Requiem for ACM 41109, the first holotype of Anchisaurus

ACM 41109, also known as AM 41/109, was until recently the holotype or defining specimen of Megadactylus polyzelus, better known as Anchisaurus. ACM 41109 includes eleven back and tail vertebrae, a partial shoulder blade, most of the right hand, part of the right forearm, much of the left leg, and parts of both ischia. These are all that remain of a basal sauropodomorph that lived about 200 million years ago. Unlike the overwhelming majority of its brethren, the mortal remains of ACM 41109 were not only buried and fossilized, but through sheer inconceivable luck were buried in a place where, 200 million years later, they would be uncovered by earth-moving operations carried out by a species that was not even a hint of a glimmer of a twinkle in the eye of one of the therapsids that lived at the same time as the dinosaur. Its rebirth in 1855 was difficult: it suffered blasting and its bones were then dispersed by workmen who were preparing improvements to the great Springfield Armory, a timely effort given the Civil War to come a few years in the future. As much of the skeleton as could be salvaged was sent to pioneering paleontologist Edward Hitchcock of Amherst College, which is how the specimen came to be labeled "ACM"; the college's museum is today the Beneski Museum of Natural History. Although the first reports came out in the 1850s, a formal name would have to wait until 1865, when the task fell to Hitchcock's son, Edward Jr. At the time, it was one of the few dinosaur specimens of any completeness known, and was part of the first wave of North American dinosaur body fossils along with Hadrosaurus, "Laelaps", Leidy's Judith River scraps, and other bits and pieces. In one of those strange comedies that happen from time to time, it went through two genus names (Megadactylus and Amphisaurus) before finally coming upon one which wasn't already in use (Anchisaurus). ACM 41109 was sufficient to define Anchisaurus polyzelus for many years. Now this function has been passed to YPM 1883, previously known as Anchisaurus colurus or Yaleosaurus, both names which now must be considered rejected. YPM 1883 was somewhat more fortunate in terms of how it was preserved and discovered, but has no particularly colorful stories behind it. The quarry that produced it is now occupied by a shopping center, and by the time it was found dozens of good dinosaur specimens had already been recovered. Perhaps ACM 41109 will now be forgotten, a handful of bones in a drawer. Yet though it no longer suits the changing needs of taxonomy, it somehow reached out beyond 200 million years and briefly become the exemplar of untold forever-vanished animals.

Further reading:

International Commission on Zoological Nomenclature (ICZN). 2015. Opinion 2361 (Case 3561): Anchisaurus Marsh, 1885 (Dinosauria, Sauropodomorpha): usage conserved by designation of a neotype for its type species Megadactylus polyzelus Hitchcock, 1865. Bulletin of Zoological Nomenclature 72(2):176–177.

Tweet, J. S., and V. L. Santucci. 2011. Anchisaurus from Springfield Armory. Brigham Young University Geology Studies 49(A):75–82.