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.

The skull of Skorpiovenator bustingorryi

Skorpiovenator, from the late Cretaceous of Argentina. Museo Municipal Ernesto Bachmann

Skorpiovenator bustingorryi. Museo Municipal Ernesto Bachmann

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. The group exhibits strongly reduced forelimbs and hands, stout hindlimbs, with a proportionally robust and short femur and tibia. 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, expanded parietal crest with a tall parietal eminence. The nasal bones of abelisaurids are distinctive on having two distinct patterns: Abelisaurus, Carnotaurus, and Majungasaurus have nasals transversely convex and extensively sculptured by highly projected rugosities, while Skorpiovenator and Rugops have nasals posteriorly expanded, with lateral and tall bony crests, which give the nasals a transversally concave profile, and conspicuous foramina over the dorsal surface.

Skull of Skorpiovenator (MMCH-PV 48). (scales bar: 5 cm)

Skull of Skorpiovenator bustingorry (MMCH-PV 48). (scale bar: 5 cm)

Skorpiovenator bustingorry is a derived abelisaurid known from a single and nearly complete skeleton (MMCh-PV 48) recovered from rocks of the famous Huincul Formation (Late Cenomanian–Early Turonian). The skull of Skorpiovenator is strongly ornamented with ridges, furrows and tubercles. Unlike other abelisaurids, in Skorpiovenator both nasals are completely unfused. But the most striking feature is an outstanding series of three large foramina in the skull roof of Skorpiovenator that appear to be an extension of the foramina row from the nasals. This feature likely represent an autapomorphy of Skorpiovenator. CT scans made on the skull of Skorpiovenator and Carnotaurus revealed an internal system of canals linked to the dorsal nasal foramina, which likely represent a correlate for a neurovascular complex. This neurovascular system was probably related to the lateral nasal vessels and perhaps innervated by the trigeminal nerve as in extant archosaurs. The biological significance of such neurovascular system could be linked to a zone of thermal exchange, which may help avoid overheat of encephalic tissues.

References:

Cerroni, M. A., Canale, J. I., Novas, F. E., & Paulina-Carabajal, A. (2020). An exceptional neurovascular system in abelisaurid theropod skull: New evidence from Skorpiovenator bustingorryi. Journal of Anatomy. doi:10.1111/joa.13258

The nature of the first dinosaur eggs

Protoceratops embryos in a curled position. Credit: M. Ellison/American Museum of Natural History

The evolution of the cleidoic egg was an important milestone in the history of the vertebrates, an innovation that enabled amniotes to colonize land. The complex structure the cleidoic egg added extramembryonic membranes (the chorion and the ammnion) and a shell that provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The shell may be either leathery or calcified. Early amniotes and more primitive tetrapods laid soft eggshells. Lizards, snakes, and pterosaurs, also laid soft eggs. Modern crocodilians and birds lay hard-shelled eggs. This feature has been interpreted as a key factor in their survival through the Cretaceous–Palaeogene extinction (approximately 66 million years ago).

Paleontological studies of this critical event have been greatly hampered by the poor early record of fossil eggs. Eggs from ornithopods, sauropodomorphs, titanosaurs and tetanuran have been reliably identified but most of these fossils are from the Cretaceous period. The bias in the egg fossil record cannot be explained solely by preferential preservation of certain nesting sites, as previously hypothesized. A new study by an international team of scientists lead by Mark Norell found that hard-shelled eggs evolved at least three times independently in dinosaurs.

Egg assigned to the basal sauropodomorph Mussaurus. Credit: Diego Pol

The first description of dinosaur eggshells was made in 1859 by Jean-Jacques Pouech, a Catholic priest and amateur naturalist. Although he did not identify the eggshell as dinosaurian, but from a gigantic bird. The egg architecture of non-avian dinosaurs, crocodilian, extant birds, and turtles, is the same: an innermost shell membrane, a biomineralized protein matrix (both arranged in multiple layers), and an outer cuticle. The new study analyzed eggs from two very different non-avian dinosaurs: Protoceratops, a small plant-eater, and Mussaurus, a long-necked herbivore.

The Protoceratops specimen, from the the Ukhaa Tolgod locality (Campanian/Upper Cretaceous) in Mongolia, comprises a clutch of at least 12 eggs and embryos. The researchers found the presence of a diffuse black and white egg-shaped halo. Raman spectroscopy revealed the presence of protein fossilization products (PFPs) and phosphate (the white layer) in the Protoceratops eggshell. Additionally, they found that the PFPs in the Protoceratops eggshells contain relatively high amounts of S-heterocycles, which are characteristic of eggshell-derived organic matter.

Simplified phylogeny showing the evolution of eggshell in Archosauria. From Norell et al., 2020

Mussaurus patagonicus was originally described from several well-preserved post-hatchling specimens associated with egg remains found at Laguna Colorada Formation (Late Triassic/Early Jurassic) in Argentina. The histological evaluation of the Mussaurus eggshells revealed a dark brown, semi-transparent, apparently multilayered carbonaceous film, comparable to the Protoceratops soft eggshell.

