Microcornus Mambetov, 1972

Malinky, John M. & Skovsted, Christian B., 2004, Hyoliths and small shelly fossils from the Lower Cambrian of North-East Greenland, Acta Palaeontologica Polonica 49 (4), pp. 551-578 : 555-556

Genus Microcornus Mambetov, 1972

Type species: Microcornus parvulus Mambetov, 1972 , Lower Cambrian , Kazakhstan .

Diagnosis. —Shell small, dorsal side with pronounced median ridge and flattened flanks, ventral side weakly convex, transverse cross−section rounded triangular. Dorsal apertural edge straight or with slight median sinus, ligula semi−circular. Initial part bulbous, delineated from adult part by constriction, and usually recurved towards dorsal side. Surface sculpture fine wrinkles parallel to apertural margin. Operculum with narrow cardinal shield (from Bengtson 1990a).

Remarks.—The diagnosis of this genus given by Mambetov (1972) emphasized the bulbous protoconch, whereas that of Bengtson (1990a) also included a variety of other features of the conch. Marek (1976b, personal communication 1993) regarded use of the protoconch in hyolith taxonomy at the generic or specific level, and the use of apparently early juvenile specimens for types, as unwise, given the morphologic changes he documented in the course of hyolith ontogeny among an assemblage from the Lower Cambrian of Nevada ( Marek 1976b). It is unfortunate and indeed remarkable that there are no specimens either from Greenland or Malyy Karatau that are intermediate in size between the Microcornus −sized specimens from both areas, and the much larger individuals reported by both Poulsen (1932) and Missarzhevsky (1969) respectively. None of the larger conchs of those authors can be positively identified as belonging to the same taxa as any of the micro−sized individuals. At least part of the difficulty in this situation arises from the incomplete preservation of many of the larger and presumably adult individuals. Despite the large number of micro− and macro−sized hyoliths documented from Siberia and South China, and now herein, it still remains impossible to determine even whether the illustrated specimens of Microcornus are juveniles or adults. In contrast, Marek (1976b) possessed individuals of a variety of different sizes which he interpreted as a series of different ontogenetic stages of Nevadalites palmeri Marek, 1976 . Marek’s (1976b) specimens, and one small and presumably juvenile representative of Nevadotheca tenuistriata ( Linnarsson, 1871; see Berg−Madsen and Malinky 1999) from the Middle Cambrian of Sweden, indicate that adult morphologic features develop early in ontogeny.

It is also worth noting that undescribed hyolithids of a small size similar to Microcornus from the Devonian of the Prague Basin may have an elongated, bullet−shaped or tubular protoconch (Malinky, unpublished) whereas those from Upper Carboniferous (Pennsylvanian) cyclothems of Midcontinent North America have no apparent protoconch ( Malinky and Mapes 1983; Malinky et al. 1986). Inclusion of tubular protoconchs in the diagnosis of Microcornus could thus extend the range of the genus from Lower Cambrian into the Lower Devonian. Qian (1989) and Demidenko (2001) each implied a highly modified concept of Microcornus when they established species of that genus that lacked the bulbous protoconch. Both Microcornus breviligulatus Qian, 1989 and M. egregius Demidenko, 2001 have a tubular protoconch, but otherwise the conchs are similar in overall form to Microcornus . The report of Microcornus sp. from the upper Lower Cambrian upper Ludwigsdorf Member of the shallow marine Charlottenberg Formation near Görlitz in eastern Germany extends the range of this genus into central Europe ( Elicki and Schneider 1992).

Stratigraphic range and distribution.—Upper Lower Cambrian (equivalent to Atdabanian–Botomian); Mongolia, Kazakhstan, South China, England, Australia, Germany, and North−East Greenland.