Wednesday, October 5, 2022

Climatic Barriers Controlled Dispersal of Earliest Dinosaurs, New Theory Says

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In addition to the discovery of Mbiresaurus raathi, Africa’s oldest known dinosaur species, Yale University paleontologist Christopher Griffin and his colleagues offer a new theory on early dinosaur migrations, including the when and where.

The new Zimbabwean assemblage is on the same paleolatitudinal climatic belt as other Carnian dinosaur-bearing assemblages and has a similar taxonomic composition: (a) the northern Zimbabwean locality geographically links other Carnian dinosaurian localities across southern Pangaea; (b) the new Zimbabwean locality is the oldest definitive dinosaurian locality in Africa, coeval with Carnian dinosaur localities from other parts of the world; (c) summary stratigraphic column of the Pebbly Arkose Formation at the type locality in Zimbabwe showing the records of Mbiresaurus raathi (red), a herrerasaurid dinosaur (yellow), a traversodontid cynodont (blue), a hyperodapedontine rhynchosaur (orange) and an aetosaur (green); (d) representative fossils from the Pebbly Arkose Formation, including (1) aetosaur paramedian osteoderm (dorsal view), (2) herrerasaurid dinosaur coracoid (lateral view), (3) cynodont dentary (lateral view) and (4) hyperodapedontine rhynchosaur maxilla (occlusal view). Scale bars – 1 cm (fossils 1, 3 and 4) and 5 cm (fossil 2). Image credit: Griffin et al., doi: 10.1038/s41586-022-05133-x.

Africa, like all the continents that exist today, was once part of the supercontinent called Pangaea.

The climate across Pangaea is thought to have been divided into strong humid and arid latitudinal belts, with more temperate belts spanning higher latitudes and intense deserts across the lower tropics of Pangaea.

Scientists previously believed that these climate belts influenced and constrained animal distribution across Pangaea.

“Because dinosaurs initially dispersed under this climatic pattern, the early dispersal of dinosaurs should therefore have been controlled by latitude,” Dr. Griffin said.

“The oldest dinosaurs are known from roughly the same ancient latitudes along the southern temperate climate belt what was at the time, approximately 50 degrees south.”

Dr. Griffin and co-authors purposefully targeted northern Zimbabwe as the country fell along this same climate belt, bridging a geographic gap between southern Brazil and India during the Late Triassic epoch.

More so, the earliest dinosaurs were restricted by climatic bands to southern Pangaea, and only later in their history dispersed worldwide.

The tempo and mode of Triassic dinosaur dispersal: (a) phylogenetic relationships of early dinosaurs, with Mbiresaurus raathi recovered as an early sauropodomorph; the ancestral geographic range of early dinosaurs was high-latitude southern Pangaea, with theropods dispersing north in the Late Carnian, followed by sauropodomorphs in the Norian; (b) the biogeographic dispersal model suggests a higher likelihood of climatic barriers to dispersal from southern Pangaea early in dinosaurian and ornithodiran evolution, with barriers dampened in the Late Carnian and Early Norian before returning for the remainder of the Triassic. Image credit: Griffin et al., doi: 10.1038/s41586-022-05133-x.

To bolster this claim, the paleontologists developed a novel data method to test this hypothesis of climatic dispersal barriers based on ancient geography and the dinosaurian family tree.

The breakdown of these barriers, and a wave of northward dispersal, coincided with a period of intense worldwide humidity, or the Carnian Pluvial Event.

After this, barriers returned, mooring the now-worldwide dinosaurs in their distinct provinces across Pangaea for the remainder of the Triassic period.

“This two-pronged approach combines hypothesis-driven predictive fieldwork with statistical methods to independently support the hypothesis that the earliest dinosaurs were restricted by climate to just a few areas of the globe,” Dr. Griffin said.

“The early history of dinosaurs was a critical group for this kind of problem,” said Virginia Tech researcher Brenen Wynd.

“Not only do we have a multitude of physical data from fossils, but also geochemical data that previously gave a really good idea of when major deserts were present.”

“This is the first time where those geochemical and fossil data have been supported using only evolutionary history and the relationships between different dinosaur species, which is very exciting.”

The study was published in the journal Nature.


C.T. Griffin et al. Africa’s oldest dinosaurs reveal early suppression of dinosaur distribution. Nature, published online August 31, 2022; doi: 10.1038/s41586-022-05133-x

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