Introducing Bagualia alba, the oldest known eusauropod.

Bagualia alba. From Pol et al., 2020

During the Early Jurassic the southwestern margin of Gondwana was affected by a voluminous magmatic episod related to the emplacement of t heKaroo-Ferrar-Chon Aike Large Igneous Provinces (LIPs). That pulse of volcanism led to global warming, with at least four times the present level of atmospheric CO2, and ocean acidification that resulted in a mass extinction of marine invertebrates and turnover among groups of marine plankton. The period is also characterized by a floristic turnover and the diversification of the conifers, especially modern families, with small coriaceous leaves. Biostratigraphic and high-precision geochronologic results indicate that a major faunal turnover of the sauropodomorph dinosaurs took place in the Early Jurassic, which led to the rise of the eusauropods. Cañadón Asfalto Basin (part of the Chon Aike Igneous Province of Patagonia) in the Chubut Province of Argentina preserves an extraordinary record of Jurassic fauna and flora that marks key events in the evolution of Mesozoic life. The recently described Bagualia alba, recovered from the base of the Cañadón Asfalto Formation, lived 179 million years ago and is the oldest known eusauropod. The name of the new specimen refers to Cañadón Bagual, the site where the fossil was found, and alba (dawn, in Spanish), for its early age.

Reconstruction of Bagualia alba. Credit: Jorge Gonzalez

Discovered in 2007 by an international team of researchers led by Argentinean paleontologist Diego Pol, the holotype of Bagualia alba (MPEF PV 3301) consists of a posterior half of a skull found in articulation with seven cervical vertebrae. It was found in close association with multiple cranial and postcranial remains belonging to at least three individuals. Body mass estimated suggests that Bagualia weighted 10 tons, approximately the size of two African elephants. Among the characters that distinguish Bagualia from other early sauropods area pointed process on the anteroventral end of the premaxilla and anterodorsal end of the dentary; orbital margin of the frontal with a close V-shape pointed medially that results in a short contribution to the orbit; supratemporal fenestra about as anteroposteriorly long as lateromedially wide; and strongly marked proatlantal facets on the laterodorsal margin of the foramen magnum. The teeth have a D-shaped cross section, apical denticles, and buccal and lingual grooves. But the most striking feature of Bagualia is the enamel layer which is extremely thick, seven times that of other pre-volcanic herbivores, and is heavily wrinkled on its outer surface.

In Patagonia, prior to the Toarcian palaeoenvironmental crisis the plant assemblage consisted of sphenophytes, dipteridacean ferns, conifers, seed ferns, Bennetitales and cycads. This diversity is indicative of more humid conditions. By contrast, the less diverse palynologycal assemblage postdating the volcanic event is indicative of seasonally dry and warm conditions, and are largely dominated by the conifers Araucariaceae, Cheirolepidiaceae and Cupressaceae. These large conifers with coriaceous leaves as the dominant trees likely acted as a strongly selective regime favouring the survival and success of eusauropods, which had powerful jaws and an oversized gut. Conversely, the disappearance of many elements of the diverse pre-Toarcian flora could have influenced the extinction of the diverse lineages of smaller non-sauropods, which lacked adaptations to high-fibre herbivory as their gracile skulls and mandibles were less mechanically efficient and their teeth were small, with thin enamel (less than 200 μm) and lacked tooth–tooth occlusion.

 

 

References:

Pol D., Ramezani J., Gomez K., Carballido J. L., Carabajal A. Paulina, Rauhut O. W. M., Escapa I. H. and Cúneo N. R., (2020) Extinction of herbivorous dinosaurs linked to Early Jurassic global warming eventProc. R. Soc. B.28720202310 http://doi.org/10.1098/rspb.2020.2310
 
Cúneo, R., Ramezani, J., Scasso, R., Pol, D., Escapa, I., Zavattieri, A. M., & Bowring, S. A. (2013). High-precision U–Pb geochronology and a new chronostratigraphy for the Cañadón Asfalto Basin, Chubut, central Patagonia: Implications for terrestrial faunal and floral evolution in Jurassic. Gondwana Research, 24(3-4), 1267–1275. doi:10.1016/j.gr.2013.01.010 

