Terrestrial floras at the Triassic-Jurassic Boundary in Europe.

Proportions of range-through diversities of higher taxonomic categories of microfloral elements over the Middle Triassic–Early Jurassic interval (From Barbacka et al., 2017)

Over the last 3 decades, mass extinction events  have become the subject of increasingly detailed and multidisciplinary investigations. Most of those events are associated with global warming and proximal killers such as marine anoxia. Volcanogenic-atmospheric kill mechanisms include ocean acidification, toxic metal poisoning, acid rain, increased UV-B radiation, volcanic darkness, cooling and photosynthetic shutdown. The mass extinction at the Triassic-Jurassic Boundary (TJB) has been linked to the eruption of the Central Atlantic Magmatic Province (CAMP), a large igneous province emplaced during the initial rifting of Pangea. Another theory is that a huge impact was the trigger of the extinction event. At least two craters impact were reported by the end of the Triassic. The Manicouagan Impact crater in the Côte-Nord region of Québec, Canada was caused by the impact of a 5km diameter asteroid, and it was suggested that could be part of a multiple impact event which also formed the Rochechouart crater in France, Saint Martin crater in Canada, Obolon crater in Ukraine, and the Red Wing crater in USA (Spray et al., 1998).

Photographs of some Rhaetian–Hettangian spores and pollen from the Danish Basin (From Lindström, 2015)

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. In the Southern Hemisphere, the vegetation turnover consisted in the replacement to Alisporites (corystosperm)-dominated assemblage to a Classopollis (cheirolepidiacean)-dominated one. But there was no mass extinction of European terrestrial plants during the TJB. The majority of genera and a high percentage of species still existed in its later stages, and replacement seems to have been local, explainable as a typical reaction to an environmental disturbance. In Greenland, for example, the replacement of Triassic wide-leaved forms with Jurassic narrow-leaved forms was linked to the reaction of plants to increased wildfire. In Sweden, wildfire in the late Rhaetian and early Hettangian caused large-scale burning of conifer forests and ferns, and the appearance of new swampy vegetation. In Austria and the United Kingdom, conifers and seed ferns were replaced by ferns, club mosses and liverworts. In Hungary, there was a high spike of ferns and conifers at the TJB, followed by a sudden decrease in the number of ferns along with an increasing share of swamp-inhabiting conifers.

Although certain taxa/families indeed became extinct by the end of the Triassic (e.g. Peltaspermales), the floral changes across Europe were rather a consequence of local changes in topography.

References:

Maria Barbacka, Grzegorz Pacyna, Ádam T. Kocsis, Agata Jarzynka, Jadwiga Ziaja, Emese Bodor , Changes in terrestrial floras at the TriassicJurassic Boundary in Europe, Palaeogeography, Palaeoclimatology, Palaeoecology (2017), doi: 10.1016/j.palaeo.2017.05.024

S. Lindström, Palynofloral patterns of terrestrial ecosystem change during the end-Triassic event — a review, Geol. Mag., 1–23 (2015) https://doi.org/10.1017/S0016756815000552

Van de Schootbrugge, B., Quan, T.M., Lindström, S., Püttmann, W., Heunisch, C., Pross, J., Fiebig, J., Petschick, R., Röhling, H.-G., Richoz, S., Rosenthal, Y., Falkowski, P. G., 2009. Floral changes across the Triassic/Jurassic boundary linked to flood basalt volcanism. Nat. Geosci. 2, 589–594. doi: 10.1038/NGEO577.

N.R. Bonis, W.M. Kürschner, Vegetation history, diversity patterns, and climate change across the Triassic/Jurassic boundary, Paleobiology, 8 (2) (2012), pp. 240–264 https://doi.org/10.1666/09071.1

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Zuul, the Gatekeeper

Skull of Zuul (Photograph: Brian Boyle/Royal Ontario Museum)

The Ankylosauria is a group of herbivorous, quadrupedal, armoured dinosaurs subdivided in two major clades, the Ankylosauridae and the Nodosauridae. Zuul crurivastator, from the Coal Ridge Member of the Judith River Formation of northern Montana, is the most complete ankylosaurid ever found in North America. The generic name refers to Zuul the Gatekeeper of Gozer (from the 1984 film Ghostbusters), and the species epithet combines crus (Latin) for shin or shank, and vastator (Latin) for destroyer, in reference to the sledgehammer-like tail club. The extraordinary preservation of abundant soft tissue in the skeleton, including in situ osteoderms and skin impressions make this specimen an important reference for understanding the evolution of dermal and epidermal structures in this clade. Until the discovery of Zuul, Laramidian ankylosaurin specimens were primarily assigned to three taxa: Euoplocephalus tutus and Ankylosaurus magniventris from northern Laramidia, and Nodocephalosaurus kirtlandensis from southern Laramidia.

