Cecilia Apaldetti

Vertebrate succession in the Ischigualasto Formation

ABSTRACT The Upper Triassic (Carnian—Norian) Ischigualasto Formation has yielded a diverse vertebrate fauna that records the initial phase of dinosaur evolution. Radioisotopic dates from ash layers within the formation provide a chrono-stratigraphic framework, and stratigraphic and sedimetological studies have subdivided the formation into four members and three abundance-based biozones. We describe two new basal dinosauromorphs, an unnamed lagerpetid and a new silesaurid, Ignotosaurus fragilis, gen. et sp. nov., which increase to 29 the number of vertebrates in the Ischigualasto fauna. We provide a census of 848 fossil specimens representing 26 vertebrate taxa logged to stratigraphic intervals of 50 m. This temporally calibrated census shows that abundance and taxonomic diversity within the Ischigualasto Formation does not change suddenly but rather appears to gradually decline in response to climatic deterioration. The only abrupt shift in faunal composition occurs at the end of the second of three biozones, when the abundant cynodont Exaeretodon is replaced by the rare dicynodont Jachaleria.

A New Basal Sauropodomorph (Dinosauria: Saurischia) from Quebrada del Barro Formation (Marayes-El Carrizal Basin), Northwestern Argentina

Background Argentinean basal sauropodomorphs are known by several specimens from different basins; Ischigualasto, El Tranquilo, and Mogna. The Argentinean record is diverse and includes some of the most primitive known sauropodomorphs such as Panphagia and Chromogisaurus, as well as more derived forms, including several massospondylids. Until now, the Massospondylidae were the group of basal sauropodomorphs most widely spread around Pangea with a record in almost all continents, mostly from the southern hemisphere, including the only record from Antarctica. Methodology/Principal Finding We describe here a new basal sauropodomorph, Leyesaurus marayensis gen. et sp. nov., from the Quebrada del Barro Formation, an Upper Triassic-Lower Jurassic unit that crops out in northwestern Argentina. The new taxon is represented by a partial articulated skeleton that includes the skull, vertebral column, scapular and pelvic girdles, and hindlimb. Leyesaurus is diagnosed by a set of unique features, such as a sharply acute angle (50 degrees) formed by the ascending process of the maxilla and the alveolar margin, a straight ascending process of the maxilla with a longitudinal ridge on its lateral surface, noticeably bulging labial side of the maxillary teeth, greatly elongated cervical vertebrae, and proximal articular surface of metatarsal III that is shelf-like and medially deflected. Phylogenetic analysis recovers Leyesaurus as a basal sauropodomorph, sister taxon of Adeopapposaurus within the Massospondylidae. Moreover, the results suggest that massospondylids achieved a higher diversity than previously thought. Conclusions/Significance Our phylogenetic results differ with respect to previous analyses by rejecting the massospondylid affinities of some taxa from the northern hemisphere (e.g., Seitaad, Sarahsaurus). As a result, the new taxon Leyesaurus, coupled with other recent discoveries, suggests that the diversity of massospondylids in the southern hemisphere was higher than in other regions of Pangea. Finally, the close affinities of Leyesaurus with the Lower Jurassic Massospondylus suggest a younger age for the Quebrada del Barro Formation than previously postulated.

A new sphenodontian (Lepidosauria: Rhynchocephalia) from the Late Triassic of Argentina and the early origin of the herbivore opisthodontians

Sphenodontians were a successful group of rhynchocephalian reptiles that dominated the fossil record of Lepidosauria during the Triassic and Jurassic. Although evidence of extinction is seen at the end of the Laurasian Early Cretaceous, they appeared to remain numerically abundant in South America until the end of the period. Most of the known Late Cretaceous record in South America is composed of opisthodontians, the herbivorous branch of Sphenodontia, whose oldest members were until recently reported to be from the Kimmeridgian–Tithonian (Late Jurassic). Here, we report a new sphenodontian, Sphenotitan leyesi gen. et sp. nov., collected from the Upper Triassic Quebrada del Barro Formation of northwestern Argentina. Phylogenetic analysis identifies Sphenotitan as a basal member of Opisthodontia, extending the known record of opisthodontians and the origin of herbivory in this group by 50 Myr.

