Under harsh Pleistocene climates, migration and other forms of seasonally patterned landscape use... more Under harsh Pleistocene climates, migration and other forms of seasonally patterned landscape use were likely critical for reproductive success of mastodons ( Mammut americanum ) and other megafauna. However, little is known about how their geographic ranges and mobility fluctuated seasonally or changed with sexual maturity. We used a spatially explicit movement model that coupled strontium and oxygen isotopes from two serially sampled intervals (5+ adolescent years and 3+ adult years) in a male mastodon tusk to test for changes in landscape use associated with maturation and reproductive phenology. The mastodon’s early adolescent home range was geographically restricted, with no evidence of seasonal preferences. Following inferred separation from the matriarchal herd (starting age 12 y), the adolescent male’s mobility increased as landscape use expanded away from his natal home range (likely central Indiana). As an adult, the mastodon’s monthly movements increased further. Landscap...
Middle Miocene (~18–12 Ma) mammalian faunas of the North American Great Plains contained a higher... more Middle Miocene (~18–12 Ma) mammalian faunas of the North American Great Plains contained a higher diversity of presumed browsers than any modern biome. This has been attributed to greater primary productivity, which commonly corresponds with densely vegetated, forested habitats today. However, several lines of evidence suggest that open biomes, which support low browser diversity today, dominated the middle Miocene. To gain a better understanding of middle Miocene habitats on the northern Great Plains, we generated a large dataset of stable carbon isotope values (δ13C) from a wide variety of fossil ungulates. Tooth enamel was sampled from four local faunas in Nebraska of late Barstovian age (14.8–12.5 Ma). Paleoenvironmental interpretations were made using a predictive model based on δ13C values in vegetation and mammalian herbivores in modern biomes. Resulting mean δ13C fossil values for local faunas plot in the upper range expected for C3 vegetation, indicating open biomes. Most mean values for genera plot in the range expected for open canopy habitats with only two genera in the closed canopy range. Of all the taxa, only the grazing horse, Neohipparion, plots in the range for likely C4 consumption. Neohipparion has the highest mean δ13C value of any taxon, with one individual plotting in the range for unequivocal C4 consumption and others in the mixed C4/C3 or C3 water-stressed range. Neohipparion appears to be the earliest known taxon that included C4 vegetation in its dietary niche. The range in carbon isotope values suggests browsers, mixed-feeders, and grazers all consumed vegetation in mostly open areas. Our model implies a fairly dry, open biome with limited riparian habitat. This interpretation is generally consistent with previous interpretations of an open savanna-like biome. A high diversity of middle Miocene browsers and other taxa appear to have lived in an open biome with no modern analog.
Geological Society of America Abstracts with Programs, 2020
The San Juan Basin (SJB) of New Mexico, USA preserves arguably the best record of early Paleocene... more The San Juan Basin (SJB) of New Mexico, USA preserves arguably the best record of early Paleocene mammalian evolution, spanning the Puercan and Torrejonian North American land mammal ages (NALMA). The lack of floral data from the SJB limits paleoclimatic and paleoenvironmental reconstruction and prevents an assessment between the mammalian and plant records. Further, although early Paleocene fossil floras are well known from the Northern Great Plains of North America, relatively little is known about floras from southern North America. The lack of southern North American data limits regional comparisons of floral diversity, composition, and plant community responses to environmental change. We present a description of early Paleocene floras from the SJB and a comparison of SJB floras to floras from the Northern Great Plains. Fossil leaves were collected from the lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation, with a focus on localities correlative to the early Puercan (Pu1) to early Torrejonian (To1) NALMAs which have been dated to ~65.8-64.0 Ma in the SJB. These floras are dominated by angiosperms, diverse (average site diversity = 32.5), dominated by species that appear to be endemic to the SJB, and display variability in morphotypes occurrences between facies. The taxonomic composition of Puercan floras appears to be distinct from Torrejonian floras suggesting a possible large scale change in plant communities in the earliest Paleocene that may be correlative with a major turnover in the mammalian fauna. Paleoclimate analyses indicate the Puercan and Torrejonian floras sample a warm and relatively wet climate, which remained similar throughout the early Paleocene. Interestingly, the SJB floras are significantly more diverse than age equivalent Northern Great Plains floras and indicate a warmer and wetter climate. These results indicate that a diverse and largely endemic plant community existed in the SJB shortly after the Cretaceous-Paleogene (K-Pg) boundary and that these floras may have experienced a rapid turnover coincident with the Puercan-Torrejonian turnover in mammalian faunas. Further, these results suggest a significant difference between northern and southern floras and a large north-south diversity gradient, which may suggest a variable response to the K-Pg extinction event.
