It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Thursday, April 04, 2024
PALEONTOLOGY
Early dinosaurs grew up fast, but they weren’t the only ones
High growth rates apparently a common feature among early Mesozoic animals
PLOS
IMAGE:
HERRERASAURUS ISCHIGUALASTENSIS IS AN EARLY SAURISCHIAN DINOSAUR. IT SHARED A BIPEDAL, RUNNING ANATOMY COMMON TO LARGE CARNIVOROUS DINOSAURS THAT WOULD EVOLVE IN THE FUTURE, BUT THIS DINOSAUR LIVED AT A TIME WHEN DINOSAURS WERE SMALL-BODIED AND RARE.
CREDIT: KRISTINA CURRY ROGERS (ILLUSTRATION BY JORDAN HARRIS, CC-BY 4.0 (HTTPS://CREATIVECOMMONS.ORG/LICENSES/BY/4.0/)
The earliest dinosaurs had rapid growth rates, but so did many of the other animals living alongside them, according to a study published April 3, 2024 in the open-access journal PLOS ONE by Kristina Curry Rogers of Macalester College, Minnesota and colleagues.
Dinosaurs grew up fast, a feature that likely set them apart from many other animals in their Mesozoic (252 to 66 million years ago) ecosystems. Some researchers have proposed that these elevated growth rates were key to the global success of dinosaurs, but little is known about the growth strategies of the earliest dinosaurs. In this study, Rogers and colleagues performed histological analysis, examining patterns of bone tissue growth in the fossilized leg bones of an array of animals in one of the earliest known Mesozoic ecosystems.
The studied fossils come from the Ischigualasto Formation of Argentina and date between 231-229 million years old. Sampled fossils include several of the earliest known dinosaurs as well as several non-dinosaur reptiles and one early relative of mammals.
The analysis found that most of the examined species had elevated growth rates, more similar to some modern-day mammals and birds than to living reptiles. The early dinosaurs all exhibited particularly fast growth, but they weren’t alone in this, as similar growth rates were seen in several of the non-dinosaur reptiles as well.
These results show that the earliest dinosaurs were already fast growers, supporting the idea that this feature was important to their later success. But apparently dinosaurs were only one of multiple lineages evolving with elevated growth rates during the Triassic (252-201 million years ago), suggesting that this feature is only part of the story of dinosaurs’ eventual global prosperity. The authors note that future studies could expand on these preliminary results by sampling a wider variety of ancient animals from additional early Mesozoic fossil sites.
The authors add: “Our sample comes from a time in which dinosaurs were the new kids on the block, restricted to relatively small, basic body plans, and evolving within a world rich with a diverse array of more specialized, non-dinosaur reptiles. We tackled the question of how all of these animals grew, and found that the earliest dinosaurs grew quickly, and that these rapid growth rates probably played a significant role in dinosaurs’ subsequent ascent within Mesozoic ecosystems; but dinosaurs weren’t unique – many of their non-dino sidekicks shared rapid growth 230 million years ago.”
Citation: Curry Rogers K, MartÃnez RN, Colombi C, Rogers RR, Alcober O (2024) Osteohistological insight into the growth dynamics of early dinosaurs and their contemporaries. PLoS ONE 19(4): e0298242. https://doi.org/10.1371/journal.pone.0298242
Author Countries: USA, Argentina
Funding: This work was supported the National Science Foundation CAREER Grant – EAR-0955716 to KCR and by the Wallace Faculty Travel Grant from Macalester College to KCR. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Osteohistological insight into the growth dynamics of early dinosaurs and their contemporaries
ARTICLE PUBLICATION DATE
3-Apr-2024
Jurassic shuotheriids reveal earliest dental diversification of mammaliaforms
MUSEUM VICTORIA
Palaeontologists have presented a new insight into the initial dental variations across mammaliaforms, providing a fresh perspective on the evolutionary past of these ancient beasts.
The discovery, involving a team of international researchers including Professor Patricia Vickers-Rich from the Monash University School of Earth, Atmosphere and Environment, is published today in the renowned journal Nature.
The research, conducted by a group of palaeontologists from prestigious institutions in New York, China and Australia, examines the tooth structure of the Jurassic shuotheriids to gain a better understanding of their phylogenetic relationships and evolutionary paths.
“Our study questions current theories and offers a new viewpoint on the evolutionary history of mammaliaforms,” said Professor Vickers-Rich.
“We provide vital insights into the phylogenetic relationships and evolutionary trajectories of shuotheriids, little known until recent discoveries in China, by explaining the intricate tooth shapes and occlusal patterns.” she said.