The endocranial anatomy of Buriolestes

The skull of Buriolestes shultzi. From Müller et al., 2018

The Santa Maria Formation in southern Brazil, comprises a succession of Middle to Late Triassic sedimentary rocks that have been long renowned for their rich tetrapod fossils including one of the oldest (and the best preserved) associations of dinosaur and dinosaur precursor. Buriolestes shultzi, discovered in 2009 and described in 2016, is one of the earliest member of Sauropodomorpha, the group known for giant quadrupedal and herbivorous forms.

Buriolestes lived in what’s now Brazil about 230 million years ago. The holotype (ULBRA-PVT280) includes a partial skull, few pre-sacral, three sacral, and 42 tail vertebrae, left scapula and forelimb lacking most of the manus, paired iliaand ischia, partial left pubis, and nearly complete left hind limb. More remains of Buriolestes were discovered between 2015 and 2018. One of thoses new specimens (CAPPA/UFSM 0035) is a nearly complete and articulated skeleton. More important, the skull is almost entirely preserved, including both lower jaws.

Bones of the skull roof of CAPPA/UFSM 0035. Frontals in (a) dorsal and (b) ventral views. Parietals in (c) dorsal and (d) ventral views. From Müller et al., 2020

The estimated body mass for this new specimen was 4.50 kg. The endocast of Buriolestes exhibits an elongated olfactory tract, differing from the short tract observed in later forms, combined to a relatively small pituitary gland. The CT scans show a well‐developed flocculus of the cerebellum, with this structure projecting into the space between the semicircular canal. The flocculus plays a key a role in coordinate eye movements, and tends to be enlarged in taxa that rely on quick movements of the head and the body. This condition is also observed in another of the earliest sauropodomorphs, Saturnalia. Additionally, the dentary traits are compatible with a faunivorous diet suggesting that early members of the Sauropodomorpha were likely predators.

 

References:

Müller, Rodrigo T.; Ferreira, José D.; Pretto, Flávio A.; Bronzati, Mario; Kerber, Leonardo (2020). “The endocranial anatomy of Buriolestes schultzi (Dinosauria: Saurischia) and the early evolution of brain tissues in sauropodomorph dinosaurs”. Journal of Anatomy. doi:10.1111/joa.13350

Müller, Rodrigo T.; Langer, Max C.; Bronzati, Mario; Pacheco, Cristian P.; Cabreira, Sérgio F.; Dias-Da-Silva, Sérgio (2018). “Early evolution of sauropodomorphs: anatomy and phylogenetic relationships of a remarkably well-preserved dinosaur from the Upper Triassic of southern Brazil”. Zoological Journal of the Linnean Society. 184 (4): 1187–1248. doi:10.1093/zoolinnean/zly009

Cabreira et al., A Unique Late Triassic Dinosauromorph Assemblage Reveals Dinosaur Ancestral Anatomy and Diet, Current Biology (2016), http://dx.doi.org/10.1016/j.cub.2016.09.040

Halloween special VIII: The Great Dying

“Out of the Aeons” is a short story by H. P. Lovecraft and Hazel Heald. The story introduces the powerful Ghatanothoa, a Great Old One and the first-born of Cthulhu, brought to Earth from the planet Yuggoth by the Mi-go, an anciente alien race, who built a colossal fortress atop the Mount Yaddith-Gho, and sealed Ghatanothoa inside the mountain. Those who worship this entity hold the god responsible for earthquaques and other natural disasters. Like Groth himself, Ghatanothoa is the harbinger of death.

The fossil record indicates that more than 95% of all species that ever lived are now extinct. Occasionally, extinction events reach a global scale with many species of all ecological types dying out in a near geological instant. These mass extinctions were originally identified in the marine fossil record and have been interpreted as a result of catastrophic events or major environmental changes that occurred too rapidly for organisms to adapt.

