A new early-diverging probainognathian cynodont and a revision of the occurrence of cf. Aleodon from the Chañares Formation, northwestern Argentina: New clues on the faunistic composition of the latest Middle-?earliest Late Triassic Tarjadia Assemblage Zone.

The Chañares Formation (Ischigualasto-Villa Unión Basin) is worldwide known by its exquisitely preserved fossil record of latest Middle-to-early Late Triassic tetrapods, including erpetosuchids, "rauisuchians," proterochampsids, gracilisuchids, dinosauromorphs, pterosauromorphs, kannemeyeriiform dicynodonts, and traversodontid, chiniquodontid and probainognathid cynodonts, coming from the Tarjadia (bottom) and Massetognathus-Chanaresuchus (top) Assemblage Zones of its lower member. Regarding cynodonts, most of its profuse knowledge comes from the traditional layers discovered by Alfred Romer and his team in the 1960s that are now enclosed in the Massetognathus-Chanaresuchus Assemblage Zone (AZ). In this contribution we focus our study on the probainognathian cynodonts discovered in levels of the Tarjadia Assemblage Zone. We describe a new chiniquodontid cynodont with transversely broad postcanine teeth (Riojanodon nenoi gen. et sp. nov.) which is related to the genus Aleodon. In addition, the specimen CRILAR-Pv 567 previously referred to cf. Aleodon is here described, compared, and included in a phylogenetic analysis. It is considered as an indeterminate Aleodontinae nov., a clade here proposed to included chiniquodontids with transversely broad upper and lower postcanines, by having a cuspidated sectorial labial margin and a lingual platform that is twice broader than a lingual cingulum. Cromptodon mamiferoides, from the Cerro de Las Cabras Formation (Cuyo Basin), was also included in the phylogenetic analysis and recovered as an Aleodontinae. The new cynodont and the record of Aleodontinae indet. reinforce the faunal differentiation between the Tarjadia and Massetognathus-Chanaresuchus Assemblage Zones, in the lower member of the Chañares Formation, and inform on the diverse chiniquodontid clade with both sectorial and transversely broad postcanine teeth.

Unexpected morphological variability in the eggshells of the South American caimans Caiman latirostris and Caiman yacare.

Eggshell morphology is a valuable indicator of the local conditions within the nests of modern crocodilians and birds. In contrast to these latter, the anatomical structure of the eggshells of most crocodilian species is practically unknown. Here, we provide the first characterization of crocodilian eggshells, using x-ray micro-CT scans. We studied eggshells of Caiman latirostris and Caiman yacare from various developmental stages that coincide with the beginning of embryonic ossification. The new 3D renderings revealed complex ornamentation, unique among crocodilians, and amphora-shaped pore canals, some of which converge in single pore openings. We also documented a high density of pore canals with a gas diffusion capacity 45 times higher than the average predicted for modern avian eggshells. The external ornamentation and the thickness of the compact layer of the eggshells (i.e. excluding ornamentation) showed ontogenetic and interspecific differences that could be related to nesting materials and nesting areas selected by each species. The shell features described here evidence a greater structural complexity than previously recognized in phylogenetically close, sympatric crocodilian species. Further comprehensive morphological analyses on other modern and fossil crocodilian eggshells using micro-CT technology will shed new light on the evolution of reproductive strategies in this intriguing archosaur clade.

First titanosaur dinosaur nesting site from the Late Cretaceous of Brazil.

Titanosaurs were successful herbivorous dinosaurs widely distributed in all continents during the Cretaceous, with the major diversity in South America. The success of titanosaurs was probably due to several physiological and ecological factors, in addition to a series of morphological traits they achieved during their evolutionary history. However, the generalist nesting behaviour using different palaeoenvironments and strategies was key to accomplish that success. Titanosaur nesting sites have been found extensively around the world, with notable records in Spain, France, Romania, India, and, especially, Argentina. Here, we describe the first titanosaur nesting site from the Late Cretaceous of Brazil that represents the most boreal nesting site for South America. Several egg-clutches, partially preserved, isolated eggs and many eggshell fragments were discovered in an Inceptisol palaeosol profile of the mining Lafarge Quarry, at the Ponte Alta District (Uberaba Municipality, Minas Gerais State), corresponding to the Serra da Galga Formation (Bauru Group, Bauru Basin). Although classical mechanical preparation and CT scans have not revealed embryonic remains in ovo, the eggs and eggshell features match those eggs containing titanosaurian embryos found worldwide. The morphology of the egg-clutches and observations of the sedimentary characteristics bolster the hypothesis that these sauropods were burrow-nester dinosaurs, as was already suggested for the group based on other nesting sites. The egg-clutches distributed in two levels along the Lafarge outcrops, together with the geopalaeontological data collected, provide clear evidence for the first colonial nesting and breeding area of titanosaur dinosaurs in Brazil.

Two Late Cretaceous sauropods reveal titanosaurian dispersal across South America.

South American titanosaurians have been central to the study of the evolution of Cretaceous sauropod dinosaurs. Despite their remarkable diversity, the fragmentary condition of several taxa and the scarcity of records outside Patagonia and southwestern Brazil have hindered the study of continental-scale paleobiogeographic relationships. We describe two new Late Cretaceous titanosaurians from Quebrada de Santo Domingo (La Rioja, Argentina), which help to fill a gap between these main areas of the continent. Our phylogenetic analysis recovers both new species, and several Brazilian taxa, within Rinconsauria. The data suggest that, towards the end of the Cretaceous, this clade spread throughout southern South America. At the same locality, we discovered numerous accumulations of titanosaurian eggs, likely related to the new taxa. With eggs distributed in three levels along three kilometres, the new site is one of the largest ever found and provides further evidence of nesting site philopatry among Titanosauria.

