New Mexico Geological Society Annual Spring Meeting — Abstracts

The Tinajas Member of the Atrasado Formation (Upper Pennsylvanian: Missourian) yields vertebrate coprolites from the Kinney Brick Quarry and the Tinajas Lagerstätten in central New Mexico, but the largest coprofauna from these strata derives from a locality in the Cerros de Amado of Socorro County (NMMNH locality L-9096). Here, most coprolite specimens are found as a weathering lag that eroded from a shale, although a small number of specimens occur in situ in thin limestone beds. The coprolites are preserved in a low energy, shallow marine environment.

The majority of coprolites are spiral in morphology--heteropolar, amphipolar and scroll. Five ichnotaxa are heteropolar spiral in morphology and include Crassocoprus mcallesteri, Kalocoprus oteroensis, two ichnospecies of Heteropolacopros and ?Speirocoprus isp. The most common ichnotaxon is Crassocoprus mcallesteri. Heteropolar microspiral coprolites are assigned to Heteropolacopros. There is a need for an ichnotaxonomic review of this widespread ichnogenus, which was originally named for a Triassic morphotype. This is the only occurrence of Kalocoprus oteroensis outside the Beeman Formation of Otero County, NM. Amphipolar coprolites are represented by Hyronocopros amphipola. There are several specimens of the scroll coprolite Bibliocoprus beemanensis. Diverse vertebrate bones occur elsewhere in the Tinajas Member, but not at this locality.

The majority of large coprofaunas from shallow marine strata are dominated by spiral morphologies. During the Mesozoic and Cenozoic, spiral coprolites are clearly attributable to chondrichthyans. Thus, there is clearly a disparity between the high taxonomic diversity of fish faunas of these ages and the low diversity of chondrichthyan-dominated coprolite ichnofaunas; this has been termed the Shark Surplus Paradox. Coprolite assemblages from the Devonian-Permian also yield abundant spiral forms, although in ichnofaunas of these ages there are more candidates for producers. The Cerros de Amado locality provides information on the distribution and ecological context of the Shark Surplus Paradox during the Paleozoic.

Distribution: Arguably the best record of Late Paleozoic vertebrate coprolites is preserved in Scotland where spiral morphotypes become abundant by the Early Devonian. However, large samples from discrete ichnofaunas are rare, so the Cerro de Amado locality documents a Late Pennsylvanian data point of a high percentage of spiral forms in an assemblage from a shallow marine environment.

Ecology: Four bromalite faunas of Missourian age in New Mexico represent an ecological transect from estuarine to shallow marine: (1) Kinney Brick Quarry Lagerstätte – estuarine; (2) Tinajas Lagerstätte – lagoonal ; (3) Cerros de Amado – nearshore marine; and (4) Sacramento Mountains – offshore marine. There are trends through these ichnofaunas (Kinney-Tinajas- Cerros de Amado-Sacramentos); (1) flattened preservation in matrix to isolated three dimensional; (2) diverse bromalites to only coprolites; and (3) increasing proportion of spiral coprolites. The ichnofauna of the Cerro de Amado locality provides evidence that at least during the Missourian the Shark Surplus Paradox is a phenomenon of shallow marine environments.