Fernando Abdala

A sauropodomorph dinosaur from the Upper Triassic (Carnian) of southern Brazil

Abstract Three newly discovered skeletons from the Carnian red beds of the Santa Maria Formation, south Brazil, represent one of the oldest dinosaurs ever found. The new taxon Saturnalia tupiniquim, is equivalent in age to the earliest dinosaurs from northwestern Argentina, being the oldest sauropodomorph dinosaur known from plentiful skeletal material. The record of Saturnalia, a 1.5-m-long gracile plant-eating animal, indicates that, like other major dinosaur lineages, the first representatives of the mainly heavy-built sauropodomorphs were gracile animals.

Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions

Abstract The enigmatic concretionary exposures that typify the Chañares Formation (Ladinian, northwestern Argentina) long have defied precise paleoenvironmental characterization. Recent work indicates that the formation accumulated in an alluvial-to-lacustrine setting within an active rift basin that received sedimentary detritus from surrounding highlands, as well as copious amounts of volcanic ash. Ash- flow sheets were emplaced presumably as secondary mass flows on alluvial surfaces characterized by small fluvial channels and shallow lakes. Thin bentonite beds intercalated in the Chañares Formation indicate that ash also accumulated via direct airfall, although this mode of emplacement accounts for a very small fraction of the overall section. A shift to widespread lacustrine deposition is recorded by the superjacent Los Rastros Formation, which preserves at least six shallowing-upward hemicycles, five of which commenced amidst explosive volcanic activity as evidenced by intercalated bentonite beds. Volcanism played an important role in the generation and preservation of the Chañares Formations exceptional tetrapod fossil record. This is especially true of the classic Los Chañares locality, where more than 100 individuals representing a diverse array of taxa (archosaurs, cynodonts, dicynodonts) are entombed in volcanogenic concretions with matrices of relic glass shards diagenetically replaced by calcite. Taphonomic attributes of the Los Chañares locality are consistent with the scenario of mass mortality, and several clues hint at the nature of the event. The killing agent was lethal to a variety of taxa, killed both adults and juveniles, and led to the concentration of taxa that under normal circumstances would tend to dissociate, such as carnivores and their potential prey. It also produced a counterintuitive bias against the preservation of large-bodied taxa, which may have been largely unsusceptible to the death event, or perhaps were excluded from the Los Chañares death assemblage via post-mortem sorting. The spatial arrangement of skeletal material in a small sample of concretions is consistent with the stranding of tetrapod carcasses along a strandline, and it is feasible that volcanism led to catastrophic flooding of the landscape via damming and/or diversion of local drainages. Uncompacted skeletal elements and relic outlines of glass shards indicate that carbonate concretions formed shortly after skeletal material was buried in reworked volcanic ash. The microbial decay of organic matter presumably catalyzed concretion diagenesis. There is no indication that bone hydroxyapatite diffused into the entombing glassy matrix and contributed to concretion formation. Bones entombed within early diagenetic concretions were safeguarded from subsequent destructive pedogenic and/or diagenetic processes, and were incorporated in exquisite quality into the fossil record.

Gomphodont cynodonts of the Chañares Formation: the analysis of an ontogenetic sequence

Abstract A revision of gomphodont cynodonts from the Chañares Formation of Argentina, is presented. Four species in two genera: Massetognathus pascuali, M. teruggii, M. major, and Megagomphodon oligodens were originally described by Romer in 1967 and 1972. Qualitative characters proposed originally as diagnostic of the species were revised, and a bivariate analysis of allometry of 17 linear skull measurements was performed upon 31 specimens. Allometry relationships account for most of the differences among specimens. There are neither size gaps nor systematic trends across the sample. The temporal region scales with a ′positive′ allometry, whereas components of the neurocranium and muzzle show ′negative′ or isometric scaling. This results in a change from short, stocky skulls with large muzzles in small specimens, to slender skulls with small muzzles and large temporal fossae characteristic of large individuals. The reduction and ontogenetic loss of the parietal foramen in a cynodont is reported for the first time. Consideration of qualitative characters together with the allometric results lead us to conclude that all forms belong to a single species in a growth series. Consequently, Massetognathus pascuali is recognized as the only valid entity, an idea previously suggested but without a justification based upon extensive analysis.