Biomechanical evidence suggests extensive eggshell thinning during incubation in the Sanagasta titanosaur dinosaurs.

The reproduction of titanosaur dinosaurs is still a complex and debated topic. Their Late Cretaceous nesting sites are distributed worldwide and their eggs display substantial morphological variations according to the parent species. In contrast to the typical 1.3-2.0 mm thick shells common to eggs of most titanosaur species (e.g., those that nested in Auca Mahuevo, Tama, Totesti or Boseong), the Cretaceous Sanagasta eggs of Argentina display an unusual shell thickness of up to 7.9 mm. Their oviposition was synchronous with a palaeogeothermal process, leading to the hypothesis that their extra thick eggshell was an adaptation to this particular nesting environment. Although this hypothesis has already been supported indirectly through several investigations, the mechanical implications of developing such thick shells and how this might have affected the success of hatching remains untested. Finite element analyses estimate that the breaking point of the thick-shelled Sanagasta eggs is 14-45 times higher than for other smaller and equally sized titanosaur eggs. The considerable energetic disadvantage for piping through these thick eggshells suggests that their dissolution during incubation would have been paramount for a successful hatching.

Deep faunistic turnovers preceded the rise of dinosaurs in southwestern Pangaea.

The Triassic period documents the origin and diversification of modern amniote lineages and the Late Triassic fossil record of South America has been crucial to shed light on these early evolutionary histories. However, the faunistic changes that led to the establishment of Late Triassic ecosystems are largely ignored because of the global scarcity of fossils from assemblages a few million years older. Here we contribute to fill this gap with the description of a new tetrapod assemblage from the lowermost levels of the Chañares Formation (uppermost Middle-lower Late Triassic epochs) of Argentina, which is older than the other vertebrate assemblages of the same basin. The new assemblage is composed of therapsids, rhynchosaurids and archosaurs, and clearly differs from that of the immediately overlying and well-known historical Chañares vertebrate assemblage. The new tetrapod association is part of a phase of relatively rapidly changing vertebrate assemblage compositions, in a time span shorter than 6 million years, before the diversification of dinosaurs and other common Late Triassic tetrapods in southwestern Pangaea.

Micro-CT scan reveals an unexpected high-volume and interconnected pore network in a Cretaceous Sanagasta dinosaur eggshell.

The Cretaceous Sanagasta neosauropod nesting site (La Rioja, Argentina) was the first confirmed instance of extinct dinosaurs using geothermal-generated heat to incubate their eggs. The nesting strategy and hydrothermal activities at this site led to the conclusion that the surprisingly 7 mm thick-shelled eggs were adapted to harsh hydrothermal microenvironments. We used micro-CT scans in this study to obtain the first three-dimensional microcharacterization of these eggshells. Micro-CT-based analyses provide a robust assessment of gas conductance in fossil dinosaur eggshells with complex pore canal systems, allowing calculation, for the first time, of the shell conductance through its thickness. This novel approach suggests that the shell conductance could have risen during incubation to seven times more than previously estimated as the eggshell erodes. In addition, micro-CT observations reveal that the constant widening and branching of pore canals form a complex funnel-like pore canal system. Furthermore, the high density of pore canals and the presence of a lateral canal network in the shell reduce the risks of pore obstruction during the extended incubation of these eggs in a relatively highly humid and muddy nesting environment.

What do giant titanosaur dinosaurs and modern Australasian megapodes have in common?

Titanosauria is a globally distributed clade of sometimes extremely large Mesozoic herbivorous sauropod dinosaurs. On the basis of current evidence these giant dinosaurs seem to have reproduced in specific and localized nesting sites. However, no investigations have been performed to understand the possible ecological and geological biases that acted for the selection of these nesting sites worldwide. In this study, observations were performed on the best-known Cretaceous nesting sites around the world. Our observations strongly suggest their eggs were incubated with environmental sources of heat, in burial conditions. Taking into account the clutch composition and geometry, the nature and properties of the sediments, the eggshells' structures and conductance, it would appear that titanosaurs adopted nesting behaviors comparable to the modern Australasian megapodes, using burrow-nesting in diverse media and mound-building strategies.

The oldest known communal latrines provide evidence of gregarism in Triassic megaherbivores.

Defecation in communal latrines is a common behaviour of extant mammals widely distributed among megaherbivores. This behaviour has key social functions with important biological and ecological implications. Herbivore communal latrines are only documented among mammals and their fossil record is exceptionally restricted to the late Cenozoic. Here we report the discovery of several massive coprolite associations in the Middle-Late Triassic of the Chañares Formation, Argentina, which represent fossil communal latrines based on a high areal density, small areal extension and taphonomic attributes. Several lines of evidence (size, morphology, abundance and coprofabrics) and their association with kannemeyeriiform dicynodonts indicate that these large synapsids produced the communal latrines and had a gregarious behaviour comparable to that of extant megaherbivores. This is the first evidence of megaherbivore communal latrines in non-mammal vertebrates, indicating that this mammal-type behaviour was present in distant relatives of mammals, and predates its previous oldest record by 220 Mya.