Thursday, September 16, 2010
Fossil of the Week
9/16 – “Pre-Scallop”
Scallops – today members of the bivalve family Pectinidae – have been alive on Earth since the early Triassic Period, approximately 240 million years ago. They are characterized by “ears” (called auricles) on the shell and a notch below one of them through which a set of elastic threads (a byssus) emerge to help hold the scallop, during at least its juvenile life, to the sea bottom. At the edge of the byssal notch in most scallops is a comb-like set of spines (the ctenolium) that separate and support the byssal threads. The direct ancestors of scallops were scallop-like bivalves of the family Entoliidae, which lived during the Paleozoic and Mesozoic Eras (400-65 million years ago). Entoliids had auricles and a byssal notch only as juveniles, but they did not have a ctenolium. The ctenolium – a defining feature of the modern family Pectinidae – is a feature that evolved within the scallop lineage.
This Fossil of the Week is Entolium aviculatum (Swallow, 1858) (PRI 14077, 2.25 cm in diameter), from the Carboniferous Period (approximately 330 million years ago) of Henry County, Missouri. This specimen is a “steinkern” or internal cast – a mold of the inside of the shell made when the mud inside the shell turned to rock, and the pieces of shell fell away; the inside of the other valve shows on the other side of this fossil. The auricles of the specimen are to the right and left of the umbo (the oldest part of the bivalve, including the larval shell) at top center. Entolium aviculatum was originally described in the genus Pecten in the scallop family Pectinidae. The genus Entolium was distinguished by F. B. Meek in 1865 (still in the scallop family Pectinidae) as different from most scallops in having indistinct auricles and no notch for the byssus (these features are now known to be present and most obvious only in juvenile entoliids). The family Entoliidae was not separated from Pectinidae until 1960 by the Russian paleontologist I. A. Korobkov. This is a good example of the complicated string of events that often ensues in naming and distinguishing species, especially in paleontology, in which the number of specimens and/or the quality of specimens may limit our knowledge (at least for a time) about the features that define families, genera, and species.
George Clinton Swallow (1817-1899) led an interesting and varied life. He was a native of Maine, was educated at Bowdoin College, and was Missouri State Geologist when he described this species. He held professorships and state geologist positions in both Missouri and Kansas, but later left paleontology to become a medical doctor. He was teaching at a medical school in Missouri when he had a dispute with the President and was dismissed. He then became a newspaper editor and state inspector of mines in Montana. In 1858, Fielding Bradford Meek (1817-1876; who described the genus Entolium) accused him of unfairly taking credit for being the first to recognize Permian fossils in North America, which ignited a long controversy over the Permian Period in Kansas. Swallow was also accused of disloyalty during the Civil War, and was jailed twice in 1862.
Text by Paula Mikkelsen