Top Fossil discoveries of 2023

An artistic reconstruction of the passage of the group of sauropods, a small theropod, and an iguanodon. Image credit: Jorge Gonzalez

This year was marked by extreme weather, earthquakes, intense volcanic activity, and political and humanitarian crisis. The end of the year was particulary dramatic for science in Argentina after the winning of the far-right presidencial candidate who is a denier of climate change and has decided to stop funding scientific research. But resilience is a key factor among humans and science is particulary adjusted to persist against all odds.

Here is my list of the top fossil discoveries of 2023.

• The Oldest Ichtyosaur from the Arctic.

Stratigraphic context and morphology of the earliest ichthyopterygian fossils. Image credit: Øyvind Hammer and Jørn Hurum

Ichthyosaurs were iconic marine reptiles that roamed the Mesozoic oceans for some 160 million years. They were characterized by an elongated body, a relatively small head, a long snout, flipper shaped limbs, and dolphin-like tail flukes. A new study from the Uppsala University and the University of Oslo with new ichthyopterygian material recoverd from the Arctic island of Spitsbergen recalibrates the time and origin of this clade.

The fossil remains designated as PMO 245.975 includes 11 articulated vertebral centra, 15 indeterminate bone fragments, limb and/or limb girdle elements. The centra of PMO 245.975 are comparable with vertebrae from ‘middle-sized’ ichthyopterygian skeletons. Aditionally, their internal organization is also entirely cancellous with a dense circumferentially oriented trabecular network (1). Those features indicates fast growth, elevated metabolism and a fully oceanic lifestyle, evidencing that the earliest ichthyopterygian ancestors must have rapidly adapted as oceanic apex predators.

• The Great Dying: a model for the current biodiversity loss.

The Permo-Triassic boundary at Meishan, China (Photo: Shuzhong Shen)

During the last 540 million years five mass extinction events shaped the history of the Earth. The Permian-Triassic mass extinction (PTME) is the most severe biotic crisis in the fossil record, with as much as 95% of the marine animal species and a similarly high proportion of terrestrial plants and animals going extinct . This great crisis occurred 252 million years ago (Ma), and is linked to the emplacement of the large igneous province of the Siberian Traps.

An international team of researchers from the California Academy of Sciences, the China University of Geosciences (Wuhan), and the University of Bristol, examined fossils from South China (a shallow sea during the Permian-Triassic transition) and recreated the ancient marine environment using simulated food webs to represent the ecosystem before, during, and after the PTME (2). The new study indicates that in the first phase of the extinction community stability slightly decreased despite the loss of more than half of taxonomic diversity, while community stability significantly decreased in the second phase (about 60,000 years after the first biodiversity crissis) because ecosystems are more resistant to environmental change when there are multiple species that perform similar functions. Once the last species in each role began to go extinct, the ecosystem rapidly collapsed.

• Dinosaurs vs mammals.

Psittacosaurus lujiatunensis–Repenomamus robustus pair (WZSSM VF000011). Scale bar equals 10 cm. From Han et al., 2023.

Dinosaurs and mammals have coexisted since the Late Triassic period. However, direct fossil evidence for their interaction is rare. A remarkable fossil (WZSSM VF000011) from the Lujiatun Member of the Lower Cretaceous Yixian Formation in China, reveals an old battle between a cat-sized mammal (Repenomamus robustus, a triconodont mammal) and a dinosaur (Psittacosaurus lujiatunensis).

Repenomamus robustus and R. giganticus are the largest known Mesozoic mammals. In 2005 a study reveled the presence of the tiny bones of a juvenile Psittacosaurus in the the stomach of a specimen of R. robustus. However, the new fossil shows a Psittacosaurus that is at least three times larger than the R. robustus. The position of the mammal atop the dinosaur, and the grasping and biting actions of the mammal, indicates that the mammal was preying on the dinosaur when the two were entombed by a volcanic debris flow (3).

• A 150-million-year-old ‘dinosaur daycare’.

Map of the tracks. Arrows indicate direction of movement of the sauropod producers. Photo credit: Sebastián Apesteguía.

Bolivia yields an outstanding dinosaur ichnological record. Different ichnosites in the Chuquisaca Department, and the Potosí Department, reveal an outstanding abundance and diversity of theropod, sauropod, ankylosaur and ornithopod footprints. A new ichnosite bearing about 350 dinosaur footprints, discovered by Dr. Gustavo Méndez Torrez, along the shore of the Santa Ana River, near the town of Entre Ríos, in the Department of Tarija, offers a glimpse into a Jurassic kindergarten. More important, with this new discovery Bolivia has dinosaur footprints from the entire Mesozoic Era (4). 

The best preserved trackway exhibits large pedes of about 95 cm and 75 cm in in diameter, sub-ovoidal to roughly sub-rectangular in outline, with three or four claw impressions, and sub-circular manus tracks with two or three digit impressions. The team was able to calculated that the animal’s hips were 3.8 meters above the ground, with a estimated length from nose to tail of 20 meters. Associated with those tracks, the researchers found a large number of small footprints, between 15 cm and 30 cm in diameter. The distribution of these trackways may represent an ichnological example of herd behavior from the Late Jurassic period.