Shuotheriids, mammal-like animals from the Jurassic Period, have perplexed scientists because of their unique dental characteristics.
Shuotheriids had what are termed “pseudotribosphenic teeth”, with a “pseudotalonid” (a basin-like structure) located in front of the trigonid in the lower molars, unlike the tribosphenic pattern seen in current therian mammals where the taloned is located behind the trigonid.
“This unique tooth pattern has hindered our comprehension of shuotheriid relationships and the first steps in the evolution of mammaliaform species,” Professor Vickers-Rich said.
The research team used advanced techniques to discover new specimens, some with complete skeletons, of Jurassic shuotheriids, enabling a thorough examination of their “pseudotribosphenic” teeth. In their analysis of this new material, the researchers were able to more fully analyse the dental structures using a variety of analysis, which suggested that shuotheriid dental structures appear to be very similar to those of docodontans.
The study suggests that shuotheriids do not have a genuine trigonid in their bottom teeth, indicating a closer relationship to docodontans than previously thought.
This reassessment of tooth architecture not only resolves unresolved interpretations but also triggers a reconsideration of the evolutionary connections within mammaliaforms.
Co-author, Dr Thomas Rich of Museums Victoria Research Institute said, “In 1982, a single Jurassic small lower jaw with four teeth was placed at one point on the mammalian family tree by Minchen Chow and myself. Now two virtually complete specimens, analysed in a different way, places all of them in a quite different place on the mammalian family tree. Additional specimens and different methods suggest different interpretations. This is how science often works.”
Based on this new data, shuotheriids appear to be in a different clade named Docodontiformes, separate from ausktribosphenids, and thus grouped with docodontans. This discovery highlights the significance of pseudotribosphenic characteristics in elucidating the first diversification of mammaliaforms.
“The study emphasises the presence of a significant variety of tooth morphology in early mammaliaforms, indicating unique ecomorphological adaptations throughout the evolutionary development within mammals,” Professor Vickers-Rich said.
“The dental modifications such as transverse broadening of posterior teeth, cusp rearrangement and even rotation indicate the many ecological niches early mammaliaforms inhabited.”
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Full list of authors and their affiliations:
Jin Meng: American Museum of Natural History, Fangyuan Mao: Chinese Academy of Sciences, Zhiyu Li: Inner Mongolia Museum of Natural History, Zhili Wang: Inner Mongolia Museum of Natural History, Chi Zhang: Institute of Vertebrate Paleontology and Paleoanthropology, Tom Rich: Museums Victoria, Melbourne, Patricia Vickers-Rich: Monash University
An international team of paleontologists led by the American Museum of Natural History and the Chinese Academy of Sciences has discovered new sets of fossils from the Jurassic period that provide fresh insights into the early evolution of mammals. The findings, detailed today in two back-to-back studies in the journal Nature, could change how scientists reconstruct the earliest branches in the mammalian tree of life.
The first paper focuses on shuotheriids, a family of mouse-sized mammals with molars that are different from those in any living mammal. The evolutionary position of these animals has been heavily debated, but they have been linked to the australosphenidans, the group that includes today’s monotremes—mammals that lay eggs, such as the platypus. Analyzing two newly discovered and well-preserved skeletal fossils of shuotheriids that lived between 168–164 million years ago in what is now Inner Mongolia, the researchers found that the molars of these animals were more like those of another extinct mammal group called the docodontans. They also determined that these two specimens belong to a new genus and species, which they named Feredocodon chowi.
“When you look at the fossil record, both for mammals and many other sorts of animals, teeth are the part of the body that you are most likely to recover,” said Jin Meng, curator in the American Museum of Natural History’s Division of Paleontology and a corresponding author on both Nature papers along with Fangyuan Mao from the Chinese Academy of Sciences. “Yet since the 1980s, the perplexing tooth shape seen in shuotheriids has been a barrier to our efforts to understand early mammal evolution. These new specimens have allowed us to solve this longstanding problem.”
The second study, also led by Meng and Mao, is based on the fossil skulls of Feredocodon chowi as well as a second new species, named Dianoconodon youngi, which lived between 201–184 million years ago. In this study, the researchers looked at structure of the middle ear, which gives modern mammals the sharpest hearing on Earth.
The modern mammalian middle ear, the area just inside the eardrum that turns vibrations in the air into ripples in the inner ear’s fluids, has three bones, or auditory ossicles—a feature that is unique to mammals. Reptiles and birds only have one middle-ear bone. Scientists know that during the early evolution of mammals from the group that includes lizards, crocodilians, and dinosaurs, bones formed the joints of the jaw were separated and became associated with hearing. The newly described specimens provide convincing fossil evidence of this transition in action.
