Triassic World: Rise of the Kingdom

Eoraptor lunensis, outcropping from the soil. Valle de la Luna (Moon Valley), Parque Provincial Ischigualasto, Provincia de San Juan, Argentina.

“Jurassic World: Fallen Kingdom” has finally been released, but don’t worry, this post is spoiler free. I just use the hype to the tell the story of the rise of dinosaurs to ecological dominance.

There have been many opinions about the origin of the dinosaurs. In the competitive model the success of dinosaurs was explained in terms of their upright posture, predatory skills, or warm-bloodedness. In the opportunistic model, dinosaurs emerged in the late Carnian or early Norian, and then diversified explosively. The current view contains some aspects of both the classic competition model and the opportunistic model. In this model, the crurotarsan-dominated faunas were replaced by a gradual process probably accelerated by the ecological perturbation of the CPE (Carnian Pluvial Episode).

Early-late Carnian (Late Triassic) palaeogeographic reconstruction showing some of the main vertebrate-bearing units (From Bernardi et al. 2018)

The CPE is often described as a shift from arid to more humid conditions (global warming, ocean acidification, mega-monsoonal conditions, and a generalised increase in rainfall). The widespread extinction caused by the CPE was followed by the first substantial diversification of dinosaurs. That diversification can in fact be
divided into three phases: (1) the possible origin in the Olenekian-Anisian (248–245 Ma) related to the turmoil of recovering life in the aftermath of the devastating Permian-Triassic mass extinction (PTME), (2) a rapid diversification of saurischians, primarily sauropodomorphs and possible theropods, termed the dinosaur diversification event (DDE), at 232 Ma, and (3) a further diversification of theropods and especially ornithischians after the end-Triassic mass extinction, 201 Ma.

In Tanzania, the Manda Beds yielded the remains of the possible oldest dinosaur, Nyasasaurus parringtoni, and Asilisaurus, a silesaurid (the immediate sister-group to Dinosauria). However the oldest well-dated identified dinosaurs are from the late Carnian of the lower Ischigualasto Formation in northwestern Argentina, dated from 231.4 Ma to 225.9 Ma. Similarly, the Santa Maria and Caturrita formations in southern Brazil preserve basal dinosauromorphs, basal saurischians, and early sauropodomorphs. In North America, the oldest dated occurrences of vertebrate assemblages with dinosaurs are from the Chinle Formation. Two further early dinosaur-bearing formations, are the lower (and upper) Maleri Formation of India and the Pebbly Arkose Formation of Zimbabwe. These skeletal records of early dinosaurs document a time when they were not numerically abundant, and they were still of modest body size (Eoraptor had a slender body with an estimated weight of about 10 kilograms).

 

Mounted skeleton of Plateosaurus engelhardti (almost complete specimen AMNH FARB 6810 from Trossingen, Germany)

The CPE is one of the most severe biotic crises in the history of life. On land, palaeobotanical evidence shows a shift of floral associations of towards elements more adapted to humid conditions (the palynological record across the CPE suggest at least 3–4 discrete humid pulses). Several families and orders make their first appearance during the Carnian: bennettitaleans, modern ferns, and conifer families (Pinaceae, Araucariaceae, Cheirolepidaceae). The oldest biological inclusions found preserved in amber also come from the Carnian; and key herbivorous groups such as dicynodonts and rhynchosaurs, which had represented 50% or more of faunas, disappeared.

The DDE likely occurred in steps. Followed the extinction of rhynchosaurs in most, or all, parts of the world, there was a burst of dinosaurian diversity in the late Carnian, represented by the upper Ischigualasto Formation and coeval units, with mostly carnivorous small- to medium-sized dinosaurs. Then, the long span of the early Norian, from 228.5–218 Ma, during which dicynodonts and sauropodomorph dinosaurs were the major herbivores. Finally, with the disappearance of dicynodonts, sauropodomorph dinosaurs became truly large in the middle and late Norian, from 218 Ma. This was followed by the extinction of basal archosaur groups during the end-Triassic mass extinction, 201 Ma, and the diversification of sauropods, larger theropods, ornithopods, and armoured dinosaurs subsequently, in the Jurassic.

 

References:

Michael J. Benton et al. The Carnian Pluvial Episode and the origin of dinosaurs, Journal of the Geological Society (2018). DOI: 10.1144/jgs2018-049

Massimo Bernardi et al. Dinosaur diversification linked with the Carnian Pluvial Episode, Nature Communications (2018). DOI: 10.1038/s41467-018-03996-1

Jessica H. Whiteside, Sofie Lindström, Randall B. Irmis, Ian J. Glasspool, Morgan F. Schaller, Maria Dunlavey, Sterling J. Nesbitt, Nathan D. Smith, and Alan H. Turner. 2015. Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years. PNAS: doi:10.1073/pnas.1505252112

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On the rise of the archosauromorphs

Proterosaurus speneri at Teyler’s Museum.

In the aftermath of the devastating Permo-Triassic mass extinction (~252 Ma), synapsid groups such as anomodonts and gorgonopsians and parareptiles such as pareiasaurs, were decimated and largely displaced by the archosauromorphs. The group, which include the ‘ruling reptiles’ (crocodylians, pterosaurs, dinosaurs, and their descendants, birds), originated during the middle–late Permian. The most basal archosauromorphs are Aenigmastropheus and Protorosaurus.

During the Triassic, the archosauromorphs achieved high morphological diversity, including aquatic or semi aquatic forms, highly specialized herbivores, massive predators, armoured crocodile-like forms, and gracile dinosaur precursors. The group constitutes an excellent empirical case to shed light on the recovery of terrestrial faunas after a mass extinction.