• Venetoraptor gassenae

Skeletal anatomy of V. gassenae gen. et sp. nov. (CAPPA/UFSM 0356). From Müller et al., 2023.

Venetoraptor gassenae, a new lagerpetid species that lived about 230 million years ago in Brazil, was about 1 meter long and weighed between 4 to 8 kilograms. The holotype (CAPPA/UFSM 0356), a partial skeleton of a single individual, including cranial and postcranial elements, was recovered from the reddish mudstones of the Buriol site, Rio Grande do Sul State, Brazil. The most striking features of Venetoraptor are the presence of a toothless beak and enlarged hands with scimitar-like claws. The generic name combines the word raptor, plunderer (Latin) in reference to its raptorial beak and grasping hands and the word Veneto in reference to ‘Vale Vêneto’, a touristic locality in Rio Grande do Sul, Brazil. The specific name honours Mrs Valserina Maria Bulegon Gassen, one of the main people responsible for the foundation of CAPPA/UFSM (5).

An international team led by Rodrigo Muller (from CAPPA/UFSM), and Martin Ezcurra (from CONICET, Argentina), published the discovery on Nature. The new study (that includes the most comprehensive dataset of Triassic Pan-Aves known to date) also found that the body plan of pterosaurs and dinosaurs evolved as part of a broader morphological diversification of ornithodirans during the Late Triassic.

• Chucarosaurus diripienda.

Chucarosaurus diripienda. Image credit: Sebastián Rozadilla.

The Argentinean record of titanosaurs is particularly abundant. The study of this diverse group of sauropod dinosaurs embrace an extensive list of important contributions, which started with Richard Lydekker’s pioneering work on Patagonian dinosaurs. Some of them, were the largest animals to ever walk the Earth: Argentinosaurus, Futalognkosaurus, and Puertasaurus surpassed lengths of 37 m and masses of 70 tons.  The best preserved skeletal elements in South American sauropodomorphs are axial and appendicular one, because distinct factors have limited the preservation of complete and articulated skeletons when the specimens are more than 10 m long.

Discovered in the Upper Cretaceous strata of the Huincul Formation, Chucarosaurus diripienda is the biggest dinosaur found in Rio Negro province. Chucarosaurus lived about 90 million years ago. The body length estimated is ∼ 30 m (98 ft) long. Body mass estimated suggests that Chucarosaurus weighted 40-50 tons. The holotype (MPCA PV 820) includes a complete left humerus, left radius, left complete metacarpal II, left ischium, left femur, left fibular shaft, proximal of right tibia. and distal end of indeterminate metapodial. The femur was roughly 1.9 metres (6.2 ft) long. The humerus is slightly twisted and relatively slender (6).

• A matter of size.

Patagotitan, skeleton cast on display at the Field Museum of Natural History, Chicago, IL. From Wikimedia Commons.

Size is an important matter to both general audiences and scientists interested in dinosaur paleontology. Dinosaurs captived our imagination as masive and fierce creatures that once roamed the Earth. Sauropod dinosaurs were the largest terrestrial vertebrates to ever walk the Earth. Their morphology is easy recognizable: a long, slender neck, a relatively small head, and a tail at the end of a large body supported by four columnar limbs. They evolved from small, gracile, bipedal forms, and it was long thought that acquisition of giant body size in this clade occurred during the Jurassic and was linked to several skeletal modifications. True sauropods only evolved in the Early Jurassic. Ingentia prima, from the Late Triassic of Argentina, weighed up to 11 tons and measured up to 32 feet (10 meters) long, was about three times the size of largest Triassic dinosaurs and similar to the size of Jurassic forms.

A new study by Michael Daniel D’Emic looked across a large dataset of sauropods dinosaurs found evidence that this group evolved superlative size at least three dozen times over the course of a hundred million years, on at least six landmasses and in five ecomorphologically disparate clades (7)

• Lips don’t lie.

Close up of “Sue” at the Field Museum of Natural History in Chicago, IL (From Wikipedia Commons)

The iconic Tyrannosaurus rex have been portrayed as a ferocious creature possessing jaws full of exposed teeth and no lips. The reason for this depiction relays in the enormous size of their teeth and their phylogenetic association to crocodylians. A new study lead by Thomas Cullen, Professor of Paleobiology at Auburn University, defies the common perceptions about the appearance of these iconic predators (8).

Based on the pattern of the enamel of dinosaur teeth and crocodiles, dental histology and morphological comparisons, the researchers suggest that  lips would have protected the teeth and helped keep them from drying out due to the exposure from the elements.

• The sea monster from the End of the World.

Rearticulated skull and jaw of NDGS 10838 in left lateral view, with left bones labeled. From Zietlow et al., 2023.

