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 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 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 time. It 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 the 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