Holmiceras Hyatt, 1894

Aubrechtová, Martina & Korn, Dieter, 2022, Taxonomy and ontogeny of the Lituitida (Cephalopoda) from Orthoceratite Limestone erratics (Middle Ordovician), European Journal of Taxonomy 799 (1), pp. 1-108 : 44-45

Genus Holmiceras Hyatt, 1894

Type species

Lituites praecurrens Holm, 1891 ; subsequent designation by Sweet (1958).

Diagnosis

Genus of the family Lituitidae with 1 to 1.5 loosely coiled volutions of the coiled conch; uncoiled part short, sigmoidal, with expansion angles of 7°–20°. Whorl profile either slightly compressed or somewhat depressed. Chamber length between 0.20 and 0.30 of the whorl height. Shell surface with annuli and/or lirae; lateral and dorsal sinuses and ventrolateral and dorsolateral projections in the apical parts of the conch, but adorally, the ventral and dorsal sinuses disappear so, that on the mature body chamber, the venter and dorsum are occupied by broad projections (after Sweet 1958).

Species included

Holmiceras benneti Flower, 1975 ; Orthoceras coreanicum Kobayashi, 1928 ; Lituites Kjerulfi Brøgger, 1882 ; Lituites praecurrens Holm, 1891 ; Holmiceras sichuanense Lai, 1986 ; Holmiceras havelense sp. nov.

Remarks

The species of Holmiceras are morphologically closely similar to species of Ancistroceras , which have a smaller and tightly coiled juvenile conch and attain greater expansion angles (usually between 20 and 30°). The conch shapes in the species of Holmiceras are also somewhat similar to those in Angelinoceras because the uncoiled conch in both genera expands rapidly. The species of Angelinoceras have much larger general dimensions, though, with coiled conch diameters of 30–50 mm. The species of the genus Lituites generally have lower expansion angles ( Fig. 30 ), larger overall conch sizes and mature aperture with five projections and deep sinuses on the ventral and lateral sides of the conch. Sweet (1958), Dzik (1984) and Fang et al. (2021) discussed the phylogenetic relationships of Holmiceras in greater detail.

Geographic and stratigraphic occurrence

Canada, Norway, Sweden, northern Germany, Korea and China; Middle to Late Ordovician.

The type species H. praecurrens is a stratigraphically significant species in the Orthoceratite Limestone of Baltoscandia, indicating a Kundan age ( Evans et al. 2014).

Figures

Fig. 30. Conch expansion rates in species of the Lituitidae Phillips, 1848.