The holotype (ROM 75860)  is a partial skeleton consisting of a nearly complete cranium, and a partially articulated postcranium. It is estimated to be over 6 metres long, and it would have weighed approximately 2500 kg. It has been dated to approximately 75 million years ago, and it was discovered accidentally on 16 May 2014 during overburden removal for a scattered tyrannosaurid skeleton, when a skid-steer loader encountered the tail club knob.

Overview of the tail of Zuul crurivastator in dorsal view, with insets of detailed anatomy (From Arbour and Evans, 2017)

The skull is almost complete, missing only the tip of the right quadratojugal horn and the ventral edge of the vomers, and is the largest ankylosaurine skull recovered from Laramidia. The skull is relatively flat dorsally, and had an elaborate ornamentation across the snout. The squamosal horns are robust and pyramid-shaped, and the quadratojugal horns had a sharp, posteriorly offset apex.

The tail club (including the 13 caudal vertebrae in the handle and the knob) is at least 210 cm long. Osteoderms are preserved not only in the anterior, flexible portion of the tail but also along the tail club handle. The first three pairs of caudal osteoderms are covered with a black film, that probably represent preserved keratin, and is similar to the texture observed at the base of bovid horn sheaths.

The discovery of Zuul fills a gap in the ankylosaurine record and further highlights that Laramidian ankylosaurines were undergoing rapid evolutionary rates and stratigraphic turnover as observed for Laramidian ceratopsids, hadrosaurids, pachycephalosaurids and tyrannosaurids.

References:

Arbour V. M., Evans D. C., (2017), A new ankylosaurine dinosaur from the Judith River Formation of Montana, USA, based on an exceptional skeleton with soft tissue preservation , Royal Society Open Science, rsos.royalsocietypublishing.org/lookup/doi/10.1098/rsos.161086

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

Jianianhualong and the evolution of feathers.

Jianianhualong tengi holotype (From Xu, X. et al., 2017)

In recent years, several discovered fossils of theropods and early birds have filled the morphological, functional, and temporal gaps along the line to modern birds. Most of these fossils are from the Jehol Biota of northeastern China, dated between approximately 130.7 and 120 million years ago. Among them are many fossils of troodontids, which are considered as the closest relatives of birds. Previous reported troodontid species include Mei long, Sinovenator changii, Sinusonasus magnodens and Jinfengopteryx elegans. Now a new troodontid, Jianianhualong tengi gen. et sp. nov., has anatomical features that shed light on troodontid character evolution.

The holotype (DLXH 1218) is a nearly complete skeleton with associated feathers, and is inferred to be an adult. It is estimated to be 112 cm in total skeletal body length with a fully reconstructed tail, and its body mass is estimated to be 2.4 kg, similar to most other Jehol troodontids, such as Sinovenator. The skull and mandible are in general well preserved, and  has a relatively short snout and highly expanded skull roof. There are probably 21 maxillary teeth and 25 dentary teeth on each side of the jaw. The vertebral column is nearly completely represented and  the tail is 54 cm long. The furcula is poorly preserved, and the humerus is 70% of femoral length. The manus is typical of maniraptoran theropods, and measures 112 mm in length. The pelvis is in general similar to those of basal troodontids, with a proportionally small ilium, a posteroventrally oriented pubis, and a short ischium. A phylogenetic analysis places Jianianhualong in an intermediate position together with several species between the basalmost and derived troodontids.

Plumage of Jianianhualong tengi (Adapted from Xu, X.  et al, 2017)

The tail frond of Jianianhualong preserves an asymmetrical feather, the first example of feather asymmetry in troodontids. Feathers were once considered to be unique avialan structures. Since the discovery of the feathered Sinosauropteryx in 1996, numerous specimens of most theropod groups and even three ornithischian groups preserving feathers have been recovered from the Jurassic and Cretaceous beds of China, Russia, Germany, and Canada. These feathers fall into several major morphotypes, ranging from monofilamentous feathers to highly complex flight feathers.

Evidence indicates that the earliest feathers evolved in non-flying dinosaurs for display or thermoregulation, and later were co-opted into flight structures with the evolution of asymmetrical pennaceous feathers in Paraves, therefore, the discovery of tail feathers with asymmetrical vanes in a troodontid theropod indicates that feather asymmetry was ancestral to Paraves.

 

 

References:

Xu, X. et al. Mosaic evolution in an asymmetrically feathered troodontid dinosaur with transitional features. Nat. Commun. 8, 14972 doi: 10.1038/ncomms14972 (2017).

Xu, X. et al. An integrative approach to understanding bird origins, Science, Vol. 346 no. 6215 (2014). DOI: 10.1126/science.1253293