomenon is known in recent coleoids and similarly may be the result of changing environmental conditions. Palaeobiogeographically, the main area occupied by the species was confined to the epicratonic seas, which spread across what is now present-day central and North-West Europe, with some occurrences in western Asia (Turkmenistan/Uzbekistan) and sporadic occurrence in a deep-water facies of Apulia (Italy). Taphonomical observations suggest that Morrisiceras shells have not undergone significant post-mortem drifting or transport after deposition. The preservation of some of the Morrisiceras morphotypes, occurring as dorso-ventrally flattened specimens in the host clays, may indicate that they were quickly buried in life position after they have landed on the sea-floor.