The Tyrannosauroids from the Southern Hemisphere.

Santanaraptor lived in South America during the Early Cretaceous about 112 million years ago (From Wikimedia Commons).

Tyrannosauroidea, the superfamily of carnivorous dinosaurs that includes the iconic Tyrannosaurus rex, was mainly distributed in the Northern Hemisphere. However, a few specimens from Australia (Timimus hermani and the articulated pubes NMV P186046) and Brazil (Santanaraptor placidus), have been referred to this clade.

Santanaraptor was unearthed in 1996 in the Romualdo Group (Santana Formation). The holotype is a juvenile partial skeleton that may have reached 1.25 metres (4.1 ft) in length, and it was presumed to be similar to Dilong and Guanlong. It was first described as a coelurosaurian theropod by Alexander Kellner in 1999, but in 2014 Thomas Holtz classified Santanaraptor as the first tyrannosauroid known from Gondwana. Delcourt and Grillo (2018), also placed Santanaraptor within Tyrannosauroidea based on the following features: the absence of an accessory ridge on the lateral surface of the cnemial crest; the absence of a horizontal groove across the astragalar condyles anteriorly; a deep fossa on the medial surface of the femoral head, lateral to the trochanteric fossa; an ischial medial apron positioned along the anterior margin of its shaft in medial view; the lesser trochanter and the greater trochanter extending to approximately the same level proximally; the proximal margin of the femur is concave in posterior view due to a greater trochanter that is elevated substantially relative to the lateral portion of the proximal surface of the head; and a shallow femoral extensor groove on the anterior surface of the distal end that is expressed as a broad concave anterior margin in distal view but present as an extensive depression on the anterior surface of the femur.

Holotypic left femur of Timimus hermani (From Wikimedia Commons)

Timimus was unearthed in 1994 from Eumeralla Formation and shares similar features with Tyrannosauroidea, but due to the incompleteness of the Timimus holotype, is difficult to properly evaluate its phylogenetic position. The same applies to NMV P186046. It was suggested (Benson et al., 2012) that NMV P186046 and the Timimus holotype may represent a single taxon given their similar phylogenetic positions and congruent sizes, although they did not come from the same site.

Time-calibrated phylogeny of Tyrannosauroidea. From Delcourt and Grillo, 2018.

Tyrannosauroidea had a Eurasian distribution, but basal lineages of the newly proposed clade, Pantyrannosauria (the most inclusive clade, containing Tyrannosaurus rex and Dilong paradoxus, but not Proceratosaurus bradleyi), were distributed across Europe (Juratyrant, Eotyrannus and Aviatyrannis), North America (Stokesosaurus), South America (Santanaraptor), Australia (Timimus), and Asia (Dilong and Xiongguanlong). It was hypothesized that all basal lineages of Pantyrannosauria were already established in the Late Jurassic before the complete separation of Gondwana and Laurasia.

The fact that Santanaraptor and Timimus were relatively small suggests that Gondwanan tyrannosauroids remained small in comparison to northern species. The presence of Santanaraptor in a semi-arid environment with marine incursion also suggests that tyrannosauroids were not only found in humid paleoenvironments.

References:

Rafael Delcourt, Orlando Nelson Grillo , Tyrannosauroids from the Southern Hemisphere: Implications for biogeography, evolution, and taxonomy. Palaeo (2018), doi: 10.1016/j.palaeo.2018.09.003

Apesteguía, S., Smith, N.D., Juárez Valieri, R., Makovicky, P.J., 2016. An Unusual New Theropod with a Didactyl Manus from the Upper Cretaceous of Patagonia, Argentina. PLoS One 11, 1–41. doi:10.1371/journal.pone.0157793

Benson, R.B.J., Rich, T.H., Vickers-Rich, P., Hall, M., 2012. Theropod fauna from southern Australia indicates high polar diversity and climate-driven dinosaur provinciality. PLoS One 7, e37122. doi:10.1371/journal.pone.0037122