Global paleogeographic map for the Permian-Triassic transition showing the location of the Siberian Traps Large Igneous Province. From Vajda et al., 2019

During the last 540 million years five mass extinction events shaped the history of the Earth. The end-Permian extinction (PTB) is the most severe biotic crisis in the fossil record, with as much as 95% of the marine animal species and a similarly high proportion of terrestrial plants and animals going extinct . This great crisis occurred 252 million years ago (Ma), and is linked to the emplacement of the large igneous province of the Siberian Traps. A new study published early this month in the journal Nature Geoscience is the first to conclusively reconstruct the entire cascade of events that lead to the PTB mass extinction.

The team lead by Hana Jurikova used the boron isotope of well preserved shells of brachiopods and paired with carbon and oxygen isotope data, generating a new record of ocean pH for the Permian/Triassic boundary. These findings indicate that the PTB mass extinction was triggered by a multimillennial-scale voluminous injection of carbon to the atmosphere by the emplacement of Siberian Traps sill intrusions. Massive volcanic eruptions with lava flows, released large quantities of sulphur dioxide, carbon dioxide, thermogenic methane and large amounts of HF, HCl, halocarbons and toxic aromatics and heavy metals into the atmosphere. The CO2 greenhouse effect resulted in strong heating and acidification of the surface ocean, which prompted the initial disappearance of all reef-building taxa. Acid rain likely had an impact on freshwater ecosystems and may have triggered forest dieback. 

 

References:

Hana Jurikova et al, Permian–Triassic mass extinction pulses driven by major marine carbon cycle perturbations, Nature Geoscience (2020). DOI: 10.1038/s41561-020-00646-4

V. Vajda et al. (2020), End-Permian (252Mya) deforestation, wildfires and flooding—An ancient biotic crisis with lessons for the present, Earth and Planetary Science Letters 529 (2020) 115875 https://doi.org/10.1016/j.epsl.2019.115875

H. P. Lovecraft & Hazel Heald, “Out of the Aeons”, Weird Tales magazine, 1935

 

Meet Niebla antiqua

Silhouette of Niebla antiqua showing the preserved bones in white. Scale bar: 0.5 meters. From Aranciaga et al., 2020.

The Abelisauridae represents the best-known carnivorous dinosaur group from Gondwana. Their fossil remains have been recovered in Argentina, Brazil, Morocco, Niger, Libya, Madagascar, India, and France. The group was erected by Jose Bonaparte with the description of  Abelisaurus Comahuensis. These theropods exhibit spectacular cranial ornamentation in the form of horns and spikes and strongly reduced forelimbs and hands. The Argentinean record of abelisauroid theropods begins in the Middle Jurassic (Eoabelisaurus mefi) and spans most of the Late Cretaceous. The clade includes Carnotaurus sastrei, Abelisaurus comahuensis, Aucasaurus garridoi, Ekrixinatosaurus novasi, Skorpiovenator bustingorryi, Tralkasaurus cuyi and Viavenator exxoni.  Niebla antiqua, a new specimen from the Late Cretaceous of Río Negro province, is an important addition to the knowledge of abelisaurid diversity.

Niebla antiqua is much smaller than other abelisaurids like Carnotaurus and Abelisaurus, with only 4–4.5 metres (13–15 ft) long. It was found near Matadero Hill, located within the Arriagada Farm, at 70 km south from General Roca city, Río Negro province, Argentina. The generic name derived from the Spanish word for “mist”, referring to the foggy days during the excavation of the specimen. The specific name “antiqua”, comes from the Latin “old” and makes reference to the age of the specimen.

