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

Florentino Ameghino, the father of Argentinian paleontology.

Portrait of Florentino Ameghino (1854-1911) by Luis De Servi (1863-1945).

Portrait of Florentino Ameghino (1854-1911) by Luis De Servi (1863-1945).

Florentino Ameghino was born on September 19, 1854. He came from a family of Italian immigrants who settled in 1854 in the town of Lujan, Buenos Aires, Argentina, where the extraction and exportation of fossils were a lucrative activity. The first well known fossil from South America, Megatherium americanum, was recovered by Fray Manuel Torres in this place and later described by George Cuvier in 1796. Fossil collectors and museum commissioners visited the Lujan area pursuing the colossal fossil bones. Darwin himself described the small city in his famous Notebook: “The houses at Luxan are all of one ground-floor, except that of the Cabildo, on the east side of the plaza, which has rooms above. They are all built with sun-burnt bricks, called adobes, not white-washed. The church is a small plain building, with a little turret, and a cupola top.”

Despite being a self-taught naturalist, Ameghino became an international authority in the field of paleontology of vertebrates, geology, and archeology. He was portrayed by his biographers as the incarnation of materialism, leftist culture and national genius. Throughout his scientific career, Ameghino was seconded by his younger brother Carlos Ameghino (1865–1936), who was a “travelling naturalist” for the Museo de La Plata. During his trips, he gathered a remarkable collection of fossil mammals, later described by Florentino.

Ameghino ́s house at Las Heras 466. (ca. 1920). From Ludueña, 2011.

Thanks to the financial support obtained by members of the Genovese community in Argentina, Ameghino traveled to Paris in 1878 and presented his archaeological and paleontological collections at the Paris Anthropological Exhibition. He studied with various French experts, including Paul Gervais and Gabriel de Mortillet, and sold part of his collection to the North American paleontologist Edward Drinker Cope. In 1879, he married Léontine Poirier. She became an important part of Ameghino’s scientific career helping him with his writings, their bookstore, and hosting her husband’s visitors in their home in La Plata. In 1880, Ameghino published La antigüedad del Hombre en el Plata (Man’s Antiquity in the la Plata Basin). He and Léontine stayed in France until 1881.

In 1884, Ameghino published Filogenia, his most important theoretical work which evidenced that Ameghino was a true Darwinist. The book was shaped by Haeckel’s reconstruction of the human ancestral tree, and Gabriel de Mortillet’s ideas. A year later, Ameghino was appointed professor of zoology at Universidad de Córdoba a position that he quit for the vice-directorship of the recently founded Museo de La Plata.

Photo from the Archives of the Museo de La Plata. Although it has no references, it is thought to portray Carlos and Florentino during the latter’s only visit to Patagonia. From Sergio F. Vizcaíno, 2011.

Between the 1880s and 1890s, Ameghino’s descriptions revolutionized scientific opinion regarding primitive mammals. He began corresponding with Hermann von Ihering, a German scientist who had settled in São Paulo, Brazil. They both studied the Tertiary geological formations in South America and elaborated the idea that all mammals had originated in Patagonia and then moved to Africa through the continental bridges connecting the ancient continents. This partnership was internationally known as Ameghino, von Ihering & Co. At the time, Ameghino had alredy published his monograph Contribución al conocimiento de los mamíferos fósiles de la República Argentina. The work was praised by Karl von Zittel in his History of Geology and Paleontology of 1899, who remarked that “the most important paleontological event of the last two decades of the nineteenth century has been the disclosure made by Florentino Ameghino of a rich Mammalian fauna in the Tertiary rocks of Patagonia.”

In 1887, Ameghino described a large, toothless jaw from the Miocene of the Province of Santa Cruz, naming it Phorusrhacos longissimus and assigning it to a new family of edentulous mammals. He used this finding as a critical evidence for his contention that modern mammalian lineages originated in Argentina and later spread across the globe. At that time, Ameghino and Francisco P. Moreno, Director of the Museo de la Plata, were in the middle of a bitter dispute. The feud between the two men was in many aspects similar to the well-known feud between E.D. Cope and O.C. Marsh, which took place in the United States at roughly the same time. Four years later, Moreno and Alcides Mercerat recognized for the first time that the mandible described by Ameghino was really that of a bird.