The transition started from an ancestral animal that had a double jaw joint, a feature with the joint of a mammal on the outside and a reptilian joint on the inside. Analyses on the older fossil (Dianoconodon youngi) show that one of its two joints, the reptilian one, was starting to lose its ability to handle the forces created by chewing. The younger specimen (Feredocodon chowi) already had a middle ear of mammals formed and adapted exclusively for hearing.
“Scientists have been trying to understand how the mammalian middle ear evolved since Darwin’s time,” said Meng. “While paleontological discoveries have helped reveal the process during the last a few decades, these new fossils bring to light a critical missing link and enrich our understanding of the gradual evolution of the mammalian middle ear.”
Reconstruction of the newly described species Feredocodon chowi
Reconstruction of the newly described species Dianoconodon youngi
Mammaliaforms are extinct and extant organisms that are closely related to mammals. Studying mammaliaforms helps scientists understand the evolutionary processes that led to various mammalian features.
In two consecutive studies in Nature, Dr. MAO Fangyuan and Dr. ZHANG Chi from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences, together with colleagues from Australia and the United States, recently reported two Jurassic mammaliaforms from China, revealing the earliest dental diversification, mandibular middle ears, and articular-quadrate joint transformation of mammaliaforms.
The studies provide key information about the evolutionary shift from reptilian jaw bones to early mammalian middle ear ossicles, presenting new perspectives on the early diversity of mammaliaforms and reshaping the early mammalian phylogeny.
Shuotheriids were Jurassic mammaliaforms with pseudotribosphenic molars that featured a "pseudotalonid" (a basin-like structure) in front of the trigonid in the lower molars. In contrast, molars in living mammals feature a tribosphenic pattern where the talonid is located behind the trigonid and "receives," i.e., interlocks with, the protocone of the upper molar for food processing/mastication.
Traditionally, shuotheriids have been phylogenetically grouped with "australosphenidans" (including the living monotremes), but this relationship is controversial and leaves some puzzling morphological, paleogeographical, and functional issues unexplained in mammalian forms.
In the first paper, the researchers examined two specimens from the Middle Jurassic Daohugou locality in Inner Mongolia and established a new genus and species of shuotheriid, Feredocodon chowi.
Based on the evidence of the complete dentitions, occlusal relationships, and the serial homology of the teeth, the researchers proposed a new interpretation: The pseudotribosphenic molars are actually homologous to the molar pattern of docodontans.
The results of phylogenetic analyses reconstructed from the revised dental characters suggest that a Morganucodon-like ancestor independently gave rise to three major groups of mammaliaforms: Docodontiformes (Docodonta and Shuotheridia), Allotheria, and Holotheria (symmetrodontans, therians, and kin). The key feature of the tooth evolution in early mammaliaforms is that the molars, which arose from the ancestral triconodont pattern as in Morganucodon, became broader and more complex to accommodate more efficient food processing. However, the evolutionary processes of the three groups took place in different directions.
In the second paper, the researchers reported the mandibular middle ears (MdME) of two species—one being the shuotheriid Feredocodon described above and the other being a Morganucodon-like animal from the Early Jurassic Lufeng Biota, named Dianoconodon youngi.
The two species showed some new morphological features that support the evolutionary shift from jaw joint bones to middle ear ossicles in early mammals. The mandibular features suggest that one of the dual jaw joints in the ancestral Morganucodon, the articular-quadrate joint, lost its load-bearing function in Dianoconodon, while the mandibular middle ear was better adapted for hearing. The postdentary bones of the shuotheriid species are more advanced, showing characteristics suitable for a purely auditory function.
The new evidence provides insight into how the ossified Meckel's cartilage functioned as a stabilizing mechanism and reveals that the medial displacement of the quadrate relative to the articular bone played a critical role in the transformation from a load-bearing jaw joint to the middle ear structures.
This research strongly supports and enhances the view that the gradual evolution of the mammalian middle ear is a classic example of vertebrate evolution.
Feredocodon chowi and Dianoconodon youngi are named in honor of Professors Minchen CHOW (ZHOU Minchen) and Chung-Chien YOUNG (YANG Zhongjian), respectively. These studies were supported by the National Natural Science Foundation of China and the Youth Innovation Promotion Association of CAS.
Primary tooth patterns of mammaliaforms within the phylogenetic frame
Mandibular middle ears and transformation of the articular-quadrate joints in mammaliaforms
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