 

Digital reconstruction of the braincase of Niebla antiqua in right lateral (A), dorsal (B), and posterior (C) views. From Aranciaga et al., 2020

Digital reconstruction of the braincase of Niebla antiqua in right lateral (A), dorsal (B), and posterior (C) views. From Aranciaga et al., 2020

 

The holotype (MPCN-PV-796) is represented by a nearly complete braincase, incomplete left dentary, isolated teeth, relatively complete scapulocoracoid, dorsal ribs and incomplete vertebrae. The braincase of Niebla is exquisitely preserved, allowing the recognition of most cranial nerves and vascular foramina. The cranial endocast has a total length of 144 mm and has an approximate volume of 64.2 cm3. The scapulocoracoid is notably similar to that of Carnotaurus with a narrow and elongate scapular blade, a glenoid surface posteriorly oriented, and a dorsoventrally expanded and wide coraco-scapular plate. Paleohistological analysis indicates that despite of its relatively small size, the holotypic specimen represents a somatically mature individual.

 

References:

Aranciaga Rolando, M., Cerroni, M. A., Garcia Marsà, J. A., Agnolín, F. l., Motta, M. J., Rozadilla, S., Brisson Eglí, Federico., Novas, F. E. (2020). A new medium-sized abelisaurid (Theropoda, Dinosauria) from the late cretaceous (Maastrichtian) Allen Formation of Northern Patagonia, Argentina. Journal of South American Earth Sciences, 102915. doi:10.1016/j.jsames.2020.102915

Introducing Oksoko avarsan

Oviraptorosaurs are a well-defined group of coelurosaurian dinosaurs characterized by short, deep skulls with toothless jaws, pneumatized caudal vertebrae, anteriorly concave pubic shafts, and posteriorly curved ischia. The most basal forms were small, similar to a chicken or a turkey, and like extant birds, they had pennaceous feathers. Their fossil record span much of the Cretaceous of Asia and North America. The most famous dinosaur of this group, Oviraptor, was discovered in 1923 by Roy Chapman Andrews in Mongolia, associated with a nest of what was thought to be Protoceratops eggs. The misconception persisted until 1990s when it was revealed that the eggs actually belonged to Oviraptor, not Protoceratops. Since then, more skeletons of Oviraptor and other oviraptorids like Citipati and Nemegtomaia have been found brooding over their eggs.

The Nemegt Basin in the Gobi Desert holds an extraordinary record of members of all three Late Cretaceous families of oviraptorosaurs: avimimids, caenagnathids,and oviraptorids. Oksoko avarsan is a newly described small oviraptorosaur, with a large, toothless beak and only two fingers on each forearm. The generic name is derived from the word Oksoko, one of the names of the triple-headed eagle in Altaic mythology. The specific name is derived from the Mongolian word avarsan, meaning rescued, because the holotype was rescued from poachers and smugglers in 2006.

The skull of Oksoko avarsan in lateral view. From Funston et al., 2020.

Preserved in an assemblage of four individuals, the holotype, MPC-D 102/110.a, is a nearly complete juvenile skeleton missing only the distal half of the tail. The excellent preservation of this assemblage provides strong evidence of gregarious behaviour.

The new taxon exhibits the following features: a dome-shaped cranial crest composed of the nasals and frontals, with a small contribution from the posteroventrally inclined parietals, nasal recesses housed in a depression; postorbital with dorsally directed frontal process; cervical vertebrae with large epipophyses; accessory ridge of brevis fossa of ilium, anteriorly curving pubis; and large proximodorsal process of distal tarsal IV. But the most striking feature of Oksoko is the functionally didactyl manus. This is the first evidence of digit loss in oviraptors. Maximum-likelihood reconstruction reveals a trend towards forelimb and digit reduction in oviraptorosaurs. This variation in forelimb length and morphology variation may have facilitated the radiation of the clade in the Late Cretaceous.

 

 

References:

Gregory F. Funston; Tsogtbaatar Chinzorig; Khishigjav Tsogtbaatar; Yoshitsugu Kobayashi; Corwin Sullivan; Philip J. Currie (2020). «A new two-fingered dinosaur sheds light on the radiation of Oviraptorosauria». Royal Society Open Science, doi:10.1098/rsos.201184

Funston, G. F., Mendonca, S. E., Currie, P. J., & Barsbold, R. (2018). Oviraptorosaur anatomy, diversity and ecology in the Nemegt Basin. Palaeogeography, Palaeoclimatology, Palaeoecology, 494, 101–120. doi:10.1016/j.palaeo.2

The skull of Carnotaurus

Carnotaurus sastrei. Credit: Gabriel Lio.