Basal Sauropodomorphs from the Ischigualasto Formation

ABSTRACT Basal sauropodomorphs from the Ischigualasto Formation include Eoraptor lunensis, Panphagia protos, and Chromogisaurus novasi. Few comparisons have been made between these taxa, because Eoraptor was only recently reassessed as a basal sauropodomorph and because Panphagia and Chromogisaurus were described nearly simultaneously. We describe in detail the fully prepared bones of the holotype of Chromogisaurus novasi, examine the evidence for its taxonomic distinction, and analyze the phylogenetic relationships among basal sauropodomorphs. Our results support Chromogisaurus novasi as a valid genus and species and provide weak phylogenetic evidence favoring a series of stem taxa at the base of Sauropodomorpha. The analysis positions Panphagia as the basal-most sauropodomorph, followed by Eoraptor, Pampadromaeus, and a clade that includes Chromogisaurus and Saturnalia.

Redescription of the Skull of Coloradisaurus brevis (Dinosauria, Sauropodomorpha) from the Late Triassic Los Colorados Formation of the Ischigualasto-Villa Union Basin, northwestern Argentina

ABSTRACT The cranial anatomy of the basal sauropodomorph Coloradisaurus brevis from the upper levels of the Norian Los Colorados Formation is here redescribed and comparisons made based on the holotype skull and mandible. Coloradisaurus brevis is diagnosed by most of the features proposed in the original description and an additional set of autapomorphies, such as presence of circular upper temporal fenestrae, laminae on the ventrolateral margins of a ventral fossa of the basisphenoid and on the ventral region of the parasphenoid, and tab-like medial process at the posteromedial end of the mandible. Coloradisaurus is placed within Massospondylidae, as in recent analyses. This position is supported primarily by postcranial characters, but some cranial features identified in this study provide additional evidence supporting this position (e.g., jugal contribution to the antorbital fenestra, frontal proportionately longer than nasals). However, the cranial anatomy of Coloradisaurus also bears several characters that are shared with plateosaurids (e.g., low mandibular articulation, broad maxillary wall on the anterior margin of antorbital fossa, broad prefrontals, projection of infratemporal fenestra behind the orbit, stepped braincase, robust septum between basipterygoid processes). Thus, Coloradisaurus is interpreted as a massospondylid that has convergently acquired characters that are otherwise only known in plateosaurids, highlighting the conflicting pattern of character distribution among basal sauropodomorphs.

Braincase of Panphagia protos (Dinosauria, Sauropodomorpha)

ABSTRACT We describe a partial braincase of the basal sauropodomorph Panphagia protos from the Upper Triassic (mid-Carnian) horizons of the Ischigualasto Formation. The disarticulated braincase from a subadult individual includes one frontal, both parietals, one prootic, and the supraoccipital. The frontal is longer anteroposteriorly than it is wide transversely, has a small anterolateral process, and an elongate oval depression for the olfactory bulb. The supraoccipital is broader transversely than it is deep dorsoventrally and lacks a pronounced median nuchal eminence. Some braincase features that characterize more derived basal sauropodomorphs are not present in Panphagia, including a broader frontal and reduced anterior tympanic and floccular recesses. Panphagia appears to represent an early stage in the evolution of sauropodomorph dinosaurs.

Novel insight into the origin of the growth dynamics of sauropod dinosaurs

Sauropod dinosaurs include the largest terrestrial animals and are considered to have uninterrupted rapid rates of growth, which differs from their more basal relatives, which have a slower cyclical growth. Here we examine the bone microstructure of several sauropodomorph dinosaurs, including basal taxa, as well as the more derived sauropods. Although our results agree that the plesiomorphic condition for Sauropodomorpha is cyclical growth dynamics, we found that the hypothesized dichotomy between the growth patterns of basal and more derived sauropodomorphs is not supported. Here, we show that sauropod-like growth dynamics of uninterrupted rapid growth also occurred in some basal sauropodomorphs, and that some basal sauropods retained the plesiomorphic cyclical growth patterns. Among the sauropodomorpha it appears that the basal taxa exploited different growth strategies, but the more derived Eusauropoda successfully utilized rapid, uninterrupted growth strategies.