Images of the type specimen of the Santacrucian sloth
Proschismotherium oppositum Ameghino, 1902. From Vizcaino et al., 2017-

Ameghino resigned from his position at the Museo de La Plata in 1888, and Moreno denied him access to the paleontological collection. From that moment, and until became head of the Museo Argentino de Ciencias Naturales in Buenos Aires in 1902, the Ameghino brothers continued with their palaeontological exploration, without any permanent official support, but they managed to get the funds to run their paleontological investigations as a private enterprise. Karl von Zittel, subsidized their explorations, receiving in exchange fossils for the collection of the Munich University. Meanwhile Moreno, in order to gain priority over his rivals, published a series of brief reports about the new palaeontological discoveries made by his field researchers.

In 1895, the critical financial situation forced Florentino Ameghino to sell his fossil bird collection, in order to support his further work in Patagonia. The collection was purchased by the London Museum by the sum of 350 £ in 1896. When Florentino became director of the Museo Nacional de Buenos Aires in 1902 the selling of fossils ceased, and he started making claims for the return of the museum’s collections. He also proposed for the most remarkable specimens of Patagonian and Pampean fossil faunas to be cast and stored in Buenos Aires and La Plata museums to be used in Argentinean schools. The same casts were sent to Museums all over the world and in exchange, Ameghino received casts of the oldest fossil mammals from Africa and the Northern Hemisphere to compare with the Patagonian faunas. It was a clever way to prevent the sale of the original fossils.

Florentino Ameghino in his archaeological deposit, 1902. Archivo General de la Nación Argentina. Inventario 4738.

Florencio Ameghino died on August 6, 1911. After his death, he became a national icon for his role in creating national science and culture. The Ameghino collection is still today the reference collection of the entire Cenozoic Era biostratigraphic system for the South American continent. The Florentino Ameghino Partido, situated in the north-west of Buenos Aires Province, was named after him, as well as various educational institutions across the country, libraries and museums, squares, schools, parks and other locations. The Ameghino Crater, located to the north of the Sinus Successus on the Moon, is also named in his honor. A very rare privilege for a paleontologist.

 

References:

Ameghino, F. 1889. Contribución al conocimiento de los mamíferos fósiles de la República Argentina. Actas de la Academia Nacional de Ciencias en Córdoba 6: 1-1028.

Podgorny, I. (2016). The Daily Press Fashions a Heroic Intellectual: The Making of Florentino Ameghino in Late Nineteenth-Century Argentina. Centaurus, 58(3), 166–184. doi:10.1111/1600-0498.12125

Ludueña, E. 2011. La Casa era de los Ameghino. Monumento Histórico Nacional. La Graphica, Luján, 176 pp.

Sergio F. Vizcaíno, Gerardo De Iuliis, Paul D. Brinkman, Richard F. Kay, and Daniel L. Brinkman (2017). On an album of photographs recording fossils in the “Old collections” of the Museo de la Plata and Ameghino’s private collection at the beginning of the XXTH century. Publicación Electrónica de la Asociación Paleontológica Argentina 17 (1): 14–23.
Vizcaíno, Sergio Fabián; Cartas para Florentino desde la Patagonia: Crónica de la correspondencia édita entre los hermanos Ameghino (1887-1902); Asociación Paleontológica Argentina; Publicación Especial – Asociación Paleontológica Argentina; 12; 1; 12-2011; 51-67 http://hdl.handle.net/11336/80957

The Spinosaurus tail

Reconstructed skeleton and caudal series of Spinosaurus aegyptiacus. From Ibrahim et al., 2020.

Spinosaurus aegyptiacus is one of the most famous dinosaur of all time. It was discovered by German paleontologist and aristocrat Ernst Freiherr Stromer von Reichenbach in 1911. This gigantic theropod possessed highly derived cranial and vertebral features sufficiently distinct for it to be designated as the nominal genus of the clade Spinosauridae. Unfortunatelly, the holotype of Spinosaurus aegyptiacus was destroyed after a British Royal Air Force raid bombed the museum and incinerated its collections. Only two photographs of the holotype of Spinosaurus aegyptiacus were recovered in the archives of the Paläontologische Museum in June 2000, after they were donated to the museum by Ernst Stromer’s son, Wolfgang Stromer, in 1995. These photographs provide additional insight into the anatomy of the holotype specimen of the animal.

