Evolution News - How The Bird-Hipped Dinosaurs Evolved

Shaking the dinosaur family tree: how did ‘bird-hipped’ dinosaurs evolve?

Two Herrerasaurus chasing a Silesaurus down a stream in the Triassic period. Two Plateosaurus are in the background.

Mohamad Haghani/Stocktrek Images

It's been another terrible week for Creationists.

First there was the news that plants have a ludicrously complex process for correcting the mistakes made when DNA is replicated, and how this shows common descent of plants and animals with the news that this process works when inserted into a human cell. Then there was yet more evidence of the malevolence of any creator who could come up with a design for a trypanosome flagellum that makes it easier for tsetse flies to inject it into us and cause sleeping sickness, and the news that a redesigned fungus is killing the frogs that would eat the mosquitoes that spread malaria. And lastly there is the news that the Theory of Evolution shows no signs of being abandoned as the best explanation for biodiversity but can even be applied to cultures with the finding that chimpanzees, like humans, form different cultural groups.

In fact, of course, most weeks are bad news weeks for Creationists because so many new scientific papers, especially in the fields of biology, archaeology and geology, refute Creationism without even trying, simply because, unlike Creationism, science is firmly grounded in observable and verifiable reality.

Now there is news that scientists from Cambridge University, UK, have shown how a group of dinosaurs, the remote ancestors of modern birds - the bird-hipped dinosaurs - evolved from a 'transitional', i.e., stem group, of dinosaurs, the silesaurs, which had been identified ten years earlier. They have published their finding in the Zoological Journal of the Linnean Society.

The Cambridge University News release explains how this discovery was made:

Artist's impression

The researchers, from the University of Cambridge and the Universidade Federal de Santa Maria in Brazil, were attempting to solve a long-standing mystery in palaeontology: where the ‘bird-hipped’ dinosaurs, or ornithischians, came from.

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

The researchers conducted an extensive analysis of early dinosaurs as well as silesaurs, a group named after Silesaurus, first described in 2003. The researchers suggest that silesaurs progressively modified their anatomy during the Late Triassic Period, so that they came to resemble ornithischians by the Early Jurassic Period.

However, these ornithischian ancestors have the hip structure of the ‘lizard-hipped’ dinosaurs, or saurischians, suggesting that the earliest bird-hipped dinosaurs were in fact lizard-hipped. The results are reported in the Zoological Journal of the Linnean Society.

Dinosaurs originated in the Late Triassic period, about 225 million years ago, and dominated life on Earth until a mass extinction event 66 million years ago. Dinosaurs have fascinated us since they were first named as such by Richard Owen in 1842.

The earliest discovered dinosaur remains were scrappy: odd-looking teeth and a few bones. By the latter half of the 19th century however, enough dinosaur remains had been found that a classification system was needed. Harry Seeley, who had been trained in Cambridge by Adam Sedgwick, developed such a classification of dinosaurs based primarily upon the shape of their hip bones: they were either saurischians (lizard-hipped) or ornithischians (bird-hipped). This classification, first published in 1888, proved reliable: all dinosaur discoveries seemed to slot neatly into one or other of these groupings.

However, in 2017, Professor David Norman from Cambridge’s Department of Earth Sciences and his former PhD students Matthew Baron and Paul Barrett argued that these dinosaur family groupings need to be rearranged, re-defined and re-named. In a study published in Nature, the researchers suggested that bird-hipped dinosaurs and lizard-hipped dinosaurs such as Tyrannosaurus evolved from a common ancestor, potentially overturning more than a century of theory about the evolutionary history of dinosaurs.

Controversy aside, it has long been recognised that the bird-hipped dinosaurs are anatomically distinct from all other types of dinosaurs, even though they have nothing to do with birds. But how they came to be has remained a long-standing problem.