"Data_faunal_list" includes occurrence data for fossil mammals from the Barstovian, Cla... more "Data_faunal_list" includes occurrence data for fossil mammals from the Barstovian, Clarendonian, and Hemphillian North American Land Mammal Ages. Data originally downloaded from the MioMap database. Only fossil localities that produced at least one species of ungulate are included here. "Data_isotopes" includes the stable carbon and oxygen isotope values from the tooth enamel of Hemphillian-aged rhinoceroses from the Great Plains.
The San Juan Basin of New Mexico preserves the most complete record of mammalian evolution throug... more The San Juan Basin of New Mexico preserves the most complete record of mammalian evolution through the lower Paleocene of North America, spanning the Puercan and Torrejonian land-mammal ages. Faunal turnover is extreme in parts of this record, raising the question of whether these faunal changes resulted from extrinsic factors, such as climate or environmental change, or from intrinsic factors such as rapid rates of evolution or competition among taxa to fill newly opened ecological niches following the K-Pg mass extinction. To test this problem we construct stable carbon and oxygen isotope stratigraphies from mammalian tooth enamel to use as proxies for environmental and climate change, respectively. The longest isotopic record is from the closely related periptychid condylarths, Carsioptychus and Periptychus, and spans all eight local biozones (Pu2-3, To1-To6). Isotopes were also sampled from Tetraclaenodon, spanning most of the Torrejonian (To2-To6). We evaluate faunal turnover 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). The stable isotope record indicates stability in carbon and oxygen values through most of the record with two exceptions in the late Torrejonian: a significant increase in carbon values in the Tetraclaenodon from To4 to To5, and a significant increase in oxygen values in Periptychus from To5 to To6. In both cases, however, covariation of values does not occur between these taxa. Carbon isotope values in Tetraclaenodon are positively and significantly correlated with faunal origination rates, although the sample is small (n=5), suggesting that carbon values in Tetraclaenodon may be reflecting underlying environmental changes. However, oxygen isotope values, which serve as a climate proxy, are not correlated with turnover metrics in either Tetraclaenodon or Periptychus. Thus, there is no clear correlation between climate and faunal change in the San Juan Basin, and preliminary results suggest that mammalian turnover resulted mostly from intrinsic factors and/or immigration. Future efforts will focus on mean annual temperature estimates from San Juan Basin floras and expanding the stable isotope record to include additional taxa to better resolve the climate and isotopic records.
Body size plays a critical role in mammalian ecology and physiology. Previous research has shown ... more Body size plays a critical role in mammalian ecology and physiology. Previous research has shown that many mammals became smaller during the Paleocene-Eocene Thermal Maximum (PETM), but the timing and magnitude of that change relative to climate change have been unclear. A high-resolution record of continental climate and equid body size change shows a directional size decrease of ~30% over the first ~130,000 years of the PETM, followed by a ~76% increase in the recovery phase of the PETM. These size changes are negatively correlated with temperature inferred from oxygen isotopes in mammal teeth and were probably driven by shifts in temperature and possibly high atmospheric CO(2) concentrations. These findings could be important for understanding mammalian evolutionary responses to future global warming.