Porfiri, J. D., Novas, F. E., Calvo, J. O., Agnolín, F. L., Ezcurra, M. D. & Cerda, I. A. 2014. Juvenile specimen of Megaraptor (Dinosauria, Theropoda) sheds light about tyrannosauroid radiation. Cretaceous Research 51: 35-55. https://doi.org/10.1016/j.cretres.2014.04.007

Holtz Jr, T.R., 2004. Tyrannosauroidea, in: Weishampel, D.B., Dodson, P., Osm (Eds.), The Dinosauria. University of California Press, Berkeley, pp. 111–136.

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A brief history of Mesozoic theropods research in Gondwana

Snout of the ceratosaurian Genyodectes serus

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. The Mesozoic Gondwanan neotheropod record includes: coelophysoids, basal averostrans, ceratosaurids, abelisauroids, megalosauroids, carcharodontosaurids, megaraptorans, basal coelurosaurs, compsognathids, alvarezsauroids, unenlagiids, and basal avialans, as well as putative tyrannosauroids, ornithomimosaur-like forms, and troodontid. Therefore, the Gondwanan fossil record has been crucial to understand the evolution and global biogeography of dinosaurs during the Mesozoic.

The first probable theropod remains from Gondwana were discovered in Colombia by Carl Degenhard, a German engineer, in 1839. At that time the word “dinosaur” did not even exist yet. Although Degenhard identified them as bird footprints, his brief description suggests that they were tracks of bipedal dinosaurs. But it was not until 1896 that the first Gondwanan theropod was named by the French palaeontologist Charles Depéret as “Megalosaurus” crenatissimus from the Upper Cretaceous of Madagascar. Several theropod remains were described from India, Africa, and South America during the 19th century. These early fragmentary discoveries lead the authors of the late XIX and early XX centuries to interpret them as belonging the same lineages present in Europe and North America.

Elaphrosaurus bambergi (Museum für Naturkunde 4960, holotype) from the Upper Jurassic of Tanzania (Janensch, 1920)

In 1901, A. Smith Woodward described Genyodectes, based on fragmentary skull bones, including portions of both maxillas, premaxillae,  parts of the supradentaries, and some teeth, discovered by Santiago Roth in Chubut, at the end of the 1880s. Genyodectes remained as the most completely known  theropod from South American until the 1970s. In 2004, O. Rauhut concluded that Genyodectes is more closely related to Ceratosaurus than the more derived abelisaurs.

Between 1915 and 1933, the most relevant Gondwanan theropod discoveries were produced by the work of the German palaeontologists Frederich von Huene, Ernst Stromer, and Werner Janensch, including for the first time the publication of very informative partial skeletons, such as those of Spinosaurus aegyptiacus and Elaphrosaurus bambergi (Stromer, 1915; Janensch, 1920). Despite its low fossil record, Spinosaurus is one of the most famous dinosaur of all time. 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. But during and after the Second World War the influence of the German palaeontology in the research of Gondwanan theropods abruptly declined.

Skull and neck of Carnotaurus sastrei

By the 1960s, the Argentine biologist Osvaldo Reig, together with Rodolfo Casamiquela and José Bonaparte, began to explore the Mesozoic rocks of Argentina looking for fossil tetrapods. In 1985, Bonaparte published a note presenting Carnotaurus sastrei as a new genus and species and briefly describing the skull and lower jaw. It was collected in the lower section of La Colonia Formation, Chubut Province. The discoveries of Bonaparte and his collaborators resulted in the recognition of the Patagonian theropod record as the most relevant and informative among Gondwanan continents. Some of the theropod species discovered in Patagonia are known on the basis of skulls and fairly complete skeletons offering insights into the anatomy and phylogeny of abelisaurids, carcharodontosaurids, and maniraptorans.