A Norian Lagerpetid Dinosauromorph from the Quebrada Del Barro Formation, Northwestern Argentina

Abstract. The early evolution of Ornithodira —the clade including pterosaurs and dinosaurs— is poorly known. Until a decade ago, the basal radiation of Dinosauromorpha, the clade including dinosaurs and birds, was poorly understood because of the poor fossil record restricted to specimens known from the Ladinian Chañares Formation in Argentina. Over the last years the discovery of several non-dinosaurian dinosauromorphs dramatically expanded this record and also demonstrated that this group —previously restricted to de Middle Triassic— survived at least well into the Norian. Although Norian non-dinosaurian dinosauromorphs have been reported from several regions around the world, the only known Norian non-dinosauriform dinosauromorphs —Dromomeron romeri Irmis, Nesbitt, Padian, Smith, Turner, Woody, and Downs and Dromomeron gregorii Nesbitt, Irmis, Parker, Smith, Turner, and Rowe— come from North America. We report here the first record from the Southern Hemisphere of a non-dinosauriform dinosauromorph, Dromomeron gigas sp. nov., from the Norian Quebrada del Barro Formation, northwestern Argentina. A phylogenetic analysis recovers Dromomeron gigas nested into the monophyletic group Lagerpetidae, and as the sister taxon to Dromomeron romeri. The inclusion of D. gigas within Lagerpetidae suggests that body size increased in this lineage over time, as was previously demonstrated for Dinosauriformes as a whole, and that lagerpetids reached a larger size than previously thought. Finally, the new finding provides new information on the basal radiation of Dinosauromorpha constituting the first record of a Norian association of dinosaurs with non-dinosauriform dinosauromorphs outside North America.

FIRST ARGENTINE MICROFOSSIL BONEBED FROM THE UPPER TRIASSIC MARAYES—EL CARRIZAL BASIN, SAN JUAN PROVINCE

Abstract The first Triassic microfossil bonebed found in Argentina is located 80 meters from the top of the Quebrada del Barro Formation in the Marayes-El Carrizal Basin, in the province of San Juan. It consists of specimens from at least 63 individuals from an anomalously high concentration of fossils distributed laterally and vertically in a meter-thick fine-grained deposit. Two new taxa from the genera Sphenodontia and Eucynodontia had previously not been found in Argentine Triassic quarries. The bonebed is preserved in a mudflow deposit interbedded with calcic-Argillisols in the medial-distal zone of a distributary fluvial system (DFS). The accumulation is characterized by small-sized skeletal fragments (skulls, jaws and vertebra; all less than 50 mm), low degree of articulation, variable degrees of subaerial exposure, tooth marks, surface dissolution, and an alkaline authigenic mineral suite. Detailed paleoenvironmental and taphonomic characterization indicate that this accumulation underwent three stages in its taphonomic history: (1) biogenic extrinsic concentration; (2) local transportation and re-deposition by a mudflow on a swampy floodplain; and (3) drying and pedogenesis. Additionally, the deposit highlights biogenic activity as a way to concentrate a paleofaunal assemblage that likely represents the original community, and mudflow deposits from crevasse splays in DFS as a potential facies for microfossil preservation.

An early trend towards gigantism in Triassic sauropodomorph dinosaurs

Dinosaurs dominated the terrestrial ecosystems for more than 140 Myr during the Mesozoic era, and among them were sauropodomorphs, the largest land animals recorded in the history of life. Early sauropodomorphs were small bipeds, and it was long believed that acquisition of giant body size in this clade (over 10 tonnes) occurred during the Jurassic and was linked to numerous skeletal modifications present in Eusauropoda. Although the origin of gigantism in sauropodomorphs was a pivotal stage in the history of dinosaurs, an incomplete fossil record obscures details of this crucial evolutionary change. Here, we describe a new sauropodomorph from the Late Triassic of Argentina nested within a clade of other non-eusauropods from southwest Pangaea. Members of this clade attained large body size while maintaining a plesiomorphic cyclical growth pattern, displaying many features of the body plan of basal sauropodomorphs and lacking most anatomical traits previously regarded as adaptations to gigantism. This novel strategy highlights a highly accelerated growth rate, an improved avian-style respiratory system, and modifications of the vertebral epaxial musculature and hindlimbs as critical to the evolution of gigantism. This reveals that the first pulse towards gigantism in dinosaurs occurred over 30 Myr before the appearance of the first eusauropods.A new sauropodomorph dinosaur taxon, Ingentia prima, and new lessemsaurid fossils from the Late Triassic of Argentina, reveal a distinctive and early pathway towards gigantism, 30 million years before the first eusauropods appeared.