Almost a century later, a partial skeleton of a subadult individual of S. aegyptiacus was discovered in the Cretaceous Kem Kem beds of south-eastern Morocco. At the time of deposition, this part of Morocco was located on the southern margin of the Tethys Ocean and it was characterized by an extensive fluvial plain dominated by northward flowing rivers and terminating in broad deltaic systems on Tethys’ southern shores. The neotype of S. aegyptiacus preserves portions of the skull, axial column, pelvic girdle, and limbs. An international team led by Nizar Ibrahim published the first description of the fossil in 2014 and suggested that Spinosaurus may have been specialised to spend a considerable portion of their lives in water.

 

Selected caudal vertebrae and chevrons of Spinosaurus. From Ibrahim et al., 2020.

Spinosaurus clearly show some adaptations to a partially or predominantly piscivorous diet (because of their morphological convergence with those of crocodilians and other fish-eating reptiles, isolated spinosaurid teeth have frequently been misinterpreted). Furthermore, the presence of a short, robust femur with hypertrophied flexor attachment and the low, flat-bottomed pedal claws are consistent with aquatic foot-propelled locomotion. Now, the description of a nearly complete and partially articulated tail of S. aegyptiacus reinforces the hypothesis that this giant theropod spent plenty of time underwater.

Proximal and distal elements of the tail are complete and preserved in three dimensions, indicating a minimal taphonomic distortion. The preserved tail is approximately 400 cm long. The zygapophyses are significantly less developed than in most tetanurans, hinting at a different functional capacity for the tail in this taxon. The neural arches are also distinctive elements of the Spinosaurus tail, while the morphology of the neural spines shows considerable variation. The elongate neural and haemal arches result in a tail shape that is markedly vertically expanded and has an extensive lateral surface area. The highly specialized morphology of the Spinosaurus tail allowed it to function as a propulsive structure for aquatic locomotion. The anterior positioning of the center of mass within the ribcage may have also enhanced balance during aquatic movement. The model proposed by Ibrahim indicates that Spinosaurus tail shape was capable of generating more than 8 times the thrust of the tail shapes of other theropods, and achieved 2.6 times the efficiency.

 

 

References:

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

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 

HONE, D. W. E. and HOLTZ, T. R. (2017), A Century of Spinosaurs – A Review and Revision of the Spinosauridae with Comments on Their Ecology. Acta Geologica Sinica, 91: 1120–1132. doi: 10.1111/1755-6724.13328

The Evolution of the Avian Brain after the K-Pg mass extinction.

Ernst Haeckel – Kunstformen der Natur (1904), plate 99: Trochilidae .

Birds are extraordinarily intelligent. Studies have shown that corvids and some parrots are capable of cognitive achievement comparable to those of great apes. They manufacture and use tools, solve puzzles, and plan for future needs. Futhermore, they share with humans and a few other animal groups a rare capacity for vocal learning. Previous studies have shown that bird brains have more neurons than mammalian brains and have very high neuronal densities.

Birds and mammals have undergone remarkable encephalization, in which brain size has increased without corresponding changes in body size. Brain size has been correlated with major evolutionary innovations like cognition, flight, environmental adaptability and enhanced sensory capabilities. But the early evolutionary history of the hyperinflated brain that distinguishes birds from other living reptiles remains elusive. Using a large dataset comprising more than 2,000 birds and non-avian dinosaurs, a team of scientist lead by Daniel Ksepka reconstructed part of that story. Their results indicate that the avian brain size was profoundly impacted by the K-Pg mass extinction event.

Avian Brain-Body Size Evolution. From Ksepka et al., 2020.

The earliest diversification of extant birds (Neornithes) occurred during the Cretaceous period and after the mass extinction event at the Cretaceous-Paleogene (K-Pg) boundary, the Neoaves, the most diverse avian clade, suffered a rapid global expansion and radiation. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates.

