Abstract
Large-diameter ichnofossils comprising three morphotypes have been identified in the Upper Triassic Ischigualasto and Los Colorados formations of northwestern Argentina. These burrows add to the global record of the early appearance of fossorial behavior during early Mesozoic time. Morphotypes 1 and 2 are characterized by a network of tunnels and shafts that can be assigned to tetrapod burrows given similarities with previously described forms. However, differences in diameter, overall morphology, and stratigraphic occurrence allow their independent classification. Morphotype 3 forms a complex network of straight branches that intersect at oblique angles. Their calcareous composition and surface morphology indicate these structures have a composite biogenic origin likely developed due to combined plant/animal interactions. The association of Morphotypes 1 and 2 with fluvial overbank lithologies deposited under an extremely seasonal arid climate confirms interpretations that the early appearance of burrowing behavior was employed by vertebrates in response to both temperature and moisture-stress associated with seasonally or perpetually dry Pangean paleoclimates. Comparisons of burrow morphology and biomechanical attributes of the abundant paleovertebrate fauna preserved in both formations permit interpretations regarding the possible burrow architects for Morphotypes 1 and 2. In the case of the Morphotype 1, the burrow constructor could be one of the small carnivorous cynodonts, Ecteninion or Probelesodon. Assigning an architect for Morphotype 2 is more problematic due to mismatches between the observed burrow morphology and the size of the known Los Colorados vertebrates.
Highlights
Over the last two decades, tetrapod-burrow casts with diameters greater than 10 cm have been identified across broad paleolatitudinal gradients of the supercontinent Pangea
In spite of several reports of burrows from South America (e.g. [11,14]), the first burrows to be described in detail from this continent are derived from the Upper Triassic Ischigualasto Formation from the Ischigualasto-Villa Union Basin of Argentina [7]
The large diameter burrows of the Ischigualasto-Villa Union Basin add to the global record of the early appearance of fossorial behavior during early Mesozoic time
Summary
Over the last two decades, tetrapod-burrow casts with diameters greater than 10 cm have been identified across broad paleolatitudinal gradients of the supercontinent Pangea. These structures have been recorded in South Africa, Antarctica, North America, Europe, and South America and are an indicator of a relatively common tetrapod behavior during Permo-Triassic time The South African burrows are important because of their internal preservation of small-sized therapsid fossils, interpreted as the remains of the burrow architect [1,2]. Two morphotypes are characterized by networks of tunnels and shafts that can be assigned to tetrapod burrows given similarities with previously described forms [4,5,6]. A third morphotype is interpreted as forming as a result of interaction between burrowing invertebrate (or vertebrate) and coeval root systems
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