The discovery of the soft nature of Protoceratops and Mussaurus eggs provides direct evidence for the independent evolution of calcified eggs in dinosaurs. This finding is supported by the recent description of several reproductives traits in theropod dinosaurs that differs considerably from that of derived ornithischians and sauropods, and may have played a key part in the Cretaceous–Palaeogene survival and radiation of modern birds.

References:

Norell, M.A., Wiemann, J., Fabbri, M. et al. The first dinosaur egg was soft. Nature (2020). https://doi.org/10.1038/s41586-020-2412-8

Neuroanatomy of Irritator challengeri

Reconstructed mount of Irritator challengeri.

The Spinosauridae is a specialized group of large tetanuran theropods known from the Berriasian to the Cenomanian of Africa, South America, Europe and Asia, characterised by a long, narrow skull, robust forelimbs with a hooked thumb claw, and tall neural spines forming a dorsal sail. The ecology of the group has been debated since the original discovery of Spinosaurus aegyptiacus in 1911. The recient description of a nearly complete and partially articulated tail of S. aegyptiacus reinforces the hypothesis that this giant theropod spent plenty of time underwater.

The holotype of Irritator challengeri (SMNS 58022; Staatliches Museum für Naturkunde Stuttgart, Stuttgart, Germany) from the Romualdo Member of the Santana Formation (Lower Cretaceous) in northeastern Brazil represents one of the few preserved spinosaurid braincases and can provide insights into neuroanatomical structures that might be expected to reflect the ecological affinities of the group.

3D renderings of the holotype fossil of Irritator challengeri (SMNS 58022) in right lateral view. From Schade et al., 2020

The skull of Irritator is remarkably narrow, especially in the region of the elongated snout. The braincase is short anteroposteriorly but deep dorsoventrally, extending ventrally far below the occipital condyle. The cranial endocast of Irritator shows weakly demarcated brain regions, elongate olfactory tracts and pronounced cranial flexures that are consistent with the inferred phylogenetic position of spinosaurids as basal tetanurans. Irritator also exhibits enlarged floccular recesses, which is an unusual feature for basal tetanurans. The flocculus of the cerebellum plays a role in coordinate eye movements, and tends to be enlarged in taxa that rely on quick movements of the head and the body. Within non-avian theropod dinosaurs, large floccular recesses are common among coelurosaurs. Additionally, lateral semicircular canal orientation suggests a downward inclined snout posture, which enables unobstructed, stereoscopic forward vision, important for distance perception and thus precise snatching movements of the snout.

 

References:

Schade, M., Rauhut, O.W.M. & Evers, S.W. Neuroanatomy of the spinosaurid Irritator challengeri (Dinosauria: Theropoda) indicates potential adaptations for piscivory. Sci Rep 10, 9259 (2020). https://doi.org/10.1038/s41598-020-66261-w

Ibrahim, N., Maganuco, S., Dal Sasso, C. et al. Tail-propelled aquatic locomotion in a theropod dinosaur. Nature (2020). https://doi.org/10.1038/s41586-020-2190-3

Sues, H., Frey, E., Martill, D., Scott, D. 2002. Irritator challengeri, a spinosaurid (Dinosauria: Theropoda) from the Lower Cretaceous of Brazil. Journal of Vertebrate Paleontology. 22, 3: 535-547 https://doi.org/10.1671/0272-4634(2002)022[0535:ICASDT]2.0.CO;2

Ibrahim, N., Sereno, P. C., Dal Sasso, C., Maganuco, S., Fabbri, M., Martill, D. M., Zouhri, S., Myhrvold, N., Iurino, D. A. (2014). Semiaquatic adaptations in a giant predatory dinosaur. Science, 345(6204), 1613–1616. doi:10.1126/science.1258750 

The growth of Tyrannosaurus rex

Tyrannosaurus rex ontogram. From Carr 2020.

Tyrannosaurus rex is the most iconic dinosaur of all timeIt was discovered in the Hell Creek Formation by Barnum Brown in 1902, and later described  by Henry Fairfield Osborn in 1905. Osborn actually named two large Hell Creek tyrannosaurids, T. rex and Dynamosaurus imperiosus. He later realized that Dynamosaurus imperiosus and Tyrannosaurus rex were synonymous, but Tyrannosaurus has priority, as it preceded Dynamosaurus in the description (Osborn, 1906). Over he past 20 years, morphological, histological and functional studies expanded our knowledge of the T. rex and its closest relatives.

Much attention has focused on how T. rex achieved such massive size and how their skeletons changed during the transition from embryo to adult. Previous studies estimate that during its growth cycle, T. rex grew at almost 2.5 kilograms per day for about 14 years. This extraordinary rapid growth rate sets T. rex apart from other tyrannosaur species like Albertosaurus and Gorgosaurus.