The iconic Carnotaurus sastrei was collected in the lower section of La Colonia Formation, Chubut Province, Argentina, by an expedition led by Argentinian paleontologist José Bonaparte. In 1985, Bonaparte published a note presenting Carnotaurus sastrei as a new genus and species and briefly describing the skull and lower jaw. The skull is almost complete (the only missing parts correspond to portions of the left epipterygoid, the right posterolateral area of the parietal and most of teeth crowns) and is exceptionally well preserved measuring 60 cm from the tip of the premaxillae to the distal tip of the paroccipital process. The most distinctive features of Carnotaurus are the two robust conical horns that extend from the frontals. The horns are dorsoventrally compressed, and 146 mm long on both sides. The dorsal surface of each horn is ornamented with a series of longitudinal grooves. A new study by Mauricio Cerroni, Fernando Novas, and Juan Canale provides some new potential autapomorphies diagnostic of Carnotaurus, such as nasolacrimal conduct with an accessory canal, ventral excavation on the quadrate and lateral fossa of the pterygoid.

Skull and neck of Carnotaurus sastrei

The skull of abelisaurids is characterized by having a short and deep cranium at the level of the snout, antorbital fenestra with reduced antorbital fossa, frontals strongly thickened and ornamented conforming well-developed cornual structures, and expanded parietal crest with a tall parietal eminence. The nasal bones of a Carnotaurus are extensively sculptured by highly projected rugosities. Previous studies showed the presence of a row of foramina probably neurovascular, along the dorsal nasal surface, a condition also shared with Rugops and Skorpiovenator. Although in Carnotaurus these foramina are much smaller in diameter.

The horns are predominantly solid and CT scans analyses reveals the presence of a small pneumatic recess on each frontal horn. Those small pneumatic recesses in the frontal horns of Carnotaurus adds new information about the variability of the pneumatic traits on the frontal bones in non-avian theropods. Due to the nature of the horns, the thickness of the skull roof, and the robust neck (with a possible well developed epaxial musculature), it was suggested that Carnotaurus would have the potential to use the horns for some kind of agonistic behaviour. The CT scans also revealed several pneumatic cavities (e.g. promaxillary and lacrimal recesses) much less developed than in Majungasaurus, the only other abelisaurid in which these structures were extensively analysed. The ossification of hyoid apparatus (including basihyal), is one the most complex and outstanding features of Carnotaurus because this element would have remained cartilaginous in most theropods.

 

References:

M.A. Cerroni , J. I. Canale & F. E. Novas (2020): The skull of Carnotaurus sastrei Bonaparte 1985 revisited: insights from craniofacial bones, palate and lower jaw, Historical Biology, DOI: 10.1080/08912963.2020.1802445

Cerroni, M.A., Paulina Carabajal, A., Novel information on the endocranial morphology of the abelisaurid theropod Carnotaurus sastrei .C .R. Palevol (2019), https://doi.org/10.1016/j.crpv.2019.09.005

 

The early eukaryote fossil record

Figure 4.

Some examples of fossils of early eukaryotes. From Porter, 2020.

The origin of the eukaryotic cell is one of the major evolutionary events in the history of life on our planet. However, the mosaicism of the eukaryotic genome is challenging. Bacteria, Archaea, and Eukarya share common ancestry but they have very distinctive features. The eukaryotic cell differs by its much simpler prokaryote relatives, by possessing not only a nucleus, but also a complex cytoskeleton, a sophisticated endomembrane system, and mitochondria, the last of these, the result of an ancient endosymbiosis with a proteobacterium. Recently, the discovery in deep marine sediment of the Asgard archaea, a group closely related to eukaryotes, could lead us to unravel the origin of eukaryotes.