POSTWEANING ONTOGENY OF THE SKULL OF DIDELPHIS ALBIVENTRIS

Abstract Postweaning development of the skull of Didelphis albiventris was studied in juveniles (3.5–8 months), subadults (7.5–9.5 months), and adults (>9.5 months). Analysis of allometry of 15 skull measurements was performed on a continuous growth series of 52–61 specimens to evaluate quantitative ontogenetic changes. Complex modifications occurred in cranial bones or regions such as the palate, processes for origin and attachment of jaw muscles, mandibular joint, frontal region, braincase, occipital bones, and petrosal. All measurements except zygomatic breadth were either positively or negatively allometric. Neurocranial components grew with strongly negative (<1) coefficients of allometry. Adult proportions of the masticatory apparatus arose from the counteraction of developmental trends; for example, the space for large temporal muscles was provided by isometric growth of the zygomatic breadth versus slower growth of the braincase. We interpret most of the postweaning developments as a function of the shift from milk suckling to active mastication.

Comparative Postnatal Ontogeny of the Skull in Dromiciops gliroides (Marsupialia: Microbiotheriidae)

Abstract Dromiciops gliroides is the single extant representative of the marsupial family Microbiotheriidae. The importance of D. gliroides stems from its peculiar cranial anatomy (specifically the configuration of the tympanic region) and dentition and from its controversial position in the phylogenetic tree of marsupials—a South American form more closely related to Australasian marsupials. We studied the postnatal ontogeny of the skull in D. gliroides by analyzing qualitative and allometric aspects of the development of cranial structures. We compared recently weaned young individuals with adults and described the bivariate and multivariate allometric trends of 14 cranial dimensions for a sample of 37–51 specimens. Most cranial components develop in a way similar to didelphids studied so far. However, some trends (e.g., growth of the orbit) seem particular to D. gliroides. The microbiotheriid bulla of D. gliroides, a structure to which five basicranial bones contribute parts, is already present in its highly derived condition in the youngest specimens of our series. We conclude that except for the bulla, most of the cranial development in D. gliroides is highly conservative and that some peculiarities may be shared with other marsupials of similarly small body size. Data on australidelphians and small-size didelphids are needed to contrast these patterns.

A new species of the traversodontid cynodont Exaeretodon from the Santa Maria Formation (Middle/Late Triassic) of southern Brazil

Abstract The skull of the traversodontid cynodont Exaeretodon riograndensis, sp. nov. from the Santa Maria Formation, Rio Grande do Sul State, Brazil is described. The excellent preservation of one specimen reveals new cranial information about the genus. The presence of prootic crests in the extensive lateral flange, anterior to the fenestra ovalis, is a noteworthy basicranial trait, unique to this new taxon among non-mammalian cynodonts. These crests are interpreted as related to the origin of the posterior pterygoid muscle representing the first evidence, among non-mammalian cynodonts to mammals transition, of the attachment of this muscle onto the prootic bone. The new species is most similar to Exaeretodon frenguellii from the Ischigualasto Formation of Argentina. Small differences in the posterior projections of the jugal onto the squamosal and in the shape of prefrontals distinguish the two species. Additionally, the number of postcanines seems to be ontogenetically less variable in the Brazilian species. The postcanine number decreases from six plus one erupting in a small individual (preorbital length: 82.9 mm) to five plus one erupting in the largest specimen (pl. 121.5 mm).

Craniodental ontogeny of the pacarana Dinomys branickii Peters 1873 (Rodentia, Hystricognathi, Caviomorpha, Dinomyidae)

We investigate the cranial, mandibular, and dental ontogeny of the pacarana Dinomys branickii, the only extant species of the rodent family Dinomyidae. A bivariate allometric study (via reduced major axis), including 21 variables from the skull and 6 from the lower jaw on samples varying from 21 to 32 specimens, was produced. Results of the analyses indicate fast growth in the rostrum and palatal length but isometric growth in nasal width, rostral height, and length of the upper diastema. Positive allometry is also present in mandibular length, in retromolar length of the lower jaw, and, especially, in height of the articular process. Negative allometry is found in the length and width of the tympanic bulla, braincase width, width of the occipital condyles, and occipital height. We document for the 1st time the presence of a 3rd premolar (dP3), represented by a tiny, rooted element, and which is lost in early postnatal ontogeny. Replacement of dP4/dp4 occurs after eruption of M3/m3. The complete eruption of P4/p4 happens when M3/m3 are fully functional. Ontogenetic changes in the postnatal ontogeny of the skull of D. branickii are mostly related to the development of the occlusal system and action of associated muscles. This is evidenced by rapid growth of the facial region, a deepening and strengthening of the temporal and occipital regions and a widening of the posterior portion of the palate which is related to the eruption and replacement of molariform teeth. Some observed that craniomandibular trends in the ontogeny of D. branickii are reflected in the size of Ctenohystrica taxa, with small species of this lineage showing features similar to those of juveniles of D. branickii and large species resembling adults of the pacarana.