Prolyelliceras alternum  Robert, 2002

Measurements	D mm	H/D	T/D	O/D	H/T
CP-666	65.8 (64)	0.35	0.33	0.40	1.05
Holotype	120	0.29	0.21	--	1.40

Age	Origin
Black shale (anoxic facies) alternum Peruvian zone Lower Albian	Cerro Yanarajo 13 km ENE Huallanca Ancash Region, Peru

Description. Evolute internal mold in black shale, without visible sutures, with the beginning of the body chamber on the last quarter of whorl. Whorls are 20% covered, rectangular section barely compressed, with truncated upper corners and a flat venter. Umbilicus wide, with a low wall sloping at 60° and rounding towards the flank. 26 straight or slightly forward-curved ribs arise from this wall, without forming an umbilical bulla. At the outer third of the flank, below the cut corner of the cross-section, a small pimple-like tubercle appears (photo on the right). On the inner whorls, it is located just below the umbilical wall of next whorl. The ribs stop at the top of the cut corner on a strong ventrolateral clavus. The siphonal line bears an equal number of clavi, but they are slightly less prominent. The three rows of ventral clavi are offset from each other, resulting in alternate ribs on each side of the flanks. The ribs are sunken between the external ventrolateral and siphonal clavi, with vague zigzag connections.

Remarks. This species resembles Lyelliceras pseudolyelli: the ribs are alternate on each side of the venter and loosely connected to one or two siphonal clavi. There are, however, two differences: a) L. pseudolyelli has umbilical tubercles, b) its siphonal clavi are more numerous than the ventrolateral ones. The holotype of P. alternum is unusually large for the genus, and the section at large diameters is higher and becomes ogival. This explains the different proportions of our smaller specimen. In the Peruvian Albian (Robert, 2002; Robert et al., 2002), the ammonite defines a zone of Lower Albian, below that of Prolyelliceras mathewsi (end of Lower Albian) and that of P. ulrichi (beginning of Middle Albian).