Lamy Amphibian Quarry Taphonomy Redux and "Forensic Taphonomy"

The Lamy amphibian quarry is a famous quarry in the Garita Creek (=Tecovas) Formation of the Dockum Group in New Mexico. The quarry is well known for its large assemblage of metoposaurs, all presumably belonging to the taxon Koskinonodon perfectum (previously known as Buettneria perfecta). Anyone who has visited the Triassic portion of the Smithsonian Museum of Natural History in Washington, D.C. has probably seen the block of metoposaur skulls from this quarry on exhibit. It was originally interpreted to represent a group of metoposaurs dying in a small body of water dessicated by drought, but this new study proposes little evidence of this. Furthermore, the quarry was reopened and new blocks collected providing new information.

This paper also coins a new term "forensic taphonomy", which is the study of the taphonomy of a site solely utilizing field notes and previous publications without actually visiting the study area first hand. This type of work had been done previously for this site and while these authors defend this type of study in general, I'm left with the overall feeling that it is bad practice to discuss something as dependent on the proper interpretation of sedimentary structures and specimen orientations as taphomony without first hand observation of the site. Therefore, I'm glad they have revised their previous work by actually getting access to and reopening the quarry.

Finally, the authors briefly discuss an important caveat when studying specimens collected from bone-beds decades ago. To highlight what was thought to be key elements of the quarry, in this case the metoposaur skulls, the exhibitors had staff cover over many of the smaller bones around the skulls such as limb bones.  Thus "forensic taphonomy" interpretations based on this display block were skewed by the preparation and exhibit technique used.  This type of technique was also used to highlight two Coelophysis skeletons from the Late Triassic Coelophysis quarry, which are on exhibit at the American Museum of Natural History. "Undesirable" bones were removed or covered over, and tails were added to these specimens to make them complete.  Recent preparation techniques discourage the use of materials to mimic bone to make specimens appear complete, and also the past practice of amalgamating different specimens in bone panel mounts such as these.  Indeed, it can be extremely difficult to study older specimens prepared in this manner, as features of the specimens may not be real.  In fact some older, well known specimen descriptions actually describe the reconstruction, not the real bones. Be careful out there.

Lucas, S. G., Rinehart, L. F., Krainer, K., Spielmann, J. A., and A. B. Heckert. 2010. Taphonomy of the Lamy amphibian quarry: A Late Triassic bonebed in New Mexico, U.S.A. Palaeogeography, Palaeoclimatology, Palaeoecology 298:388–398. doi:10.1016/j.palaeo.2010.10.025

Abstract - Located in Santa Fe County, New Mexico, USA, the Lamy amphibian quarry is a Late Triassic (Adamanian) bonebed stratigraphically low in the Garita Creek Formation of the Chinle Group. Well known for its mass accumulation of metoposaurid amphibians, it was initially interpreted as a drought-induced death assemblage. Based on microstratigraphic and sedimentological studies, additional and extensive collecting at the quarry and a revised understanding of the bonebed, we provide a detailed taphonomic analysis of the Lamy amphibian quarry that identifies it as a low diversity multitaxic and monodominant bonebed in pedogenically modified floodplain mudstone. The Lamy bonebed shows no evidence of drought and is characterized by a high density of completely dissociated bones that show clear alignment by current and sorting (enrichment of Voorhies Group II and III elements). The bones show no significant abrasion or weathering (stage 0), preserve virtually no evidence of scavenging and show no evidence of trampling. Based on skull lengths, the metoposaurid assemblage has a type I survivorship curve and lacks juveniles. We thus posit that the following sequence of events formed the Lamy amphibian bonebed: (1) aggregation (cause unknown) of a large number of metoposaurid amphibians at a site different from the location of the bonebed, though not distant; (2) catastrophic mass mortality; (3) complete disarticulation and disassociation of the skeletons; and (4) rapid transport of the disarticulated bones onto a floodplain surface that was undergoing pedogenesis. The Lamy amphibian bonebed is representative of the Late Triassic metoposaurid bonebeds from Morocco and the western USA, which are monodominant and nearly monotaxic. They indicate that aggregation (probably of breeding populations) and mass death of metoposaurids were relatively common across the riverine floodplains of Late Triassic Pangea.