Large mammal burrows in late Miocene calcic paleosols from central Argentina: paleoenvironment, taphonomy and producers

Femora nutrient foramina and aerobic capacity in giant extinct xenarthrans

Nutrient foramina are small openings in the periosteal surface of the mid-shaft region of long bones that traverse the cortical layer and reach the medullary cavity. They are important for the delivery of nutrients and oxygen to bone tissue and are crucial for the repair and remodeling of bones over time. The nutrient foramina in the femur's diaphysis are related to the energetic needs of the femur and have been shown to be related to the maximum metabolic rate (MMR) of taxa. Here, we investigate the relationship between nutrient foramen size and body mass as a proxy to the aerobic capacity of taxa in living and extinct xenarthrans, including living sloths, anteaters, and armadillos, as well as extinct xenarthrans such as glyptodonts, pampatheres, and ground sloths. Seventy femora were sampled, including 20 from extant taxa and 50 from extinct taxa. We obtained the blood flow rate (Q̇) based on foramina area and performed PGLS and phylogenetic ANCOVA in order to explore differences among mammalian groups. Our results show that, among mammals, taxa commonly associated with lower metabolism like living xenarthrans showed relatively smaller foramina, while the foramina of giant extinct xenarthrans like ground sloths and glyptodonts overlapped with non-xenarthran placentals. Consequently, Q̇ estimations indicated aerobic capacities comparable to other placental giant taxa like elephants or some ungulates. Furthermore, the estimation of the MMR for fossil giant taxa showed similar results, with almost all taxa showing high values except for those for which strong semi-arboreal or fossorial habits have been proposed. Moreover, the results are compatible with the diets predicted for extinct taxa, which indicate a strong consumption of grass similar to ungulates and in contrast to the folivorous or insectivorous diets of extant xenarthrans. The ancestral reconstruction of the MMR values indicated a lack of a common pattern for all xenarthrans, strongly supporting the occurrence of low metabolic rates in extant forms due to their particular dietary preferences and arboreal or fossorial habits. Our results highlight the importance of considering different evidence beyond the phylogenetic position of extinct taxa, especially when extinct forms are exceptionally different from their extant relatives. Future studies evaluating the energetic needs of giant extinct xenarthrans should not assume lower metabolic rates for these extinct animals based solely on their phylogenetic position and the observations on their extant relatives.

Early Cretaceous lepidosaur (sphenodontian?) burrows

Scarce fossil tetrapod burrows have been recorded in Cretaceous rocks, which is probably linked to the dominant equable climates that existed for most of this period. The occurrence of Cretaceous tetrapod burrows from Patagonia (Chubut Province, Argentina) dated between 118 and 115 million years ago, gives insights into their paleoecology and paleoenvironment. The rocks containing the tetrapod burrows are of pyroclastic origin and represent eolian dunes and ash-fall deposits, some reworked by fluvial currents and others showing soil development. Fossil burrow casts preserved in a paleosol are composed by a ramp with a slightly curved or straight path in plan-view and lacking bifurcation, a rounded termination with no enlargement, showing a reniform cross-section, and are assigned to the ichnospecies Reniformichnus katikatii. The strongly flattened cross-sectional shape of the burrow casts and comparison with modern lizard burrows suggest that the producers were lepidosaurs (body mass = 50-323 g). Among Cretaceous fossorial lepidosaurs from Patagonia, the best candidate is an eilenodontine sphenodontian. Sphenodontians burrowed in the fossil soils where also arthropods, earthworms and shrubby plants thrived. The rare occurrence of tetrapod burrows in Cretaceous rocks is linked to stressing conditions related to frequent arrival of volcanic ash and a semiarid seasonal climate.

New radiometric 40Ar-39Ar dates and faunistic analyses refine evolutionary dynamics of Neogene vertebrate assemblages in southern South America

The vertebrate fossil record of the Pampean Region of Argentina occupies an important place in South American vertebrate paleontology. An abundance of localities has long been the main basis for constructing the chronostratigraphical/geochronological scale for the late Neogene-Quaternary of South America, as well as for understanding major patterns of vertebrate evolution, including the Great American Biotic Interchange. However, few independently-derived dates are available for constraining this record. In this contribution, we present new 40Ar/39Ar dates on escorias (likely the product of meteoric impacts) from the Argentinean Atlantic coast and statistically-based biochronological analyses that help to calibrate Late Miocene-Pliocene Pampean faunal successions. For the type areas of the Montehermosan and Chapadmalalan Ages/Stages, our results delimit their age ranges to 4.7-3.7 Ma and ca. 3.74-3.04 Ma, respectively. Additionally, from Buenos Aires Province, dates of 5.17 Ma and 4.33 Ma were recovered for "Huayquerian" and Montehermosan faunas. This information helps to better calibrate important first appearances of allochthonous taxa in South America, including one of the oldest records for procyonids (7.24-5.95 Ma), cricetids (6.95-5.46 Ma), and tayassuids (> 3.74 Ma, oldest high-confidence record). These results also constrain to ca. 3 Ma the last appearances of the autochthonous sparassodonts, as well as terror birds of large/middle body size in South America. South American faunal turnover during the late Neogene, including Late Pliocene extinctions, is interpreted as a consequence of knock-on effects from global climatic changes and initiation of the icehouse climate regime.

Comparative morphometric analysis of lungs of the semifossorial giant pouched rat (Cricetomys gambianus) and the subterranean Nigerian mole rat (Cryptomys foxi)

Lungs of the rodent species, the African giant pouched rat (Cricetomys gambianus) and the Nigerian mole rat (Cryptomys foxi) were investigated. Significant morphometric differences exist between the two species. The volume of the lung per unit body mass was 2.7 times larger; the respiratory surface area 3.4 times greater; the volume of the pulmonary capillary blood 2 times more; the harmonic mean thickness of the blood-gas (tissue) barrier (τht) ~29% thinner and; the total pulmonary morphometric diffusing capacity (DLo2) for O2 2.3 times more in C. foxi. C. gambianus occupies open burrows that are ventilated with air while C. foxi lives in closed burrows. The less morphometrically specialized lungs of C. gambianus may be attributed to its much larger body mass (~6 times more) and possibly lower metabolic rate and its semifossorial life whereas the 'superior' lungs of C. foxi may largely be ascribed to the subterranean hypoxic and hypercapnic environment it occupies. Compared to other rodents species that have been investigated hitherto, the τht was mostly smaller in the lungs of the subterranean species and C. foxi has the highest mass-specific DLo2. The fossorial- and the subterranean rodents have acquired various pulmonary structural specializations that relate to habitats occupied.