Maip macrothorax, the shadow of the death

Maip macrothorax. Image credit: Agustín Ozán

Patagonia has yielded the most comprehensive fossil record of Cretaceous theropods from Gondwana, including Megaraptora, a clade of medium-sized and highly pneumatized theropods characterized by their elongate skulls, and the formidable development of their manual claws on digits I and II. The enigmatic nature of this group has been a matter of discussion since the description of Megaraptor namunhaiquii in 1990s . Other representatives of the clade are Aoniraptor libertatem, Aerosteon riocoloradensis, Australovenator wintonensis, Murusraptor barrosaensis, Tratayenia rosalesi and Orkoraptor burkei. The phylogenetic position of Megaraptora is still controversial. But despite the lack of consensus, megaraptorans themselves remain a well-supported clade. Now, a new megaraptoran theropod dinosaur from the Upper Cretaceous of the Santa Cruz Province, Argentina, sheds light on on these enigmatic predators and their evolutionary radiation.

Maip macrothorax is a large-bodied megaraptorid from lower Maastrichtian Chorrillo Formation in Santa Cruz Province, Argentina. The holotype (MPM 21,545) includes the axis (only lacking both prezygapophyses and its right postzygapophysis), several dorsal and caudal vertebrae, three incomplete cervical ribs, numerous incomplete or fragmentary dorsal ribs, numerous gastral elements, left coracoid, distal end of a second metatarsal, and fragments of the scapula. The generic name, Maip, is derived from an evil entity in Aonikenk mythology that represents “the shadow of the death”. The specific name, macro, derives from the Greek word makrós (meaning long), and the Latin word thorax (meaning chest) in reference to its wide thoracic cavity (which has, approximately, more than 1.20 m width).

Axis of Maip in lateral (A, A´), anterior (B, B´), posterior (C, C´), dorsal (D, D´) and ventral (E, E´). Scale bar: 5 cm. From Aranciaga et al., 2022

The new specimen was discovered in 2019, but due the outbreak of the COVID-19 pandemic in early 2020 the dig was temporarily interrupted. The most striking feature of Maip is its large size. Maip macrothorax was between nine and 10 meters (30-33 feet) and weight about 5 tons. Several vertebrae and ribs of Maip show striations or rugosities interpreted as the attachment sites for the costovertebral and costotransversarium ligaments, a condition not commonly observed in other theropods.

The new study, lead by Aranciaga Rolando, recovered two new clades comprising some derived megaraptorids from South America. The first one, Clade A, comprises Megaraptor, Murusraptor, most of the Cenomanian–Turonian Patagonian forms with 6 or 7 m in length. The second one, Clade B, includes Orkoraptor, Tratayenia, Aerosteon and Maip, most of the Santonian through Maastrichtian megaraptorids from South America, with 8 or 10 m in length. This clade is supported by two synapomorphies: dorsal vertebrae with a bifurcated lamina anterior to the transverse process and forming an accessory fossa, and round and large articular facets of pre- and postzygapophyses of proximal caudal vertebrae. Additionally, the work suggests that after the Turonian, megaraptorids showed an increase in the body size and (with other theropod groups) replaced carcharodontosaurids in the role of apex predators within the Southern continents in the course of the Late Cretaceous.

 

References:

Aranciaga Rolando, A.M., Motta, M.J., Agnolín, F.L. et al. A large Megaraptoridae (Theropoda: Coelurosauria) from Upper Cretaceous (Maastrichtian) of Patagonia, Argentina. Sci Rep 12, 6318 (2022). https://doi.org/10.1038/s41598-022-09272-z

Novas, F.E., et al., Evolution of the carnivorous dinosaurs during the Cretaceous: The evidence from Patagonia, Cretaceous Research (2013), http://dx.doi.org/10.1016/j.cretres.2013.04.001

Introducing Guemesia ochoai

Right lateral view of the braincase of Guemesia ochoai. Scale bar: 5 cm. From Agnolín et al, 2022.

