Shake the Dinosaur Family Tree: How You Did Bi

Photo: Top Left: Lesothosaurus Top Right: Heterodontosaurus Foreground: Scutellosaurus
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Credit: John Seebeck

Researchers have conducted a new analysis of the origins of ‘hipped’ dinosaurs – the group that includes such iconic species as Triceratops They found that they likely evolved from a group of animals known as silesaurs, which were first identified two decades ago.

The researchers, from the University of Cambridge and the Federal University of Santa Maria in Brazil, have been trying to solve an ancient palaeontological mystery: where the ‘hips’ dinosaurs, or ornithischians, came from.

Currently, there is a gap of more than 25 million years in the fossil record, which makes it difficult to find the branch of the dinosaur family tree to which ornithischians belonged.

The researchers conducted a comprehensive analysis of the early dinosaurs as well as the plesiosaur, a group named after them Celsauruswas first described in 2003. Researchers suggest that silesaurs gradually modified their anatomy during the Late Triassic, becoming similar to ornithischians by the early Jurassic.

However, these Ornithic ancestors had the hip skeleton of ‘lizard-hiped’ dinosaurs, or saurians, suggesting that early hipped dinosaurs were in fact lizard-typed. The Results Reported on Journal of Zoology of the Linnian Society.

Dinosaurs originated in the late Triassic period, about 225 million years ago, and dominated life on Earth until the mass extinction event 66 million years ago. Dinosaurs have fascinated us since Richard Owen first named them in 1842.

The remains of the first dinosaurs discovered were scattered: strange-looking teeth and some bones. However, by the latter half of the nineteenth century, sufficient dinosaur remains had been found to require a classification system. Harry Seely, trained in Cambridge by Adam Sedgwick, developed such a classification of dinosaurs based on the shape of their hip bones: they were either Saurish (lizard-hips) or Ornithic (bird-hips). This classification, first published in 1888, has proven to be reliable: all dinosaur discoveries seem to fall neatly into one or another of these groups.

However, in a 2017 paper, Professor David Norman of the Department of Earth Sciences at Cambridge and former PhD students Matthew Barron and Paul Barrett argued that these dinosaur family groups need to be rearranged, redefined, and renamed. in study Posted in temper natureresearchers suggested that dinosaurs with hips and dinosaurs with lizards’ hips, such as dinosaur From a common ancestor, potentially overturning more than a century of theory about the evolutionary history of dinosaurs.

Regardless of the controversy, it has long been recognized that dinosaurs with two hips were anatomically distinct from all other types of dinosaurs, even though they had nothing to do with birds. How they became, however, remained a long-standing problem.

“It appears to have originated with all other dinosaurs in the late Triassic period, but it showed a unique set of features that could not be incorporated into the evolutionary succession of their dinosaur cousins,” said Norman, who is a fellow at Christ’s College. “It was as if they had suddenly appeared out of nowhere.”

Recently work She started pointing to a more diverse and puzzling picture of Ornithian origins. From a genetics perspective – how the dinosaur family tree branched out over time – Ornithic remains are expected to first appear in the fossil record about 225 million years ago.

“However, the more we searched in the rocks of that age, the less we found,” Norman said. The first indisputable Ornithic remnants date back to less than 200 million years ago, which means that there is an Ornithian hiatus of more than 25 million years. So far, all attempts to bridge that gap have failed.”

One solution to this dilemma can be traced back to A Discovery In the early years of this century, when the skeleton of an unusual dinosaur-like animal was discovered in the late Triassic period in Poland. Jerzy Dzek’s recipe and name Celsaurus (“Silesia lizard”).

Celsaurus It has long slender legs that give it an erect, dinosaur-like posture – and the thigh bones are arranged like a sauerish – but it appears to have a toothless, beak-like area at the front of its lower jaw. This was no different from the toothless beak-like structure known to the predator found in all ornithischian dinosaur skulls, despite the lack of the unique predatory bone.

His teeth were also bound at the top of the roots, and the crowns of the teeth were leaf-shaped in profile: a type of tooth-shaped seen in many early birds. Dzik speculated about a possible ornithischian similarity to Celsaurusbut the proposal was rejected or ignored by most researchers.

In the years since, more CelsaurusCreature-like creatures have been discovered, mostly in South America. Many of these specimens were fragments, but the toothless tip of the lower jaw and leaf-shaped teeth were common.

The accumulation of these samples has attracted the attention of many researchers. Their analyzes indicated that plesiosaurs were a close relative of true dinosaurs. Either they were placed on a branch just before the emergence of true dinosaurs or, in some cases, they emerged as a sister group. Ornithisia….. in 2020, Mauricio Garcia and Rodrigo Muller from the Federal University of Santa Maria in Brazil Suggestion Those silesaur-like creatures can sit on a branch dinosaur which led to Ornithisia.

“This work caught our attention in Cambridge,” Norman said. “A few years ago, I devised a research project aimed squarely at the problem of how to work Ornithisia He became, and Matt was a research student on the project.”

Norman and Baron began collaborating with Rodrigo and Mauricio, to extend the original analysis to a group of ornithischian dinosaurs, as well as the ancestors of the dinosaurs. The result of their collaboration is a family tree that depicts celiosaurs as a series of animals on the trunk of a branch resulting in Ornithisia.

“The Celsaurians gradually modified their anatomy during the late Triassic period, so that they became more like ornithischians,” Norman said. “We have been able to trace this transition through the development of the toothless beak, the development of the coarse leaf-shaped teeth typical of those seen in herbivores, alterations in the shoulder bones, changes in the proportions of the pelvic bones, and finally the restructuring of the muscular attachment regions of the hind legs.” .

The research suggests that ornithischians did not arise out of nowhere. Instead, they first appeared in the late Triassic period in the guise of silesaurs who gradually rearranged their anatomy over time until ornithischians evolved with the onset of the Jurassic.

However, another aspect of this interpretation is that the earliest ornithologists of the late Triassic period did not have any of the anatomical characteristics of true avian birds: they lacked a predator and, most importantly, retained the early saurish-hip build.

“So, the early ornithologists were, technically, Sureshnes,” Norman said. “From a taxonomic perspective, classifying salsaurs as early ornithologists seems inconsequential. But, from Darwin’s perspective, the unique anatomical characteristics of ornithologists are It was To evolve from somewhere, and where it is better than their closest relatives: their surish cousins! “