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

The massive volcanic eruptions in Siberia at the end of the Permian, covered more than 2 millions of km 2 with lava flows, releasing more carbon in the atmosphere. High amounts of fluorine and chlorine increased the climatic instability, which means that the Mesozoic began under extreme hothouse conditions. Isotope studies and fossil record, indicates that temperatures in Pangaea interiors during the Early Triassic oscillated between 30 and 40 degrees Celsius, with heat peaks in the Induan and during the Early and Late Olenekian. It was suggested that during that time there was a moderate oxygen depletion that caused the low body size of the amphibian and reptilian life-forms found in those rocks.

After the mass extinction event, a distributed archosauromorph ‘disaster fauna’ dominated by proterosuchids, established for a short time. In South Africa, Proterosuchus occurs only between 5 and 14 m above the PT boundary and a similar pattern has been documented for the synapsid Lystrosaurus. During the Olenekian (1–5 million years after the extinction), archosauromorphs underwent a major phylogenetic diversification with the origins or initial diversification of major clades such as rhynchosaurs, archosaurs, erythrosuchids and tanystropheids.

Stenaulorhynchus stockleyi, a rhynchosaur from the Middle Triassic (From Wikimedia Commons)

The Mid Triassic is marked by the return of conifer-dominated forests, and the end of an interval of intense carbon perturbations, suggesting the recovery and stabilization of global ecosystems. The Anisian (5–10 Myr after the extinction) is characterized by a high diversity among the archosauromorphs with the appearance of large hypercarnivores, bizarre and highly specialized herbivores, long-necked marine predators, and gracile and agile dinosauromorphs. This phylogenetic diversity of archosauromorphs by the Middle Triassic paved the way for the ongoing diversification of the group (including the origins of dinosaurs, crocodylomorphs, and pterosaurs) in the Late Triassic, and their dominance of terrestrial ecosystems for the next 170 million years.

 

 

References:

Ezcurra MD, Butler RJ. 2018 The rise of the ruling reptiles and ecosystem recovery from the Permo-Triassic mass
extinction. Proc. R. Soc. B 285: 20180361. http://dx.doi.org/10.1098/rspb.2018.0361

Ezcurra MD. (2016) The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms. PeerJ 4:e1778 https://doi.org/10.7717/peerj.1778

Holz, M., Mesozoic paleogeography and paleoclimates – a discussion of the diverse greenhouse and hothouse conditions of an alien world, Journal of South American Earth Sciences (2015), doi: 10.1016/j.jsames.2015.01.001

Life finds a way.

 

Site M0077 in the Chicxulub crater as seen using gravity data. From Lowery et al., 2018.

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. In 1981, 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, making the feature one of the largest confirmed impact structures on Earth.

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. The event also produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. The decrease of sunlight caused a drastic short-term global reduction in temperature (15 °C on a global average, 11 °C over the ocean, and 28 °C over land). While the surface and lower atmosphere cooled, 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. Additionally, the vapour produced by the impact  could have led to global acid rain and a dramatic acidification of marine surface waters.

The Cretaceous/Palaeogene mass extinction eradicated almost three-quarters of the plant and animal species on Earth including non-avian dinosaurs, pterosaurs, marine reptiles, and ammonites. Global forest fires might have raged for months. Photosynthesis stopped and the food chain collapsed. Marine environments lost about half of their species, and almost 90% of Foraminifera species went extinct. But life always finds a way, and 30,000 years after the impact, a thriving ecosystem was present within the Chicxulub crater.

The evidence comes from the recent joint expedition of the International Ocean Discovery Program and International Continental Drilling Program. The team sampled the first record of the few hundred thousand years immediately after the impact within the Chicxulub crater. This sample includes foraminifera, calcareous nannoplankton, trace fossils and geochemical markers for high productivity. The lowermost part of the limestone sampled also contains the lowest occurrence of Parvularugoglobigerina eugubina, the first trochospiral planktic foraminifera, which marks the base of Zone Pα. This biozone was defined at Gubbio (Italy) to precisely characterise the Cretaceous/Paleogene boundary.

3 Early Danian foraminifer abundances and I/(Ca+Mg) oxygenation proxy. From Lowery et al., 2018.

P. eugubina was a low to middle latitude taxon with an open-ocean affinity and has an extremely variable morphology. Other foraminifer of the same genus (P. extensa, P. alabamensis) and Guembelitria cretacea were found at the same core. The nannofossil assemblage includes opportunistic groups that can tolerate high environmental stress such as Thoracosphaera and Braarudosphaera, but unlike the foraminifera, there are no clear stratigraphic trends in overall nannoplankton abundance. Discrete, but clear trace fossils, including Planolites and Chondrites, characterize the upper 20cm of the transitional unit. Nevertheless, the study also shows that photosynthetic phytoplankton struggled to recover for millions of years after the event.

Core samples also revealed that porous rocks in the center of the Chicxulub crater had remained hotter than 300 °C for more than 100,000 years. The high-temperature hydrothermal system was established within the crater but the appearance of burrowing organisms within years of the impact indicates that the hydrothermal system did not adversely affect seafloor life. These impact-generated hydrothermal systems are hypothesized to be potential habitats for early life on Earth and other planets.

 

Reference:

Christopher M. Lowery et al. Rapid recovery of life at ground zero of the end-Cretaceous mass extinction, Nature (2018). DOI: 10.1038/s41586-018-0163-6

Charles G. Bardeen, Rolando R. Garcia, Owen B. Toon, and Andrew J. Conley, On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections, PNAS 2017 ; published ahead of print August 21, 2017 DOI: 10.1073/pnas.1708980114

Brugger J.G. Feulner, and S. Petri (2016), Baby, it’s cold outside: Climate model simulations of the effects of the asteroid impact at the end of the CretaceousGeophys. Res. Lett.43,  doi:10.1002/2016GL072241.