Upper Cretaceous and lower Paleocene rocks in the San Juan Basin, New Mexico, contain a robust re... more Upper Cretaceous and lower Paleocene rocks in the San Juan Basin, New Mexico, contain a robust record of dinosaur and mammal evolution, diversification, and extinction. Despite this rich fossil record, the ages and durations of the Upper Cretaceous Naashoibito Member of the Kirtland Formation and the lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation have been relatively poorly constrained. These poor age constraints have limited the ability to accurately correlate these vertebrate records to others across North America and to assess rates of speciation and extinction of dinosaurs in the Cretaceous and mammals before and after the Cretaceous-Paleogene (K-Pg) mass extinction. Further, the ages of the Naashoibito and the Ojo Alamo are contentious and have been interpreted to range from Campanian to early Paleocene. Here we present new geochronologic results that combine magnetostratigraphy and 40Ar/39Ar dating of detrital sanidine from sedimentary units and sanidine phenocry...
. Paleogene hyperthermals, including the Paleocene-Eocene Thermal Maximum (PETM) and several othe... more . Paleogene hyperthermals, including the Paleocene-Eocene Thermal Maximum (PETM) and several other smaller events, represent global perturbations to Earth's climate system and are characterized by warmer temperatures, shifts in floral and faunal communities, and hydrologic changes. These events are identified in the geologic record globally by negative carbon isotope excursions (CIEs), resulting from the input of isotopically light carbon into Earth's atmosphere. Much about the causes and effects of hyperthermals remains uncertain, including whether all hyperthermals are caused by the same underlying processes, how biotic effects scale with the magnitude of hyperthermals, and why CIEs are larger in paleosol carbonates relative to marine records. Resolving these questions is crucial for their full interpretation and application to future climate scenarios. The Fifteenmile Creek area of the central Bighorn Basin, Wyoming U.S.A., exposes an early Eocene floodplain sedimentary sequence that preserves paleosol carbonates and an extensive fossil mammal collection. Previous analysis of faunal assemblages revealed two pulses of mammal turnover and changes in diversity interpreted to correlate with the ETM2 and H2 hyperthermals that immediately follow the PETM. This was, however, based on long distance correlation of chemostratigraphic records. We present new carbon isotope stratigraphy using micrite δ13C values from paleosol carbonate nodules preserved in and between richly fossiliferous localities at Fifteenmile Creek to identify the stratigraphic positions of ETM2 and H2. Additionally, we used differential GPS elevations to establish a new stratigraphic framework that assists in correlation and is independent from the biostratigraphy and previous composite lithostratigraphic sections from the area. Carbon isotope results show that the ETM2 and H2 hyperthermals, and possibly the subsequent I1 hyperthermal, are recorded at Fifteenmile Creek. ETM2 and H2 overlap with the two previously recognized pulses of mammal turnover. Comparisons between the new chemostratigraphy and fossil record suggest that the recorded amplitude of these faunal changes may be muted as a result of some stratigraphic averaging of fossils. The CIEs for these hyperthermals are also smaller in magnitude than in more northerly Bighorn Basin records. We suggest that basin-wide differences in soil moisture and/or vegetation could contribute to variable CIE amplitudes in this and other terrestrial records.
Under harsh Pleistocene climates, migration and other forms of seasonally patterned landscape use... more Under harsh Pleistocene climates, migration and other forms of seasonally patterned landscape use were likely critical for reproductive success of mastodons ( Mammut americanum ) and other megafauna. However, little is known about how their geographic ranges and mobility fluctuated seasonally or changed with sexual maturity. We used a spatially explicit movement model that coupled strontium and oxygen isotopes from two serially sampled intervals (5+ adolescent years and 3+ adult years) in a male mastodon tusk to test for changes in landscape use associated with maturation and reproductive phenology. The mastodon’s early adolescent home range was geographically restricted, with no evidence of seasonal preferences. Following inferred separation from the matriarchal herd (starting age 12 y), the adolescent male’s mobility increased as landscape use expanded away from his natal home range (likely central Indiana). As an adult, the mastodon’s monthly movements increased further. Landscap...