It seemed to be that they originated with all other dinosaurs in the Late Triassic but exhibited a unique set of features that could not be fitted into an evolutionary succession from their dinosaur cousins. It was as if they just suddenly appeared out of nowhere.

However, the more we’ve looked in rocks of that age, the less we’ve found. The first unarguable ornithischian remains date from less than 200 million years ago, meaning there is a 25+ million-year ornithischian gap. So far, all attempts to fill that gap have failed.

Professor David Norman, Senior author.
Christ’s College.
Cambridge University, UK

Recent work has begun to indicate a more varied and puzzling picture of ornithischian origins. From a phylogenetic perspective – how the dinosaur family tree branches over time – it is predicted that ornithischian remains should first appear in the fossil record about 225 million years ago.

One solution to this conundrum can be traced back to a discovery in the early years of this century, when the skeleton of an unusual Late Triassic dinosaur-like animal was discovered in Poland. It was described by Jerzy Dzik and named Silesaurus (the ‘Silesian lizard’).

Skeleton of Silesaurus

Silesaurus has long slender legs that gave it an upright dinosaur-like posture – and its hip bones are arranged like a saurischian – but it seemed to have a toothless, beak-like region at the front of its lower jaw. This was not unlike the toothless beak-like structure known as a predentary that is found in all ornithischian dinosaur skulls, although the uniquely ornithischian predentary bone was not present.

Skull of an early Ornithischian

Its teeth were also constricted at the top of the roots, and the crowns of the teeth were leaf-shaped in profile: a type of tooth shape seen in many early ornithischians. Dzik speculated about the possible ornithischian similarities of Silesaurus, but the suggestion was dismissed or ignored by most researchers.

Illustration of teeth from Silesaurus and Ornithischian

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

The accumulation of these specimens attracted the attention of several researchers. Their analyses suggested that silesaurs were close relatives of true dinosaurs. Either they were placed on a branch just before the origin of true dinosaurs or, in some instances, they appeared as a sister group to Ornithischia. In 2020, Mauricio Garcia and Rodrigo Müller from the Universidade Federal de Santa Maria in Brazil proposed that silesaur-like creatures could sit on the branch of Dinosauria that led to Ornithischia.

This work attracted our attention in Cambridge. A few years ago, I devised a research project aimed directly at the problem of how the Ornithischia came to be, and Matt was the research student on the project.

Professor David Norman.

Norman and Barron began to collaborate with Rodrigo and Mauricio, enlarging the original analysis to include a range of ornithischian dinosaurs, as well as dinosaur ancestors. The outcome of their collaboration is a family tree that depicts silesaurs as a succession of animals on the stem of the branch leading to Ornithischia.

Dinosaur family tree

Silesaurians progressively modified their anatomy during the Late Triassic, so that they come to resemble ornithischians. We have been able to trace this transition through the development of the toothless beak, the development of leaf-shaped coarse-edged teeth typical of those seen in the herbivorous ornithischians, modifications to the shoulder bones, changes in the proportions of the pelvic bones, and finally a restructuring of the muscle attachment areas on the hind legs.

Professor David Norman.

The research suggests that ornithischians did not arise from nowhere. Rather, they first appeared in the Late Triassic in the guise of silesaurs that gradually rearranged their anatomy with the passage of time until recognisable ornithischians had evolved by the beginning of the Jurassic Period.

Dinosaur transition

However, there is another aspect to this explanation, which is that the earliest ornithischians of the Late Triassic had none of the anatomical characteristics of true ornithischians: they lacked a predentary and, most importantly, retained the early saurischian hip construction.

So, the very earliest ornithischians were, technically, saurischian. From a taxonomic perspective, classifying silesaurs as early ornithischians seems counterintuitive. But, taking a Darwinian perspective, the unique anatomical characteristics of ornithischians had to evolve from somewhere, and where better than from their nearest relatives: their saurischian cousins!

Professor David Norman.