References:

Martín D. Ezcurra, and Federico L. Agnolín (2017). Gondwanan perspectives: Theropod dinosaurs from western Gondwana. A brief historical overview on the research of Mesozoic theropods in Gondwana. Ameghiniana 54: 483–487. Published By: Asociación Paleontológica Argentina https://doi.org/10.5710/102.054.0501

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 

Buffetaut, E. 2000A forgotten episode in the history of dinosaur ichnology; Carl Degenhardt’s report on the first discovery of fossil footprints in South America (Colombia, 1839). Bulletin de la Société Géologique de France 171: 137140Google Scholar

 

Vegaviidae, a new clade of southern diving birds

Vegavis iaai by Gabriel Lio. / Photo: CONICET

The fossil record of Late Cretaceous–Paleogene modern birds in the Southern Hemisphere is fragmentary.  It includes Neogaeornis wetzeli from Maastrichtian beds of Chile, Polarornis gregorii and Vegavis iaai from the Maastrichtian of Antarctica, and Australornis lovei from the Paleogene of New Zealand. The phylogenetic relationships of these taxa have been variously interpreted by different authors. In a more recent analysis, Polarornis, Vegavis, Neogaeornis, and Australornis, are including in a new clade: Vegaviidae.

Vegaviids share a combination of characters related to diving adaptations, including compact and thickened cortex of hindlimb bones, femur with anteroposteriorly compressed and bowed shaft, deep and wide popliteal fossa delimited by a medial ridge, tibiotarsus showing notably proximally expanded cnemial crests, expanded fibular crest, anteroposterior compression of the tibial shaft, and a tarsometatarsus with a strong transverse compression of the shaft.

Histological sections of Vegavis iaai (MACN-PV 19.748) humerus (a), femur (b), polarized detail of humerus (c). Scale bar equals 10 mm for (a), (b) and 5 mm for (c). From Agnolín et al., 2017

The recognition of Polarornis, Vegavis, Neogaeornis, Australornis, and a wide array of isolated specimens as belonging to the new clade Vegaviidae reinforces the hypothesis that southern landmasses constituted a center for neornithine diversification, and emphasizes the role of Gondwana for the evolutionary history of Anseriformes and Neornithes.

The most informative source for anatomical comparison among Australornis, Polarornis, Vegavis as well as other southern avian is a recently published Vegavis skeleton (MACN-PV 19.748). Vegavis overlaps with Australornis in the proximal portion of the humerus, proximal part of the coracoid, scapula, and ulna; with Polarornis in the humerus, femur, and proximal end of the tibia; and with Neogaeornis in the tarsometatarsus.

Phylogeny with geographical distribution of Vegaviidae. From Agnolín et al., 2017.

The humerus is probably the most diagnostic element among anseriforms. In Vegavis and Australornis the humerus is notably narrow and medially tilted on its proximal half, and the deltopectoral crest extends for more than one third of the humeral length. The femur is well known both in Vegavis and Polarornis, and share a combination of characters absent in other Mesozoic or Paleogene birds, including strongly anteriorly bowed and anteroposteriorly compressed shaft (especially near its distal end)

Osteohistological analysis of the femur and humerus of V. iaai. shows a highly vascularized fibrolamellar matrix lacking lines of arrested growths, features widespread among modern birds. The femur has some secondary osteons, and shows several porosities, one especially large, posterior to the medullar cavity. The humerus exhibits a predominant fibrolamellar matrix, but in a portion of the anterior and medial sides of the shaft there are a few secondary osteons, some of them connected with Volkman’s canals, and near to these canals, there are a compact coarse cancellous bone (CCCB) with trabeculae. This tissue disposition and morphology suggests that Vegavis had remarkably high growth rates, a physiological adaptation that may be critical for surviving in seasonal climates at high latitudes, and  may also constitute the key adaptation that allowed vegaviids to survive the K/T mass extinction event.

