New Mexico Geological Society Annual Spring Meeting — Abstracts

The early Paleocene was a time of rapid faunal and floral reorganization following the end-Cretaceous mass extinction. The mammal record is punctuated by intervals of substantial turnover, yet it is unclear if these changes are due to extrinsic factors, such as climate change, or intrinsic factors, such as rapid evolution. The San Juan Basin (SJB) contains one of the best records of early Paleocene mammalian and floral evolution and thus provides an ideal location to examine ecosystem response and change in the first four million years of the Cenozoic.

Dates from magnetostratigraphy and ⁴⁰Ar/³⁹Ar sanidine provide temporal constraints to key fossil horizons and indicate that the type middle and late Puercan faunas occurred within 350 ka and 500 ka of the K-Pg boundary, respectively. These age constraints show that rates of morphological change for some mammals was extremely high. For example, mass estimates for the largest Puercan mammal, Ectoconusditrigonus, are up to 100 kg, far exceeding the size of the largest Late Cretaceous mammals (~5 kg for Bubodens magnus) – an exceptionally rapid evolutionary increase in body size.

We evaluated the mammalian turnover record based on our revised timescale, taking into consideration possible biases from unequal interval durations, sample sizes, and species richnesses. We corrected for interval duration and sample size, which were significantly correlated with origination or extinction rates. Results indicate that origination rates far outpace extinction rates, with strong peaks of origination in the late Torrejonian (To5 & To6).

We constructed a stable isotopic record (oxygen and carbon) for the early Paleocene from mammalian tooth enamel to use as a barometer for environmental and climate change. Preliminary results of this record indicate relative stability in carbon and oxygen values through most of the early Paleocene except for a significant increase in carbon values in Tetraclaenodon from To4 to To5. However, carbon values in Tetraclaenodon do not covary with those of Periptychus through this interval and there is no clear correlation between climate and faunal change in the SJB.

The SJB preserves a superb early Paleocene floral record that is relatively diverse and dominated by angiosperms. Many taxa appear to be endemic to the SJB and these floras are significantly more diverse than contemporaneous floras from the Northern Great Plains. Analyses from fossil leaves, as with the stable isotope record, show little variability in paleoenvironment or paleoclimate between the Puercan and Torrejonian. However, there is major turnover in plant morphotypes and mammal species between the Puercan and Torrejonian suggesting the possibility of synchronous floral and faunal change between Puercan and earliest Torrejonian time.

In conclusion, our results indicate that ecologically complex mammal and plant communities were present in the SJB within a few hundred thousand years of the K-Pg mass extinction. Both mammalian faunas and floras were relatively short-lived and turned over rapidly. Because similar climates and environments appear to have persisted through the early Paleocene, we suggest that plant and mammal turnover may be related to interspecific competition and/or immigration, rather than significant environmental or climatic change.