Middle Miocene (~18–12 Ma) mammalian faunas of the North American Great Plains contained a higher... more Middle Miocene (~18–12 Ma) mammalian faunas of the North American Great Plains contained a higher diversity of presumed browsers than any modern biome. This has been attributed to greater primary productivity, which commonly corresponds with densely vegetated, forested habitats today. However, several lines of evidence suggest that open biomes, which support low browser diversity today, dominated the middle Miocene. To gain a better understanding of middle Miocene habitats on the northern Great Plains, we generated a large dataset of stable carbon isotope values (δ13C) from a wide variety of fossil ungulates. Tooth enamel was sampled from four local faunas in Nebraska of late Barstovian age (14.8–12.5 Ma). Paleoenvironmental interpretations were made using a predictive model based on δ13C values in vegetation and mammalian herbivores in modern biomes. Resulting mean δ13C fossil values for local faunas plot in the upper range expected for C3 vegetation, indicating open biomes. Most mean values for genera plot in the range expected for open canopy habitats with only two genera in the closed canopy range. Of all the taxa, only the grazing horse, Neohipparion, plots in the range for likely C4 consumption. Neohipparion has the highest mean δ13C value of any taxon, with one individual plotting in the range for unequivocal C4 consumption and others in the mixed C4/C3 or C3 water-stressed range. Neohipparion appears to be the earliest known taxon that included C4 vegetation in its dietary niche. The range in carbon isotope values suggests browsers, mixed-feeders, and grazers all consumed vegetation in mostly open areas. Our model implies a fairly dry, open biome with limited riparian habitat. This interpretation is generally consistent with previous interpretations of an open savanna-like biome. A high diversity of middle Miocene browsers and other taxa appear to have lived in an open biome with no modern analog.
Geological Society of America Abstracts with Programs, 2020
The San Juan Basin (SJB) of New Mexico, USA preserves arguably the best record of early Paleocene... more The San Juan Basin (SJB) of New Mexico, USA preserves arguably the best record of early Paleocene mammalian evolution, spanning the Puercan and Torrejonian North American land mammal ages (NALMA). The lack of floral data from the SJB limits paleoclimatic and paleoenvironmental reconstruction and prevents an assessment between the mammalian and plant records. Further, although early Paleocene fossil floras are well known from the Northern Great Plains of North America, relatively little is known about floras from southern North America. The lack of southern North American data limits regional comparisons of floral diversity, composition, and plant community responses to environmental change. We present a description of early Paleocene floras from the SJB and a comparison of SJB floras to floras from the Northern Great Plains. Fossil leaves were collected from the lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation, with a focus on localities correlative to the early Puercan (Pu1) to early Torrejonian (To1) NALMAs which have been dated to ~65.8-64.0 Ma in the SJB. These floras are dominated by angiosperms, diverse (average site diversity = 32.5), dominated by species that appear to be endemic to the SJB, and display variability in morphotypes occurrences between facies. The taxonomic composition of Puercan floras appears to be distinct from Torrejonian floras suggesting a possible large scale change in plant communities in the earliest Paleocene that may be correlative with a major turnover in the mammalian fauna. Paleoclimate analyses indicate the Puercan and Torrejonian floras sample a warm and relatively wet climate, which remained similar throughout the early Paleocene. Interestingly, the SJB floras are significantly more diverse than age equivalent Northern Great Plains floras and indicate a warmer and wetter climate. These results indicate that a diverse and largely endemic plant community existed in the SJB shortly after the Cretaceous-Paleogene (K-Pg) boundary and that these floras may have experienced a rapid turnover coincident with the Puercan-Torrejonian turnover in mammalian faunas. Further, these results suggest a significant difference between northern and southern floras and a large north-south diversity gradient, which may suggest a variable response to the K-Pg extinction event.
"Data_faunal_list" includes occurrence data for fossil mammals from the Barstovian, Cla... more "Data_faunal_list" includes occurrence data for fossil mammals from the Barstovian, Clarendonian, and Hemphillian North American Land Mammal Ages. Data originally downloaded from the MioMap database. Only fossil localities that produced at least one species of ungulate are included here. "Data_isotopes" includes the stable carbon and oxygen isotope values from the tooth enamel of Hemphillian-aged rhinoceroses from the Great Plains.