The term ‘FECA’, the first eukaryotic common ancestor, is often used to refer to the initial lineage of total group eukaryotes, just after its split from its closest living relative. By contrast, ther term ‘LECA’, the last eukaryotic common ancestor, refers to the ancestor only of extant eukaryotes (all known ones) plus extinct post-LECA lineages. Eukaryogenesis is the interval between FECA and LECA, when the characters that define the crown group evolved. LECA is generally believed to have lived during the Mesoproterozoic era, about 1.6 to 1 billion years ago, or possibly somewhat earlier. The age of FECA is even more uncertain. Based on the earliest widely acceptable eukaryote fossils, FECA had arrived some time before 1.9 Ga. Some models suggest a younger age for LECA. Hence, Mesoproterozoic rocks dominantly preserve stem group eukaryotes.

The Asgard archaea and the origin of eukaryotes. Credit: Nature Publishing Group.

To understand the paths from FECA to LECA, it is necessary to identify eukaryote characters correctly in the deep fossil record. Therefore, the key to reconstructing the origin of eukaryotes lies in the integration of modern cell biology, molecular phylogeny, and the fossil record.

So, how do we recognize ancient fossils as eukaryotic? Size is a relevant parameter. On average, eukaryotic cells are substantially larger than those of prokaryotes, with diameters ranging from 10 to 100 μm. Another feature widespread among all eukaryotic supergroups is the formation of resistant-walled structures known as cysts. These forms are recognized in the fossil record by the presence of openings, spines or complex ornamentation. Some prokaryotes can be large too, and they can have processes and preservable walls. But none of them present these three characters at the same time. By contrast, eukaryotes exhibit these features in combination. Shuiyousphaeridium, one of the oldest evidence of eukaryotes, is a large, spiny, ornamented, organic walled microfossil found in latest Paleoproterozoic rocks. This form is an extinct genus of acritarch discovered in 1993.

Shuiyousphaeridium macroreticulatum from the Mesoproterozoic Ruyang Group, China. From Knoll et al., 2006.

Acritarchs are a heterogeneous and polyphyletic group of organic-walled microfossils of unknown affinity, consisting of a central cavity enclosed by a wall of single or multiple layers, with a great variability of shapes and ornamentations. The wall is made by sporopollenine or a very similar compound and the size range is about 5 to 200 micrometers. The acritarchs were dominant forms of eukaryotic phytoplankton during the NeoProterozoic and the Paleozoic. These forms diversified markedly, in parallel with the Cambrian and Ordovician radiations of marine invertebrates.

The term was first introduced by Evitt in 1963 and means “undecided origin” (from the Greek akritos = undecided, and arche = origin”), and replaced the older group “hystricosphaerid”. Based on their morphology, acritarchs are divided in nine groups: sphaeromorph, acanthomorphs, polygonomorphs, netromorphs, diacromorphs, prismatomorphs, oomorphs, herkomorphs, and pteromorphs.

Diagram showing the different group of Acritarchs. Imagen from UCL.

The iconic Grypania spiralis has been questioned as eukaryotic. This coiled ribbon-like impression, was first discovered in the Greyson Shale and Chamberlain Shale of the Mesoproterozoic Ravalli Group, in Montana, western USA. An alternative interpretation suggest that Grypania was a giant cyanobacterium.

The Tirohan Dolomite of the Lower Vindhyan (~1.6 Ga) in central India contains well-preserved fossils interpreted as probable crown-group rhodophytes (red algae). Rafatazmia chitrakootensis, is a nonbranching filamentous alga, 58–175 μm in width, and has uniserial rows of large cells. Ramathallus lobatus,  is a lobate sessile alga with pseudoparenchymatous thallus. Both represent crown-group multicellular rhodophytes, or a very ancient side branch.

Microscope images of the fossil Bangiomorpha pubescens. Credit: Nick Butterfield/University of Cambridge.