Abelisauroidea is the best known carnivorous dinosaur group from Gondwana. The clade was erected by the legendary paleontologist Jose Bonaparte with the description of Abelisaurus Comahuensis. These theropods exhibit spectacular cranial ornamentation in the form of horns and spikes, and strongly reduced forelimbs. Abelisauroidea has been divided into two main branches: the Noasauridae and the Abelisauridae. The Noasauridae are known from Cretaceous beds in northern Argentina, Madagascar, India, and Niger. They are small and slender with sizes that range from 1 to 3 metres in length. The Abelisaurids are medium to large, robust animals, such as the Carnotaurus and the Majungasaurus of Madagascar. The group exhibits short, round snouts; thickened teeth; short, stocky arms; and highly reduced forearms. The Argentinean record of abelisauroid theropods begins in the Middle Jurassic with Eoabelisaurus mefi, and spans most of the Late Cretaceous. The clade includes Carnotaurus sastrei, Aucasaurus garridoi, Ekrixinatosaurus novasi, Skorpiovenator bustingorryi, Tralkasaurus cuyi, Viavenator exxoni, Niebla antiqua, and Llukalkan aliocranianus. All of them are from the Cretaceous of Patagonia.

Map indicating the locality that yielded Guemesia ochoai. From Agnolín et al, 2022.

Guemesia ochoai is the first definitive abelisaurid theropod from Northwestern Argentina. The holotype (IBIGEO-P 103) is represented by a nearly complete braincase, with parietals, frontals, supraoccipital, basioccipital, exoccipital-opisthotic complex, basisphenoid-parasphenoid, prootics, laterosphenoids, and orbito-sphenoids. It was recovered from Los Blanquitos Formation (Campanian, Late Cretaceous), near Amblayo town, Salta province, Argentina. The name honours General Martin Miguel de Güemes who defended northwestern Argentina during the War of Independence, and Javier Ochoa, technician of the Museo Regional “Florentino Ameghino,” who discovered the specimen. The braincase of Guemesia is small compared to other abelisaurids. Unfortunately, the absence of postcranial elements makes impossible to perform histological analysis and to corroborate if the specimen reached somatic maturity. The cranial endocast has a total length of 73 mm from the base of the olfactory tract to the foramen magnum, and the volume is 47.6 cm3 (almost 70% smaller than the cranial endocasts of Carnotaurus and Viavenator). But the most striking feature of Guemesia is a row of foramina close to the midline of the fused frontals that is unknown in other abelisaurids. These rows may be linked to a zone of thermal exchange.

References:

Federico L. Agnolín et al, First definitive abelisaurid theropod from the Late Cretaceous of Northwestern Argentina, Journal of Vertebrate Paleontology (2022). DOI: 10.1080/02724634.2021.2002348

Introducing Stegouros elengassen

Life reconstruction of Stegouros elengassen. Image credit: Luis Perez Lopez

Ankylosauria is a clade of herbivorous, armored ornithischian dinosaurs subdivided in two major clades: the Ankylosauridae and the Nodosauridae. The most derived members of this group are characterized by shortened skulls, pyramidal squamosal horns, and tail clubs. Fossil evidence of armored dinosaurs from Gondwana is scarce. They were present primarily in Asia and North America. Stegouros elengassen, a new specimen from the Late Cretaceous Dorotea Formation of southern Chile, offers new evidence that contributes to the understanding of the relationships among the ankylosaurs from Gondwana.

Stegouros lived about 72 to 75 million years ago, and reached 2 meters in lenght (six feet). The generic name is derived from the the Greek word “stego” (roof ) and the Greek word “uros”
(tail) in reference to the covered tail. The specific name “elengassen” comes from an armoured
beast in the mythology of the Aónik’enk people.

Skeletal anatomy of the S. elengassen holotype. From Soto-Acuña et al., 2021.