The new findings reveal at least seven brain-body scaling events in birds right after the mass extinction event. The initial shift in the expansive neoavian radiation appears to have been driven by larger brains and smaller bodies, since the evolution of large brains provide a buffer against frequent or unexpected environmental changes via enhanced capacity for flexible behavioral responses. Birds only reached their apex in relative brain size during the Neogene when crown corvids and crown parrot radiated. Song-birds (including corvids), and hummingbirds (Trochilidae) are the only major groups of birds known to be capable of vocal learning, an ability controlled by additional brain pathways not found in other birds.

 

References:

Ksepka et al., Tempo and Pattern of Avian Brain Size Evolution, Current Biology (2020), https://doi.org/10.1016/j.cub.2020.03.060

Kabadayi, C., & Osvath, M. (2017). Ravens parallel great apes in flexible planning for tool-use and bartering. Science, 357(6347), 202–204. doi:10.1126/science.aam8138 

Lee, M.S.Y., Cau, A., Naish, D., and Dyke, G.J. (2014). Dinosaur evolution.Sustained miniaturization and anatomical innovation in the dinosaurian an-cestors of birds. Science345, 562–566 DOI: 10.1126/science.1252243

 

The end-Triassic extinction: A tale of Death and Global Warming.

A basaltic lava flow section from the Middle Atlas, Morocco. From Wikimedia Commons.

For the last 540 million years, five mass extinction events shaped the history of the Earth. The End-Triassic Extinction (ETE) is typically attributed to climate change associated with degassing of basalt flows from the central Atlantic magmatic province (CAMP) emplaced during the initial rifting of Pangea. Most mammal-like reptiles and large amphibians disappeared, as well as early dinosaur groups. In the oceans, this event eliminated conodonts and nearly annihilated corals, ammonites, brachiopods and bivalves.

The emplacement of CAMP started c. 100,000 years before the end-Triassic event and continued in pulses for 700,000 years. Three negative organic C-isotope excursions (CIEs) have being recognized at the end-Triassic: the Marshi, the Spelae, and the top-Tilmanni CIEs. A recent study published in Nature estimated that a single short-lived magmatic pulse would have released about 5 × 1016 mol CO2, roughly the same total amount of projected anthropogenic emissions over the 21st century, causing an increase of about 2 °C in global temperatures, and an oceanic pH decrease of about 0.15 units over 0.1 kyrs, suggesting that the end-Triassic climatic and environmental changes, driven by CO2 emissions, may have been similar to those predicted for the near future.

A normal fern spore compared with mutated ones from the end-Triassic mass extinction event. Image credit: S LINDSTRÖM, GEUS

These massive volcanic eruptions with lava flows, also released large quantities of sulphur dioxide, thermogenic methane and large amounts of HF, HCl, halocarbons and toxic aromatics and heavy metals into the atmosphere, resulting in global warming, and ozone layer depletion. The high concentrations of pCO2 are indicative of ocean acidification suggesting that this may have been a marine extinction mechanism especially in relation to the scleractinian corals. Mutagenesis observed in plants and their reproductive cells (spores and pollen) were likely caused by mercury, the most genotoxic element on Earth .

The new study confirms the abundance of CO2 (up to 105 Gt volcanic CO2 degassed during CAMP emplacement) and indicates that at least part of this carbon has a middle- to lower-crust or mantle origin, suggesting that CAMP eruptions were rapid and potentially catastrophic for both climate and biosphere. Since the industrial revolution, the wave of animal and plant extinctions that began with the late Quaternary has accelerated. Australia has lost almost 40 percent of its forests, and almost 20% of the Amazon has disappeared in last five decades.Calculations suggest that the current rates of extinction are 100–1000 times above normal, or background levels. If we want to stop the degradation of our planet, we need to act now.

 

References:

Capriolo, M., Marzoli, A., Aradi, L.E. et al. Deep CO2 in the end-Triassic Central Atlantic Magmatic Province. Nat Commun 11, 1670 (2020). https://doi.org/10.1038/s41467-020-15325-6

Sofie Lindström et al. Volcanic mercury and mutagenesis in land plants during the end-Triassic mass extinction, Science Advances (2019). DOI: 10.1126/sciadv.aaw4018}

Davies, J., Marzoli, A., Bertrand, H. et al. End-Triassic mass extinction started by intrusive CAMP activity. Nat Commun 8, 15596 (2017). https://doi.org/10.1038/ncomms15596