A window into Late Triassic biodiversity.

Reconstruction of the paleocommunity of Cerro Las Lajas. Credit: Lucas Fiorelli.

The Ischigualasto Formation, formed along the western margin of Argentina during the breakup of Gondwana, represents one of the most continuous continental Triassic succesions in South America, and it is known worldwide for its tetrapod assemblage, which include the oldest known record of dinosaurs. The most accepted hypothesis gives the name “Ischigualasto” a Quechua origin, meaning “place where the moon sets”. A second hypothesis suggested that the name “Ischigualasto” has Diaguita roots and means “place of death”. Adolf Stelzner in 1889 published the first data on the geology of Ischigualasto, but it was not until 1911, that Bondenbender briefly refers to the fossils of the site. The Ischigualasto Formation consists of four lithostratigraphic members which in ascending order include the La Peña Member, the Cancha de Bochas Member, the Valle de la Luna Member, and the Quebrada de la Sal Member. The northernmost known outcrops of the Ischigualasto formation are exposed at a site know as Hoyada del Cerro Las Lajas, in La Rioja Province, consisting of more than 1,000 m of fluvial-channel and flood overbank deposits with high volcanic input. This site is known as the place where the holotype and only known specimen of Pisanosaurus mertii (PVL 2577) was found.

Ischigualasto Formation in the Hoyada del Cerro Las Lajas locality. From Desojo et al., 2020

In 1962, José F. Bonaparte, Rafael Herbst, Galileo J. Scaglia, and Martín Vince carried out an expedition to the site. Bonaparte’s field notes indicate that they collected rhynchosaur and cynodont material at the site, but never described. In 2013, on the occasion of the XVII Argentine Conference of Vertebrate Paleontology, a group of researchers lead by Julia Desojo, from the National University of La Plata Museum, improvised a brief exploration to the site. Over the course of three more expeditions between 2016 to 2019, the team collected fossils and rocks from various layers of the Las Lajas outcrop, and more than 100 new fossil specimens, including Teyumbaita, a extinct genus of hyperodapedontine rhynchosaur, only previously known in the Late Triassic beds of the Santa Maria Supersequence in southern Brazil.

Teyumbaita. From Desojo et al., 2020

The team analyzed samples of volcanic ash collected from several layers of the Las Lajas outcrops and found that the layers were deposited between 230 million and 221 million years ago. They also found a correlation between the Hyperodapedon and Teyumbaita biozones at the Hoyada del Cerro Las Lajas, respectively, to the lower and upper parts of the Scaphonyx-Exaeretodon-Herrerasaurus biozone in the Hoyada de Ischigualasto and to the upper Hyperodapedon Assemblage Zone of the Santa Maria Supersequence in southern Brazil. Teyumbaita-rich faunas of both Brazil and Argentina persisted into the Norian, before it was eventually replaced by tetrapod assemblages that witnessed the humidity increase of southwestern Pangaean climate.

Reconstructed skeleton reflecting the traditional interpretation of Pisanosaurus (Royal Ontario Museum)

Pisanosaurus mertii was originally described by Argentinian paleontologist Rodolfo Casamiquela in 1967, based on a poorly preserved but articulated skeleton from the upper levels of the Ischigualasto Formation. The holotype and only known specimen (PVL 2577) is a fragmentary skeleton including partial upper and lower jaws, seven articulated dorsal vertebrae, four fragmentary vertebrae of uncertain position in the column, the impression of the central portion of the pelvis and sacrum, an articulated partial hind limb including the right tibia, fibula, proximal tarsals and pedal digits III and IV, the distal ends of the right and left femora, a left scapular blade (currently lost), a probable metacarpal III, and the impressions of some metacarpals (currently lost). The new study constrains the age of Pi. mertii as ca. 229 Ma, showing that this species is latest Carnian. Additonally, certain key anatomical features, like the external mandibular fossa and the anteroposteriorly short cervical vertebrae, indicate that Pisanosaurus is the earliest preserved Ornithiscian specimen.

 

References:

Desojo, J.B., Fiorelli, L.E., Ezcurra, M.D. et al. The Late Triassic Ischigualasto Formation at Cerro Las Lajas (La Rioja, Argentina): fossil tetrapods, high-resolution chronostratigraphy, and faunal correlations. Sci Rep 10, 12782 (2020). https://doi.org/10.1038/s41598-020-67854-1

Federico L. Agnolín & Sebastián Rozadilla (2017): Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina, Journal of Systematic Palaeontology DOI: 10.1080/14772019.2017.1352623

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