The holotype (CPAP–3165), represented by a nearly complete skeleton, semi-articulated, was discovered in 2017 at the lower section of the Dorotea Formation. The new specimen exhibits an unusual combination of characters. The skull and teeth, are classically Ankylosauria, but the pelvis and slender limb bones resemble Stegosauria. The most striking feature of Stegouros is the short, bizarre tail covered by seven pairs of large osteoderms, that are fused into a flat composite unit resembling a battle axe.

Phylogenetic analyses with five different datasets indicate that Stegouros was closer to Ankylosauria than to Stegosauria. It was grouped with the basal ankylosaurs Kunbarrasaurus (from the late Lower Cretaceous of Australia) and Antarctopelta (from the Late Cretaceous of Antarctica) forming a monophyletic clade that split earliest from all other Ankylosauria. The study, led by Chilean paleontologists, proposes the clade Parankylosauriato include the first ancestor of Stegouros—but not Ankylosaurus—and all descendants of that ancestor (Soto-Acuña et al., 2021).

 

 

References:

Soto-Acuña, S., Vargas, A.O., Kaluza, J. et al. Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile. Nature (2021). https://doi.org/10.1038/s41586-021-04147-1

The realm of the Tyrant

Close up of “Sue” at the Field Museum of Natural History in Chicago, IL, 2009 (From Wikimedia Commons)

After the extinction of many carnivorous crurotarsan lineages (phytosaurs, ornithosuchids, and rauisuchians) at the Triassic–Jurassic boundary, theropod dinosaurs became the primary large-bodied flesh-eaters in terrestrial ecosystems. The group reached a great taxonomic and morphological diversity during the Jurassic and Early Cretaceous. Some major groups include Ceratosauria, Megalosauroidea, Spinosauridae; Carnosauria, and Coelurosauria. In the last decades, the study of Gondwanan non-avian theropods has been highly prolific, showing that the group reached a great taxonomic and morphological diversity comparable to that of Laurasia. Notwithstanding, there is a qualitative difference between Jurassic and Early Cretaceous assemblages relative to the latest Cretaceous (Campano-Maastrichtian) assemblages with abelisaurids dominating Gondwanan continents, and tyrannosaurids ruling Asiamerican ecosystems. 

Tyrannosaurus rex, the most iconic dinosaur of all time, and its closest relatives known as tyrannosaurids, comprise the clade Tyrannosauroidea, a relatively derived group of theropod dinosaurs, more closely related to birds than to other large theropods such as allosauroids and spinosaurids. The clade originated in the Middle Jurassic, approximately 165 million years ago, and was a dominant component of the dinosaur faunas of the American West shortly after the emplacement of the Western Interior Seaway (about 99.5 Mya). Over the past 20 years, new discoveries from Russia, Mongolia and China helped to build the Tyranosaurs family tree.

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

All large-bodied carnivorous theropod dinosaurs passed through a wide range of body sizes. Therefore, the ecological niche of any given individual shifted throughout its lifetime. From the Jurassic through the early Late Cretaceous, this transformation occurred in the context of ecosystems in which the juveniles and subadults potentially competed with other theropod species with medium adult body sizes. But sometime after the Turonian something changed.

A new study by Thomas Holtz, a principal lecturer in the University of Maryland’s Department of Geology, surveyed the record of 60 dinosaur communities from the Jurassic and Cretaceous periods, revealing a drop-off in diversity of medium-sized predator species (50–1000 kg) in communities dominated by tyrannosaurs. On the other hand, the study also showed that the diversity of prey species did not decline. The proposed explanation for this phenomenon is the “tyrannosaurid niche assimilation hypothesis”. In this scheme, juvenile and subadult members of Tyrannosauridae were the functional equivalent of earlier middle-sized theropod carnivores. This absence of other potential mid-sized competitors in Campano-Maastrichtian Asiamerica could be a factor in some evolutionary transformations in Tyrannosauridae such as bite force and agility.