 

Skulls of the basal tyrannosauroids Guanlong (A), Dilong (B); Skulls of juvenile (C) and adult (D)Tyrannosaurus. All scale bars equal 10 cm. (Adapted from Brusatte et. al., 2010)

A new study by Thomas Carr reconstructs the growth series of T. rex. using an ontogram, the ontogenetic equivalent of cladogram, that shows the distinctives growth stages. The ontogram is composed of 21 growth stages. The growth series of T. rex was recovered using cladistic analysis based on an expanded dataset that includes chronological age, size, and size-independent cranial and postcranial characters. Five growth categories (juvenile, subadult, young adult, adult, senescent adult) were diagnosed based on histology, synontomorphies (the equivalent of a synapomorphy), and, in part, size and mass.

Tyrannosaurids are united by a conservative pattern of growth in which the skulls of juveniles were entirely reshaped during ontogeny. The sequence has been reconstructed by cladistic analysis. During the growth of an individual species, the skull and the jaws deepened, pneumatic bones inflated, the ornamented structures enlarged and coarsened, the sutural surfaces deepened and became more rugose, and the teeth became larger and thicker. In the post-cranial skeleton the most notably change is the shortned of the forearm. The new study shows that the number of growth changes generally decreases during adulthood. Additionally, changes to the pectoral girdle and pes are dominant early in ontogeny. Similarily, the transition from a long and low skull to a stout and deep skull occurred rapidly within a 2-year time span.

 

References:

Carr TD. 2020. A high-resolution growth series of Tyrannosaurus rex obtained from multiple lines of evidence. PeerJ 8:e9192 https://doi.org/10.7717/peerj.9192

Brusatte SL, Norell MA, Carr TD, Erickson GM, Hutchinson JR, et al. (2010) Tyrannosaur paleobiology: new research on ancient exemplar organisms. Science 329: 1481–1485. doi: 10.1126/science.1193304

Introducing Overoraptor, a new theropod dinosaur from the Upper Cretaceous of Patagonia

Silhouette of Overoraptor chimentoi gen. et sp. nov. (MPCA-Pv805) showing selected skeletal elements. From Motta et al., 2020.

The fossil record of basal paravians in Gondwana is restricted to a relatively small number of taxa. South American paravians are included within the clade Unenlagiidae. Overoraptor chimentoi, a new specimen paravian from Cenomanian-Turonian beds of Patagonia differs morphologically from unenlagiids and other non-avialan paravians. The new taxon comes from the Huincul Formation. This geological unit has yielded remains of different theropod clades including several abelisaurid theropods like Skorpiovenator, Tralkasaurus and Ilokelesia.

Overoraptor was a gracile theropod that reached about 1.3 m in total length. The name derived from the Spanish word “overo”, meaning piebald, in reference to the coloration of the fossil bones (a pattern of light and dark spots), and the word “raptor” from the Latin for thief. The species name honors Dr. Roberto Nicolás Chimento, who discovered the specimen. The holotype (MPCA-Pv 805) and paratype (MPCA-Pv 818) specimens of O. chimentoi were found in a quarry in association with disarticulated crocodilian and turtle bones.

Comparative image of the right scapulacoracoid of Bambiraptor, Buitreraptor, Overoraptor, and Archaeopteryx in lateral view. Image not to scale. From Motta et al., 2020.

Overoraptor exhibits the following features: posterior caudal centra with a complex system of lateral longitudinal ridges concavities (also present in Buitreraptor and Rahonavis); the scapula is proximally stout; the glenoid fossa is cup-shaped (also observed in Archaeopteryx and Jeholornis, but is absent in unenlagiids); the ulna is robust, and the posterior margin of the ulna is longitudinally convex so that the ulna is bowed as in most basal paravian; the radial process of Overoraptor has a saddle-shaped radial cotyle proximally (a condition also present in modern birds and some basal paravians as Bambiraptor); metacarpal I with extensive medioventral crest; metatarsal II with longitudinal lateroventral crest on distal half, ending distally in a posterior tubercle; absence of a distal ginglymoid articular surface on metatarsal III; and the foot exhibits the characteristic raptorial digit II. The unusual combination of a plesiomorphic hindlimb, with features that are correlated with cursorial habits (the characteristic raptorial digit II, unfused metatarsals, and poorly curved ungual phalanx I); and the more derived forelimb, with features that show some adaptations related to active flight, placed Overoraptor, together with Rahonavis in a clade that is sister to Avialae.

 

References:

Motta, M.J., Agnolín, F., Brissón Egli, F. et al. New theropod dinosaur from the Upper Cretaceous of Patagonia sheds light on the paravian radiation in Gondwana. Sci Nat 107, 24 (2020). https://doi.org/10.1007/s00114-020-01682-1