Bellerophontaceans from Hancock, Maryland--Gastropod (Torted) or Monoplacophoran (Untorted)?: ABSTRACT

Abstract

Mollusk specimens from the Briery Gap Sandstone Member of the Foreknobs Formation in Hancock, Maryland, were previously assigned to the gastropod genus Bellerophon. In recent years, the controversy of whether Bellerophon belongs to the molluscan class of torted gastropod or that of untorted monoplacophoran has resurfaced. Because of the need for better understanding of molluscan evolutionary history, resolving this controversy is very important for an interpretation of fossil phylogenies as a whole, thus increasing the effectiveness of fossils as biostratigraphic tools. In determining whether or not these animals were torted or untorted, past emphasis on muscle scar patterns has been proven inadequate due to the lack of specimens exhibiting suitable scars. The emphasis is presently being directed toward other aspects of shell morphology, such as apertural slits and secondary shell layers known as inductura. The position of the inductura relative to the slit is significant and implies that the animal is oriented in a particular fashion within its shell. This position, in turn, helps to determine whether the animal was torted or not and hence whether it is a gastropod or a monoplacophoran. Because original shell material was absent, recreating the shell was necessary in evaluating the morphology. This was achieved by replicating the shell using liquid latex. The pouring, hardening, and extraction of latex from external molds yielded replicas exhibiting detailed shell features. Observed was the location of the lateral inductural deposits opposite the apertural slit, implying that the animal's coiled shell was carried over its extended foot as a result of torsion, and therefore the animal belongs to the class Gastropoda. End_of_Article - Last_Page 509------------