The San Juan Basin of New Mexico preserves the most complete record of mammalian evolution throug... more The San Juan Basin of New Mexico preserves the most complete record of mammalian evolution through the lower Paleocene of North America, spanning the Puercan and Torrejonian land-mammal ages. Faunal turnover is extreme in parts of this record, raising the question of whether these faunal changes resulted from extrinsic factors, such as climate or environmental change, or from intrinsic factors such as rapid rates of evolution or competition among taxa to fill newly opened ecological niches following the K-Pg mass extinction. To test this problem we construct stable carbon and oxygen isotope stratigraphies from mammalian tooth enamel to use as proxies for environmental and climate change, respectively. The longest isotopic record is from the closely related periptychid condylarths, Carsioptychus and Periptychus, and spans all eight local biozones (Pu2-3, To1-To6). Isotopes were also sampled from Tetraclaenodon, spanning most of the Torrejonian (To2-To6). We evaluate faunal turnover 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). The stable isotope record indicates stability in carbon and oxygen values through most of the record with two exceptions in the late Torrejonian: a significant increase in carbon values in the Tetraclaenodon from To4 to To5, and a significant increase in oxygen values in Periptychus from To5 to To6. In both cases, however, covariation of values does not occur between these taxa. Carbon isotope values in Tetraclaenodon are positively and significantly correlated with faunal origination rates, although the sample is small (n=5), suggesting that carbon values in Tetraclaenodon may be reflecting underlying environmental changes. However, oxygen isotope values, which serve as a climate proxy, are not correlated with turnover metrics in either Tetraclaenodon or Periptychus. Thus, there is no clear correlation between climate and faunal change in the San Juan Basin, and preliminary results suggest that mammalian turnover resulted mostly from intrinsic factors and/or immigration. Future efforts will focus on mean annual temperature estimates from San Juan Basin floras and expanding the stable isotope record to include additional taxa to better resolve the climate and isotopic records.
Body size plays a critical role in mammalian ecology and physiology. Previous research has shown ... more Body size plays a critical role in mammalian ecology and physiology. Previous research has shown that many mammals became smaller during the Paleocene-Eocene Thermal Maximum (PETM), but the timing and magnitude of that change relative to climate change have been unclear. A high-resolution record of continental climate and equid body size change shows a directional size decrease of ~30% over the first ~130,000 years of the PETM, followed by a ~76% increase in the recovery phase of the PETM. These size changes are negatively correlated with temperature inferred from oxygen isotopes in mammal teeth and were probably driven by shifts in temperature and possibly high atmospheric CO(2) concentrations. These findings could be important for understanding mammalian evolutionary responses to future global warming.
Upper Cretaceous and lower Paleocene rocks in the San Juan Basin, New Mexico, contain a robust re... more Upper Cretaceous and lower Paleocene rocks in the San Juan Basin, New Mexico, contain a robust record of dinosaur and mammal evolution, diversification, and extinction. Despite this rich fossil record, the ages and durations of the Upper Cretaceous Naashoibito Member of the Kirtland Formation and the lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation have been relatively poorly constrained. These poor age constraints have limited the ability to accurately correlate these vertebrate records to others across North America and to assess rates of speciation and extinction of dinosaurs in the Cretaceous and mammals before and after the Cretaceous-Paleogene (K-Pg) mass extinction. Further, the ages of the Naashoibito and the Ojo Alamo are contentious and have been interpreted to range from Campanian to early Paleocene. Here we present new geochronologic results that combine magnetostratigraphy and 40Ar/39Ar dating of detrital sanidine from sedimentary units and sanidine phenocry...
