Oxytropidoceras (Mirapelia) douglasi  Knechtel, 1947

Measurements	D mm	H/D	T/D	O/D	H/T
CP-198	59.7 (55.4)	0.45	0.35	0.24	1.27

Age	Origin
Pariatambo Formation Peruvian ulrichi zone Beginning of Middle Albian	Pongo de Rentema Amazonas Region Peru

Description. A deformed specimen in marbled limestone with a black shell, a chipped siphonal keel, and the imprint of another ammonite (on the right in the frontal view). A few sutures are visible before the body chamber, preserved on the last quarter of whorl. The involute shell grows rapidly in height. The tall, vertical umbilical wall transitions to the flank with a rounded edge. There are no distinct ventrolateral shoulders. At the beginning of last whorl, the section is ogival, with maximum thickness at the inner third. The simple, sharp-crested, slightly sigmoid ribs originate thin and proverse at the umbilical suture. They gradually widen, and their crests become rounded and flattened, before sagging at the base of the keel. Their anterior slope is steeper than the posterior one. On the body chamber, the ribs become more flexed, their ends widen further, which reduces the interspaces, and the section thickens and becomes elliptical, with a maximum thickness at mid-flank. There are 28 ribs in total.

Remarks. A species from Peru with an elliptical whorl section lacking distinct ventral shoulders, a marked umbilical wall, whorls that grow rapidly in height, and ribs with broad and flat ends. Compared to O. (M.) mirapelianum, it has a thicker section, more convex flanks, and more flexuous ribs. Its Peruvian ulrichi zone corresponds to our lyelli zone. I bought this ammonite as an Oxytropidoceras karsteni Stieler, 1920, but this species of the subgenus Venezoliceras is quite different: Stieler (p. 396) and Renz (1982) show a taller, rectangular section, with more numerous, thin and straight ribs, and an forward angular fold on the ventrolateral shoulders. According to Cooper (1982), O. (M.) douglasi would be a variant of O. (M.) buarquianum (White, 1887).