 

References:

Agnolín, F.L., Egli, F.B., Chatterjee, S. et al. Sci Nat (2017) 104: 87. https://doi.org/10.1007/s00114-017-1508-y

Jordi Alexis Garcia Marsà, Federico L. Agnolín & Fernando Novas (2017): Bone microstructure of Vegavis iaai (Aves, Anseriformes) from the Upper Cretaceous of Vega Island, Antarctic Peninsula, Historical Biology, DOI: 10.1080/08912963.2017.1348503

A Permian lagerstätte from Antarctica.

 

Vertebraria solid-stele and polyarch roots colonised by fungal spores (From Slater et al., 2014)

Vertebraria solid-stele and polyarch roots colonised by fungal spores (From Slater et al., 2014)

A lagerstätte (German for ‘storage place’) is a site exhibiting an extraordinary preservation of life forms from a particular era. The term was originally coined by Adolf Seilacher in 1970. One of the most notable  is Burgess Shale in the Canadian Rockies of British Columbia. The site, discovered by Charles Walcott in 1909, highlight one of the most critical events in evolution: the Cambrian Explosion (540 million to 525 million years ago). The factors that can create such fossil bonanzas are: rapid burial (obrution), stagnation (eutrophic anoxia), fecal pollution (septic anoxia), bacterial sealing (microbial death masks), brine pickling (salinization), mineral infiltration (permineralization and nodule formation by authigenic cementation), incomplete combustion (charcoalification), desiccation (mummification) and freezing. The preservation of decay-resistant lignin of wood and cuticle of plant leaves  is widespread, but exceptional preservation also extends to tissues.

The Toploje Member chert of the Prince Charles Mountains preserves the permineralised remains of a terrestrial ecosystem before the biotic decline that began in the Capitanian and continued through the Lopingian until the Permo-Triassic transition (Slater et al., 2014). During the late Palaeozoic and early Mesozoic, Antarctica occupied a central position within Gondwana and played a key role in floristic interchange between the various peripheral regions of the supercontinent.

permian

Singhisporites hystrix, a megaspore with ornamented surface.

The fossil micro-organism assemblage includes a broad range of fungal hyphae and reproductive structures. The macrofloral diversity in the silicified peats is relatively low and dominated by the constituent dispersed organs of arborescent glossopterid and cordaitalean gymnosperms.  The fossil palynological assemblage includes a broad range of dispersed bisaccate, monosaccate, monosulcate and polyplicate pollen. The roots (Vertebraria), stems (Australoxylon) and leaves (Glossopteris) of the arborescent glossopterid exhibited feeding traces caused by arthropods, but the identification is  difficult since plant and arthropod cuticles look similar in thin section. Tetrapods are currently unknown from Permian strata of the Prince Charles Mountains as either body fossils or ichnofossils (McLoughlin et al., 1997, Slater et al., 2014).

Times of exceptional fossil preservation are coincident with mass extinctions, oceanic anoxic events, carbon isotope anomalies, spikes of high atmospheric CO2, and transient warm-wet paleoclimates in arid lands (Retallack 2011). The current greenhouse crisis delivers several factors that can promote exceptional fossil preservation, such as eutrophic and septic anoxia, microbial sealing, and permineralization.

References:

Benton, M.J., Newell, A.J., (2013), Impacts of global warming on Permo-Triassic terrestrial ecosystems. Gondwana Research.

Rees, P.M., (2002). Land plant diversity and the end-Permian mass extinction. Geology 30, 827–830.

Retallack, G., (2011), Exceptional fossil preservation during CO2 greenhouse crises?, Palaeogeography, Palaeoclimatology, Palaeoecology 307: 59–74.

Slater, B.J., et al., (2014), A high-latitude Gondwanan lagerstätte: The Permian permineralised peat biota of the Prince Charles Mountains, Antarctica, Gondwana Research. http://dx.doi.org/10.1016/j.gr.2014.01.004

Seilacher, A., (1970) “Begriff und Bedeutung der Fossil-Lagerstätten: Neues Jahrbuch fur Geologie und Paläontologie“. Monatshefte (in German) 1970: 34–39.