One of the oldest multicellular organisms is Bangiomorpha pubescens. This extraordinary fossil provides the earliest unambiguous record of photosynthetic eukaryotic life. The individual filaments of the fossil are up to 2 mm long, and are composed of a single series of cells, or of several series running side by side, or a combination of the two, as in modern Bangia. The age of its first appearance was ~ 1.047 Ga. Other early multicellular eukaryotes include Palaeastrum, and Proterocladus. Those much younger fossils appeared only 800 million years ago.

While some questions still remains unanswered, the continued studies of the fossil record and biomarker assemblages may allow us to identify the environmental conditions that allowed the appearance of complex life.

References:

Porter SM. (2020) Insights into eukaryogenesis from the fossil record. Interface Focus 10: 20190105. http://dx.doi.org/10.1098/rsfs.2019.0105

Bengtson S, Sallstedt T, Belivanova V, Whitehouse M (2017) Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae. PLoS Biol 15(3): e2000735. doi:10.1371/journal.pbio.2000735

Knoll AH. (2014) Paleobiological perspectives on early eukaryotic evolution. Cold Spring Harb. Perspect. Biol. 6, 1-14. doi:10.1101/cshperspect.a016121

Yonas I. Tekle, Laura Wegener Parfrey, Laura A. Katz, (2009) Molecular Data Are Transforming Hypotheses on the Origin and Diversification of Eukaryotes, BioScience(2009),59(6):471 http://dx.doi.org/10.1525/bio.2009.59.6.5

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

Ichnological evidence of rapid recovery after the K-Pg event.

 

Chicxulub impact crater, Yucatan. Credit; D. VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY

Mass extinctions had shaped the global diversity of our planet several times during the geological ages. The fossil record indicates that more than 95% of all species that ever lived are now extinct. The Cretaceous/Palaeogene (K-Pg) mass extinction eradicated almost three-quarters of the plant and animal species on Earth including non-avian dinosaurs, pterosaurs, marine reptiles, and ammonites. The extinction was caused by paleoenvironmental changes associated with the impact of an asteroid. In 1981, Pemex (a Mexican oil company) identified Chicxulub as the site of this massive asteroid impact. The crater is more than 180 km (110 miles) in diameter and 20 km (10 miles) in depth. The impact released an estimated energy equivalent of 100 teratonnes of TNT, induced earthquakes, shelf collapse around the Yucatan platform, and widespread tsunamis that swept the coastal zones of the surrounding oceans. The event also produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. Global forest fires might have raged for months. Photosynthesis stopped and the food chain collapsed. Marine environments lost about half of their species, and almost 90% of Foraminifera species went extinct.

New evidence from the International Ocean Discovery Program (IODP) Expedition 364 showed a surprisingly rapid initial tracemaker community recovery after the K-Pg mass extinction event. The trace fossil assemblage mainly consists of Chondrites, Zoophycos, Planolites, and Thalassinoides, characterizing a multitiered ichnofauna from the Zoophycos ichnofacies. Trace fossil assemblages can be divided according the palaeoenvironmental scheme into a number of ichnofacies named after a characteristic trace fossil. The Zoophycos Ichnofacies is dominated by trace fossils belonging to the ethological class fodinichnia, consisting of both simple and complex burrows.

Sedimentological and ichnological features through the studied cores, from the Chicxulub impact crater, Yucatán Peninsula. From Rodríguez-Tovar et al., 2020.

Previous studies revealed that porous rocks in the center of the Chicxulub crater remained hotter than 300 °C for more than 100,000 years. The high-temperature hydrothermal system was established within the crater but the appearance of burrowing organisms within years of the impact indicates that the hydrothermal system did not adversely affect seafloor life. These impact-generated hydrothermal systems are hypothesized to be potential habitats for early life on Earth and other planets.

