Conservative evolution in nautiloid shell morphology: Evidence from the Pennsylvanian nautiloidMetacoceras mcchesneyifrom Ohio, USA

Abstract

Morphometric analyses of shell morphology in the Pennsylvanian nautiloidMetacoceras mcchesneyiMurphy, 1970 (Cephalopoda, Mollusca) recovered from coal mines in Madison Township, Columbiana County, Ohio, USA, reveal the ontogenetic change from hatching to maturity as well as intra-specific variation of shell morphology. The shell shape ofM. mcchesneyihas isometric relationships, and the umbilicus diameter between umbilical shoulders has a positive allometric relationship with shell diameter. These show that the relative whorl shape was constant through the ontogeny, but the umbilicus became relatively broader with growth. The siphuncle position moved from a ventro-central position toward the center with growth until 420° of the total rotational angle had been attained. A constriction was recognized on the early whorl at 9.5 mm in shell diameter, and the interval angles of succeeding septa were changed at the 5th septum, indicating that hatching occurred at this diameter. The ventral apertural wall, the disappearance of ornamentation toward the last preserved aperture, and the last whorl separating from the previous whorl indicate thatM. mcchesneyiattained maturity at ca. 70 mm in shell diameter. Most characteristics of shell morphology inM. mcchesneyi(the relative shell shape and ornamentation through ontogeny) are comparable to those in modern and younger fossil nautilids, irrespective of taxonomy and age, supporting the conclusion that evolutionary rates of shell morphology are conservative in nautilid history.