 

References:

Thomas R. Holtz, Theropod guild structure and the tyrannosaurid niche assimilation hypothesis: implications for predatory dinosaur macroecology and ontogeny in later Late Cretaceous Asiamerica, Canadian Journal of Earth Sciences (2021). DOI: 10.1139/cjes-2020-0174

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

Zanno, L., Makovicky, P. Neovenatorid theropods are apex predators in the Late Cretaceous of North America. Nat Commun 4, 2827 (2013). https://doi.org/10.1038/ncomms3827

 

Meet Niebla antiqua

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

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

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

 

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

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

 

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

 

References:

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

Introducing Oksoko avarsan

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

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

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

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

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

 

 

References:

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

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

Introducing Dineobellator notohesperus

Life reconstruction of Dineobellator notohesperus. Artwork by Sergey Krasovskiy

 

The iconic Velociraptor mongoliensis, described by Osborn in 1924, belongs to the Dromaeosauridae, a family of highly derived small to mid-sized theropod dinosaurs closely related to birds. Their fossils have been found in North America, Europe, Africa, Asia, South America and Antarctica. They first appeared in the mid-Jurassic Period, but their fossil record in North America is very poor near the time of their extinction prior to the Cretaceous-Paleogene boundary. The group is characterized by the presence of long, three-fingered forelimbs that ended in sharp, trenchant claws and a tail stiffened by the elongated prezygapophyses.

The description of Dineobellator notohesperus, a new specimen discovered in 2008 in New Mexico, offers a glimpse into the biodiversity of Dromaeosaurids at the end of the Cretaceous. The generic name is derived from the Navajo word Diné, in reference to the people of the Navajo Nation, and the Latin suffix bellator, meaning warrior. The specific name is derived from the Greek word noto, meaning southern, or south; and the Greek word hesper, meaning western.

 

Skeletal reconstruction of Dineobellator notohesperus. From Jasinski et al., 2020

 

The holotype (SMP VP-2430), similar in size to Velociraptor and Saurornitholestes, includes elements of the skull, axial, and appendicular skeleton. The nearly complete right humerus measures 185.78 mm, with an estimated total length of 215 mm. The presence of quill knobs in Dineobellator provides further evidence for feathers throughout Dromaeosauridae. This new specimen co-existed with numerous other theropods, including caenagnathids, ornithomimids, troodontids, and tyrannosaurids.

Dineobellator exhibits some features in the forelimbs that suggest greater strength capabilities in flexion, in conjunction with a relatively tighter grip strength in the manual claws, while the possession of opisthocoelous proximal caudal vertebrae may have increased the agility of Dineobellator and thus may have implications for its predatory behavior, particularly with respect to the pursuit of prey.

 

References:

Jasinski, S.E., Sullivan, R.M. & Dodson, P. New Dromaeosaurid Dinosaur (Theropoda, Dromaeosauridae) from New Mexico and Biodiversity of Dromaeosaurids at the end of the Cretaceous. Sci Rep 10, 5105 (2020). https://doi.org/10.1038/s41598-020-61480-7

Senter, P., Kirkland, J. I., DeBlieux, D. D., Madsen, S. & Toth, N. New dromaeosaurids (Dinosauria: Theropoda) from the Lower Cretaceous of Utah, and the evolution of the dromaeosaurid tail. PLoS One 7, e36790 (2012). https://doi.org/10.1371/journal.pone.0036790

Osborn, Henry F. (1924a). “Three new Theropoda, Protoceratops zone, central Mongolia”. American Museum Novitates. 144: 1–12. http://hdl.handle.net/2246/3223

 

Introducing Asteriornis maastrichtensis

 

Three-dimensional image of the skull of Asteriornis maastrichtensis.
Image credit: Daniel J. Field, University of Cambridge

The earliest diversification of extant birds (Neornithes) occurred during the Cretaceous period. 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. A genome-scale molecular phylogeny indicates that nearly all modern ordinal lineages were formed within 15 million years after the extinction, suggesting a particularly rapid period of both genetic evolution and the formation of new species. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates. The description of a new neornithine from the Late Cretaceous of Belgium shed new light on the evolution of birds.