Mosasaurs were large carnivorous aquatic lizards with a global distribution that lived during the Cretaceous Period. Their first fossil remains were discovered in a chalk quarry near Maastricht, in the Netherlands, and were initially identified as a whale. A few decades later, Georges Cuvier, the ‘Father of Paleontology’, confirmed the animal’s identity as some kind of gigantic extinct lizard, with some similarities in the morphology of the bones to those of contemporary monitor lizard. Jormungandr walhallaensis (named after the Norse sea serpent), from the Pembina Member of the Pierre Shale Formation in Cavalier County, North Dakota, is a new genus and species of mosasaurine mosasaur.

Discovered in 2015, the holotype (NDGS 10838) comprises a partial skull, seven cervical vertebrae with three hypapophyseal peduncles, 11 ribs, and five anterior dorsal vertebrae.  Jormungandr walhallaensis is estimated to be about 7 meters (24 feet) long, and lived about 80 million years ago. The new taxon shares some features with Plotosaurini (a sister genus to Mosasaurus) and Clidastes (a smaller and more primitive form of mosasaur, part of the Mosasaurinae subfamily) suggesting it may represent a transitional form between the two (9).

• The last meal.

Stomach contents of Gorgosaurus. From Therrien et al., 2023.

Discoverd in 2009 at the Dinosaur Provincial Park, east of the Canadian city of Calgary, the skeleton of a young Gorgosaurus libratus, a tyrannosaurid theropod dinosaur that lived during the Late Cretaceous Period, preserves the articulated hindlimbs of two yearling Citipes, a genus of caenagnathid dinosaurs, inside its abdominal cavity. The fossil, described by a team lead by Francois Therrien of the Royal Tyrrell Museum, provides direct fossil evidence of diet and feeding behavior in young tyrannosaurids (10).

The new study suggests that tyrannosaurids underwent a major ecological and dietary shift over the course of their life span. Previos studies indcate that tyrannosaurids are united by a conservative pattern of growth in which the skulls of juveniles were entirely reshaped during ontogeny: 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.

Reference:

Kear, B. P., Engelschiøn, V. S., Hammer, Ø., Roberts, A. J., & Hurum, J. H. (2023). Earliest Triassic ichthyosaur fossils push back oceanic reptile origins. Current Biology: CB, 33(5), R178–R179. https://doi.org/10.1016/j.cub.2022.12.053 (1)

Huang, Y., Chen, Z. Q., Roopnarine, P. D., Benton, M. J., Zhao, L., Feng, X., & Li, Z. (2023). The stability and collapse of marine ecosystems during the Permian-Triassic mass extinction. Current Biology. DOI: 10.1016/j.cub.2023.02.007 (2)

Han, G., Mallon, J.C., Lussier, A.J. et al. An extraordinary fossil captures the struggle for existence during the Mesozoic. Sci Rep 13, 11221 (2023). https://doi.org/10.1038/s41598-023-37545-8 (3)

Méndez Torrez, G., Lovera Cruz, L., Céspedes-Llave, A. Á., Esperante, R., Gutiérrez Berrios, C., & Apesteguía, S. (2023). First Jurassic to earliest Cretaceous dinosaur footprints for Bolivia at the Castellón formation (Tacurú Group), Tarija. Historical Biology, 1-20. https://doi.org/10.1080/08912963.2023.2235373 (4)

Müller, R.T., Ezcurra, M.D., Garcia, M.S. et al. New reptile shows dinosaurs and pterosaurs evolved among diverse precursors. Nature 620, 589–594 (2023). https://doi.org/10.1038/s41586-023-06359-z (5)

F.L. Agnolin et al. A new gigant titanosaur (Dinosauria, Sauropoda) from the Upper Cretaceous of Northwestern Patagonia, Argentina. Cretaceous Research, published online February 2, 2023; doi: 10.1016/j.cretres.2023.105487 (6)

D’Emic MD. The evolution of maximum terrestrial body mass in sauropod dinosaurs. Curr Biol. 2023 May 8;33(9):R349-R350. doi: 10.1016/j.cub.2023.02.067 (7)

Cullen, Thomas M. et al, Theropod dinosaur facial reconstruction and the importance of soft tissues in paleobiology, Science (2023). DOI: 10.1126/science.abo7877. (8)

Zietlow, Amelia R. et al, Jormungandr walhallaensis: a new mosasaurine (Squamata: Mosasauroidea) from the Pierre Shale Formation (Pembina Member: Middle Campanian) of North Dakota, Bulletin of the American Museum of Natural History (2023). https://digitallibrary.amnh.org/items/13b0485f-c73f-47f9-8d1d-0d4ab6aaedfb (9).

François Therrien et al, Exceptionally preserved stomach contents of a young tyrannosaurid reveal an ontogenetic dietary shift in an iconic extinct predator, Science Advances (2023). DOI: 10.1126/sciadv.adi0505 (10)