. Paleogene hyperthermals, including the Paleocene-Eocene Thermal Maximum (PETM) and several othe... more . Paleogene hyperthermals, including the Paleocene-Eocene Thermal Maximum (PETM) and several other smaller events, represent global perturbations to Earth's climate system and are characterized by warmer temperatures, shifts in floral and faunal communities, and hydrologic changes. These events are identified in the geologic record globally by negative carbon isotope excursions (CIEs), resulting from the input of isotopically light carbon into Earth's atmosphere. Much about the causes and effects of hyperthermals remains uncertain, including whether all hyperthermals are caused by the same underlying processes, how biotic effects scale with the magnitude of hyperthermals, and why CIEs are larger in paleosol carbonates relative to marine records. Resolving these questions is crucial for their full interpretation and application to future climate scenarios. The Fifteenmile Creek area of the central Bighorn Basin, Wyoming U.S.A., exposes an early Eocene floodplain sedimentary sequence that preserves paleosol carbonates and an extensive fossil mammal collection. Previous analysis of faunal assemblages revealed two pulses of mammal turnover and changes in diversity interpreted to correlate with the ETM2 and H2 hyperthermals that immediately follow the PETM. This was, however, based on long distance correlation of chemostratigraphic records. We present new carbon isotope stratigraphy using micrite δ13C values from paleosol carbonate nodules preserved in and between richly fossiliferous localities at Fifteenmile Creek to identify the stratigraphic positions of ETM2 and H2. Additionally, we used differential GPS elevations to establish a new stratigraphic framework that assists in correlation and is independent from the biostratigraphy and previous composite lithostratigraphic sections from the area. Carbon isotope results show that the ETM2 and H2 hyperthermals, and possibly the subsequent I1 hyperthermal, are recorded at Fifteenmile Creek. ETM2 and H2 overlap with the two previously recognized pulses of mammal turnover. Comparisons between the new chemostratigraphy and fossil record suggest that the recorded amplitude of these faunal changes may be muted as a result of some stratigraphic averaging of fossils. The CIEs for these hyperthermals are also smaller in magnitude than in more northerly Bighorn Basin records. We suggest that basin-wide differences in soil moisture and/or vegetation could contribute to variable CIE amplitudes in this and other terrestrial records.
The San Juan Basin of New Mexico (USA) contains one of the few records of superposed Late Cretace... more The San Juan Basin of New Mexico (USA) contains one of the few records of superposed Late Cretaceous and early Paleocene terrestrial faunas and floras. However, the ages and durations of the Upper Cretaceous Naashoibito Mbr of the Kirtland Fm, and the lower Paleocene Ojo Alamo Ss and Nacimiento Fm are poorly constrained. This limits the ability to correlate these fossil records to the global time scale and hinders efforts to examine such factors as diversity and survivorship of plants and animals across the K-Pg boundary. The ages of the Naashoibito Mbr and the Ojo Alamo Ss are especially contentious and age interpretations range from Campanian to early Paleocene.
Here we present new geochronologic results that combine magnetostratigraphy and 40Ar/39Ar dating of detrital sanidine from sedimentary units and sanidine phenocrysts from a volcanic ash to constrain the ages of the Naashoibito Mbr, the Ojo Alamo Ss, and the lower Nacimiento Fm. Coupled detrital sanidine dates, magnetostratigraphy, and mammal biochronology indicate that the Naashoibito Mbr correlates to chrons C30n-C29r, suggesting a relatively short depositional history. These results indicate that the youngest Cretaceous sedimentary rocks in the San Juan Basin were likely deposited within the last ~400 kyr of the Cretaceous and that there is a significant unconformity in the Kirtland Fm between the Naashoibito Mbr and the underlying De-Na-Zin Mbr. Further, it suggests that the Naashoibito Mbr is correlative in age to much of the Hell Creek Fm in the Northern Great Plains. Magnetostratigraphy of the Ojo Alamo Ss and the Nacimiento Fm at both De-na-zin Wash and near Mesa de Cuba indicate that there is no evidence for significant diachroneity at the base of the Nacimiento Fm across the basin. The Ojo Alamo Ss was deposited in chron C29r and the lower Nacimiento Fm in chrons C29r-C28r. Deposition of basal Paleocene strata in the basin began <300 kyr after the K-Pg boundary. Magnetostratigraphy and an 40Ar/39Ar sanidine date from an ash within the Nacimiento Fm demonstrate that the middle Puercan Land Mammal Age (Pu2) interval zone began within ~350 kyr of the K-Pg boundary. An 40Ar/39Ar date from a probable volcanic ash coincident with the first occurrence of Pu3 mammals suggests that only ~150 kyr separated the type Pu2 and Pu3 faunas. This new chronostratigraphic framework indicates that diverse, ecologically complex mammal faunas were present locally within no more than a few hundred thousand years after the extinction of non-avian dinosaurs.