Asteriornis maastrichtensis is a small member of the clade Pangalloanserae, the group that includes Galliformes and Anseriformes, with an estimated body weight of about 400 grams. The holotype (NHMM, 2013 008) includes a nearly complete, articulated skull with mandibles, and associated postcranial remains preserved in four blocks. The new specimen, dated between 66.8 and 66.7 million years ago, was collected in 2000 by Maarten van Dinther. The generic name is derived from the name of the Asteria, the Greek goddess of falling stars, and the Greek word ornis for bird. The specific name maastrichtensis honors the provenance of the holotype, the Maastricht Formation (the type locality of the Late Cretaceous Maastrichtian stage).

Artist’s reconstruction of Asteriornis maastrichtensis.
Illustration: Phillip Krzeminski

Asteriornis exhibits caudally pointed nasals that overlie the frontals and meet at the midline of the skull, and a slightly rounded, unhooked tip of the premaxilla. The new specimen reveals a previously undocumented combination of ‘galliform’ and ‘anseriform’ features that emphasizes the modular nature of the skull and bill of crown birds. The narrow and elongate hindlimbs and provenance from nearshore marine sediments suggest that Asteriornis might have had a shorebird-like ecology.

 

References:

Field, D.J., Benito, J., Chen, A. et al. Late Cretaceous neornithine from Europe illuminates the origins of crown birds. Nature 579, 397–401 (2020). https://doi.org/10.1038/s41586-020-2096-0

Introducing Akainacephalus johnsoni

Skeletal reconstructions of Akainacephalus johnsoni. From Wiersma and Irmis, 2018

The Ankylosauria is a group of herbivorous, quadrupedal, armoured dinosaurs subdivided in two major clades, the Ankylosauridae and the Nodosauridae. The group is predominantly recorded from the Late Cretaceous (Turonian—late Maastrichtian) of Asia and the last Cretaceous (early Campanian—late Maastrichtian) of western North America (Laramidia). Ankylosauridae were present primarily in Asia and North America, and the most derived members of this clade are characterized by shortened skulls, pyramidal squamosal horns, and tail clubs.

Akainacephalus johnsoni, a new genus and species of an ankylosaurid dinosaur from the upper Campanian Kaiparowits Formation of southern Utah, represents the most complete ankylosaurid specimen from southern Laramidia to date, and reveals new details about the diversity and evolution of this clade. The genus name is derived from the Greek akaina, meaning “thorn” or “spine,” referring to the thorn-like cranial caputegulae of the holotype; and “cephalus,” the Greek meaning for head. The specific epithet honors Randy Johnson, volunteer preparator at the Natural History Museum of Utah.

Skull of Akainacephalus johnsoni. From Wiersma and Irmis, 2018

The holotype (UMNH VP 20202) is a partial skeleton comprising a complete skull, both mandibles, predentary, four dorsal, four dorsosacral, three sacral, one caudosacral, and eight caudal vertebrae, dorsal ribs, a complete tail club, both scapulae, left coracoid, right humerus, right ulna, partial left ilium, left femur, left tibia, left fibula, phalanx, two partial cervical osteoderm half rings, and 17 dorsal and lateral osteoderms of various sizes and morphologies.

The most striking feature of Akainacephalus johnsoni is the skull ornamentation comprising several symmetrical rows of small pyramidal and conical caputegulae along the dorsolateral surface of the skull. The postorbital horns are dorsoventrally tall, backswept, and project laterally in dorsal view. The quadratojugal horns display an  asymmetrical triangular morphology with a vertically positioned apex. Only a partial squamosal horn is preserved, but is largely broken.