Creationists might like to dwell on Professor Norman's final sentence quoted above, or maybe they would rather not think about it. Of course, it shows the fact that this team of renowned palaeontologists have no doubt about the value of the Darwinian TOE as the basis for understanding the fossil record and no doubt at all that the 'bird-hipped' dinosaurs were the result of an evolutionary process as captured by snapshot fossils showing the transition over time from earlier ancestors.

But then having their pet notions refuted just about every day by serious scientists is something they've had to get used to over the years. Creationists who want to retain their refuted notions should avoid reading the team's paper in the Zoological Journal of the Linnean Society:

Abstract

The origin and evolutionary relationships of ornithischian dinosaurs are topics that have undergone a series of substantial revisions. At present there are several competing hypotheses concerning the relationship between Ornithischia and the other principal clades of Dinosauria. Some hypotheses have posited a tree topology within Dinosauria that imply a ‘ghost-lineage’ for Ornithischia (whose representatives make their first unambiguous appearance in the Hettangian) that extends through a substantial portion of Triassic time. In contrast, other hypotheses have placed conventionally Triassic dinosauromorph (stem-lineage Dinosauria) taxa within the clade Ornithischia. Recently, a large-scale phylogenetic analysis recovered an array of taxa, known as ‘silesaurids’, as a paraphyletic assemblage of taxa (referred to in this article using the informal terms silesaurs or silesaurians) on the branch leading to the clade Ornithischia. This latter hypothesis of relationships would account for the apparent absence of Triassic ornithischians, because stem-lineage ornithischians (silesaurs in this article) are exclusively Triassic. However, the analysis that produced this novel topology used a dataset that, in its original form, did not include all early representatives of Ornithischia (sensu lato), and did not incorporate all the anatomical characters that have been suggested to unite Ornithischia with other dinosaurian clades (Theropoda and Sauropodomorpha). Nor did the initial study go on to expand upon some important taxonomic, palaeobiological and evolutionary implications of a topology that links a paraphyletic array of silesaurs to the clade Ornithischia. The present article addresses these latter issues by expansion and re-analysis of the original dataset. The results find further support for the hypothesis that silesaurs comprise a paraphyletic grouping of taxa on the stem of Ornithischia and that successive silesaur taxa acquire anatomical characters anagenetically in a process that culminates in the assembly of what may be described as a ‘traditional’ ornithischian. The overall topology of the consensus tree remains but little changed from the original analysis, despite the addition of new taxa and characters. To provide stability to this area of the tree and to preserve the most important of the relevant taxonomic names, we suggest a revised taxonomic framework for ornithischians that is consistent with this new topology. We retain the name Ornithischia for the total-group (traditional Ornithischia and its stem-lineage), while we resuscitate a name originally proposed by Richard Owen, Prionodontia (= ‘coarse edged teeth’) for the clade containing only the so-called traditional ornithischian (= ‘bird-hipped’) dinosaurs. We also erect Parapredentata as a more exclusive subclade in Ornithischia. This novel taxonomic framework is intended to provide phylogenetic clarity and a degree of stability in Ornithischia and Dinosauria as further analyses and new data continue to refine and re-shape the tree. The data presented in this study represent a stage in our attempt to establish an early dinosaur dataset in which character definitions and character scores are agreed upon and used consistently.

David B Norman, FLS, Matthew G Baron, Mauricio S Garcia, Rodrigo Temp Müller,
Taxonomic, palaeobiological and evolutionary implications of a phylogenetic hypothesis for Ornithischia (Archosauria: Dinosauria)
Zoological Journal of the Linnean Society, 2022;, zlac062, DOI: 10.1093/zoolinnean/zlac062

Copyright © 2022 The Linnean Society of London. Published by Oxford University Press.
Reprinted under the terms of the Copyright, Designs and Patents Act 1988, s60.

Clearly, there is a great deal for Creationists to ignore here, but it's just a small drop in the ocean compared to the vast ammounts of scientific data they have been ignoring for years now.

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