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

MICROFOSSILS AND THE OCEAN HISTORY.

Forams from deep-sea. Credit: Miriam Katz, Rensselaer Polytechnic Institute. (Originally published by Micropress.)

Forams from deep-sea. Credit: Miriam Katz, Rensselaer Polytechnic Institute. (Originally published by Micropress.)

Microfossils from deep-sea are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments.

The importance of microfossils as tool for paleoclimate reconstruction was recognized early in the history of oceanography. John Murray, naturalist of the CHALLENGER Expedition (1872-1876) found that differences in species composition of planktonic foraminifera from ocean sediments contains clues about the temperatures in which they lived.

Following this pioneering work, Schott working on sediments of the METEOR Expedition (1925-1927) introduced quantitative counting of species within the fossil assemblages on the sea floor and realized that surface water temperature changed as the climate fluctuated between glacial and interglacial conditions.

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

In 1955, Emiliani, who was then a student of Harold Urey at the University of Chicago,  published a paper entitled “Pleistocene temperatures” where introduced isotope stratigraphy to paleoceanography. He used the density of a heavy oxygen isotope in planktonic foraminifera from deep sea cores to outline oxygen isotope stages for the Quaternary, believing these would reflect surface temperature changes and the ice volume changes.  He concluded that the last glacial cycled had ended about 16,000 years ago, and found that temperature increased steadily between that time and about 6000 years ago. Many of Emiliani’s findings are still valid today, however in 1970 several improvements to Emiliani’s work were made, such as a revision of the temperature scale.

Oxygen isotope records have also been obtained from well-preserved microfossil materials in the Late Cretaceous  when bottom waters appear to have been much warmer than at present.

This concepts of paleotemperature reconstruction, as first developed for planktic foraminifera, apply to other groups of microfossils. Diatoms and radiolarians are susceptible to different set of dissolution parameters than calcareous fossils, resulting in a different distribution pattern at the sea floor and have been used for temperature estimates in the Pacific and in the Antartic Oceans, especially where calcareous fossils are less abundant. Diatom assemblage are also used in reconstructions of paleoproductivity.

Climatic modes and sea-level fluctuations indicated by calcareous nannofossils of the Oligocene deposits from the Romanian Carpathians. (Melinte, 2004)

Climatic modes and sea-level fluctuations indicated by calcareous nannofossils of the Oligocene deposits from the Romanian Carpathians. (Melinte, 2004)

The calcareous nannoplankton represents good proxy for the sea-level fluctuations. The group exhibit  a clear latitudinal distribution pattern, for instance, the presence of mixed nannofloral assemblages (taxa of low-middle latitudes together with high ones) are indicative of the sea-level rise,  while endemic assemblages characterize periods of low sea-level.

By studying cores from those ocean sediments, its possible determine the ages of the rocks, the ocean environment and some atmospheric conditions using the information  provided by the microfossils present in that core, as well as stable isotope analysis and magnetic stratigraphy.

Each layer of the core recorded the geological history of the ocean basins, changing climates, evolving biota and the events that could altered the course of Earth history.

References:

Armstrong, Howard A. and Martin D. Brasier.  Microfossils.  Blackwell Publishing, 2005.

Berger, W. H., Sea level in the late Quaternary: patterns of variation and implications, Int J Earth Sci (Geol Rundsch) (2008) 97:1143–1150