The San Juan Basin of New Mexico preserves the most complete record of mammalian evolution throug... more The San Juan Basin of New Mexico preserves the most complete record of mammalian evolution through the lower Paleocene of North America, spanning the Puercan and Torrejonian land-mammal ages. Faunal turnover is extreme in parts of this record, raising the question of whether these faunal changes resulted from extrinsic factors, such as climate or environmental change, or from intrinsic factors such as rapid rates of evolution or competition among taxa to fill newly opened ecological niches following the K-Pg mass extinction. To test this problem we construct stable carbon and oxygen isotope stratigraphies from mammalian tooth enamel to use as proxies for environmental and climate change, respectively. The longest isotopic record is from the closely related periptychid condylarths, Carsioptychus and Periptychus, and spans all eight local biozones (Pu2-3, To1-To6). Isotopes were also sampled from Tetraclaenodon, spanning most of the Torrejonian (To2-To6). We evaluate faunal turnover 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). The stable isotope record indicates stability in carbon and oxygen values through most of the record with two exceptions in the late Torrejonian: a significant increase in carbon values in the Tetraclaenodon from To4 to To5, and a significant increase in oxygen values in Periptychus from To5 to To6. In both cases, however, covariation of values does not occur between these taxa. Carbon isotope values in Tetraclaenodon are positively and significantly correlated with faunal origination rates, although the sample is small (n=5), suggesting that carbon values in Tetraclaenodon may be reflecting underlying environmental changes. However, oxygen isotope values, which serve as a climate proxy, are not correlated with turnover metrics in either Tetraclaenodon or Periptychus. Thus, there is no clear correlation between climate and faunal change in the San Juan Basin, and preliminary results suggest that mammalian turnover resulted mostly from intrinsic factors and/or immigration. Future efforts will focus on mean annual temperature estimates from San Juan Basin floras and expanding the stable isotope record to include additional taxa to better resolve the climate and isotopic records.
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Here we present new geochronologic results that combine magnetostratigraphy and 40Ar/39Ar dating of detrital sanidine from sedimentary units and sanidine phenocrysts from a volcanic ash to constrain the ages of the Naashoibito Mbr, the Ojo Alamo Ss, and the lower Nacimiento Fm. Coupled detrital sanidine dates, magnetostratigraphy, and mammal biochronology indicate that the Naashoibito Mbr correlates to chrons C30n-C29r, suggesting a relatively short depositional history. These results indicate that the youngest Cretaceous sedimentary rocks in the San Juan Basin were likely deposited within the last ~400 kyr of the Cretaceous and that there is a significant unconformity in the Kirtland Fm between the Naashoibito Mbr and the underlying De-Na-Zin Mbr. Further, it suggests that the Naashoibito Mbr is correlative in age to much of the Hell Creek Fm in the Northern Great Plains. Magnetostratigraphy of the Ojo Alamo Ss and the Nacimiento Fm at both De-na-zin Wash and near Mesa de Cuba indicate that there is no evidence for significant diachroneity at the base of the Nacimiento Fm across the basin. The Ojo Alamo Ss was deposited in chron C29r and the lower Nacimiento Fm in chrons C29r-C28r. Deposition of basal Paleocene strata in the basin began <300 kyr after the K-Pg boundary. Magnetostratigraphy and an 40Ar/39Ar sanidine date from an ash within the Nacimiento Fm demonstrate that the middle Puercan Land Mammal Age (Pu2) interval zone began within ~350 kyr of the K-Pg boundary. An 40Ar/39Ar date from a probable volcanic ash coincident with the first occurrence of Pu3 mammals suggests that only ~150 kyr separated the type Pu2 and Pu3 faunas. This new chronostratigraphic framework indicates that diverse, ecologically complex mammal faunas were present locally within no more than a few hundred thousand years after the extinction of non-avian dinosaurs.