The trace fossil assemblages indicate that recovery occurred within several years after the K-Pg transition with scarce, small, Planolites (a walled burrow tube made by a deposit feeder). Followed by a first phase of diversification with Planolites, Chondrites, and Palaeophycus, as well as a shallow indeterminate infauna. This community stabilized, with changes only in relative abundance until ∼640000- 700,000 years into the Paleocene. The appearance of Zoophycos marks the beginning of the highest diversity, abundance, and size of traces, and reveals that the Zoophycos ichnofacies was completely established and maintained to the top of the studied interval ∼1.25 m.y. after the K-Pg event. Comparison between the end-Permian mass extinction and the K-Pg record indicates similar overall patterns of recovery after both events, although the K-Pg recovery was significantly faster.

 

References:

Francisco J. Rodríguez-Tovar et al, Rapid macrobenthic diversification and stabilization after the end-Cretaceous mass extinction event, Geology (2020). DOI: 10.1130/G47589.1

 

The impact winter model and the end of the age of the dinosaurs

“Lucifer’s Hammer killed the dinosaurs,” said US physicist Luis Alvarez, in a lecture on the geochemical evidence he and his son found of a massive impact at the end of the Cretaceous period. A year later, Pemex (a Mexican oil company) identified Chicxulub as the site of this massive asteroid impact. The crater is more than 180 km (110 miles) in diameter and 20 km (10 miles) in depth. The impact released an estimated energy equivalent of 100 teratonnes of TNT, induced earthquakes, shelf collapse around the Yucatan platform, and widespread tsunamis that swept the coastal zones of the surrounding oceans. Global forest fires might have raged for months. Photosynthesis stopped and the food chain collapsed. The decrease of sunlight caused a drastic short-term global reduction in temperature. This phenomenon is called “impact winter”. Cold and darkness lasted for a period of years. Three-quarters of the plant and animal species on Earth disappeared, including non-avian dinosaurs, pterosaurs, marine reptiles, ammonites, and planktonic foraminifera.

Early work speculated that the eruption of the Deccan Traps large igneous province was the main abiotic driver of the K/Pg mass extinction. However, in the late ’70, the discovery of anomalously high abundance of iridium and other platinum group elements in the Cretaceous/Palaeogene (K-Pg) boundary led to the hypothesis that an asteroid collided with the Earth and caused one of the most devastating events in the history of life.

Geologic (A) and paleontological (B) records of the K/Pg mass extinction. From Chiarenza et al., 2020.

The Deccan Traps in central India is formed from a series of short (∼100-ky) intermittent eruption pulses, with two main phases: one toward the end of the Cretaceous, and the other starting just before the boundary and continuing through the earliest Paleogene. A new study from Imperial College London, the University of Bristol and University College London, lead by Dr Alessandro Chiarenza, compared the climatic perturbations generated by Deccan volcanism and the asteroid impact. The new study found that the extreme cooling caused by the asteroid impact created the conditions for the dinosaur extinction worldwide. Additionally, they found that the Deccan’s influence after the event might have been of greater importance in determining ecological recovery rates after the asteroid-induced cooling, rather than delaying it.

Previous studies suggested that while the surface and lower atmosphere cooled (15 °C on a global average, 11 °C over the ocean, and 28 °C over land), the tropopause became much warmer, eliminate the tropical cold trap, and allow water vapor mixing ratios to increase to well over 1,000 ppmv in the stratosphere. Those events accelerated the destruction of the ozone layer. During this period, UV light was able to reach the surface at highly elevated and harmful levels.

References:

Alfio Alessandro Chiarenza, Alexander Farnsworth, Philip D. Mannion, Daniel J. Lunt, Paul J. Valdes, Joanna V. Morgan, and Peter A. Allison. Asteroid impact, not volcanism, caused the end-Cretaceous dinosaur extinction. PNAS, 2020 DOI: 10.1073/pnas.2006087117

P.M. Hull et al., “On impact and volcanism across the Cretaceous-Paleogene boundary,” Science (2019). Vol. 367, Issue 6475, pp. 266-272 https://science.sciencemag.org/content/367/6475/266

Alvarez, L., W. Alvarez, F. Asaro, and H.V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction: Experimental results and theoretical interpretation. Science 208:1095–1108.