Life reconstruction of Akainacephalus johnsoni (Image credit: Andrey Atuchin and the Denver Museum of Nature & Science)

The unique anatomical features of Akainacephalus johnsoni indicate a close taxonomic relationship with Nodocephalosaurus kirtlandensis, that clearly distinguish them from other Late Cretaceous Laramidian (although both taxa are temporally separated by nearly three million years). Because both taxa a more closely related to Asian ankylosaurids, the geographic distribution of Late Cretaceous ankylosaurids throughout the Western Interior could be the result of several geologically brief intervals of lowered sea level that allowed Asian ankylosaurid dinosaurs to immigrate to North America several times during the Late Cretaceous. The dispersal of ankylosaurids into Laramidia is coeval with the dispersal of other dinosaur clades, like tyrannosaurids and ceratopsians. The climate gradients and the fluctuations in sea level, may have helped reinforced Campanian provincialism.

 

References:

Wiersma JP, Irmis RB. (2018) A new southern Laramidian ankylosaurid, Akainacephalus johnsoni gen. et sp. nov., from the upper Campanian Kaiparowits Formation of southern Utah, USA. PeerJ 6:e5016 https://doi.org/10.7717/peerj.5016

Arbour, V. M.; Currie, P. J. (2015). “Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs”. Journal of Systematic Palaeontology: 1–60. doi: 10.1080/14772019.2015.1059985

The bizarre Halszkaraptor escuilliei

H. escuilliei MPC D-102/109. From Cau et al., 2017.

Maniraptoran lineages evolved novel ecomorphologies during the Cretaceous period, including active flight, gigantism, cursoriality and herbivory. This group share the following characteristics: large brain but a reduced skull in comparison to their body size, beaks, and smaller teeth. Now, a well-preserved maniraptoran from Mongolia, revealed a mosaic of features, most of them absent among non-avian maniraptorans but shared by reptilian and avian groups with aquatic or semiaquatic ecologies. This new theropod, Halszkaraptor escuilliei gen. et sp. nov., adds an amphibious ecomorphology to those evolved by maniraptorans.

The holotype, MPC (Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia) D-102/109, is an articulated and almost complete skeleton preserved three-dimensionally. The generic name, honours Halszka Osmólska (1930–2008) for her contributions to theropod palaeontology. The species name, ‘escuilliei’ refers to François Escuillié, who returned the holotype to Mongolia.

Reconstruction of Halszkaraptor escuilliei. Photograph: Lukas Panzarin/Andrea Cau

Halszkaraptor is related to other enigmatic Late Cretaceous maniraptorans from Mongolia in a novel clade at the root of Dromaeosauridae. It was the size of a mallard. Originally poached from Ukhaa Tolgod, Mongolia, the fossil was in private collections in Japan and England for an unknown amount of time, and later it  was transferred to the Royal Belgian Institute of Natural Sciences (RBINS). Thanks to a cooperation agreement between the Ministry of Education, Culture and Science of Mongolia, the Belgian Science Policy Office and the RBINS, the specimen returned to the Institute of Paleontology and Geology, Mongolian Academy of Science.

The skeleton is almost complete. The skull is lightly built, and is still articulated with the first cervical vertebra. The preorbital region forms 60% of basicranial length, and each premaxilla is elongate, bearing eleven teeth, the highest number among dinosaurs. The presacral vertebrae include 10 cervicals and 12 dorsals. The neck forms 50% of snout–sacrum length.

Skull of H. escuilliei. From Cau et al., 2017

The forelimb is relatively shorter than in most dromaeosaurids. The ulna is flattened and possesses an acute posterior margin. The hand has a morphology that is unique among theropods, with a progressive elongation of the lateral fingers, with the third being the longest and most robust. The 76 mm long femur has a robust greater trochanter. The metatarsus lacks cursorial adaptations and measures 80% of femoral length. The feet are complete and articulated, although some elements are poorly visible.

Based on the neck hyperelongation for food procurement, the forelimb proportions that may support a swimming function, and postural adaptations convergent with short-tailed birds, Halszkaraptor may represent the first case among non-avian dinosaurs of a double locomotory module.

References:

Cau, A.; Beyrand, V.; Voeten, D.; Fernandez, V.; Tafforeau, P.; Stein, K.; Barsbold, R.; Tsogtbaatar, K.; Currie, P.; Godrfroit, P.; “Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs”. Nature. doi:10.1038/nature2467