Fish Fossils of Tujiaaspis vividus Breathe New Life into Fin and Limb Evolutionary Hypothesis
A trove of fossils, unearthed in rock from China dating back some 436 million years, has revealed for the first time that the mysterious galeaspids, members of an extinct clade of jawless fish, possessed paired fins.
The discovery, by an international team led by Prof. ZHU Min from the Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP) of the Chinese Academy of Sciences and Prof. Philip Donoghue from the University of Bristol, shows the primitive condition of paired fins before they separated into pectoral and pelvic fins, the forerunner of arms and legs.
The findings were published in Nature on Sept. 28.
Fish fossils of Tujiaaspis vividus breathe new life into fin and limb evolutionary hypothesis. Fig. 1 Life reconstructions of Tujiaaspis vividus (Image by ZHENG Qiuyang)
Until now, the only surviving galeaspid fossils were heads, but these new fossils comprise whole bodies. They were found in rocks in Hunan Province and Chongqing and were named Tujiaaspis after the indigenous Tujia people who live in the region.
Fig. 3 The holotype specimen and its interpretative drawing of Tujiaaspis vividus from 436 million years old rocks of Chongqing, China (Image by Gai, et al.)
Theories abound about the evolutionary beginnings of vertebrate fins and limbs—the evolutionary precursors of arms and legs—and are mostly based on comparative embryology. There is a rich fossil record of early vertebrate , but they either had separated paired fins or they didn’t. There has been little evidence for the gradual evolution of fins.
According to first author GAI Zhikun, a professor at IVPP, “The anatomy of galeaspids has been something of a mystery since they were first discovered more than half a century ago. Tens of thousands of fossils are known from China and Vietnam, but almost all of them are just heads—nothing has been known about the rest of their bodies—until now.”
The new fossils are spectacular, preserving the whole body for the first time and revealing that these animals possessed paired fins that extended all the way from the back of the head to the very tip of the tail. This is a great surprise since scientists had thought galeaspids lack paired fins altogether.
“Tujiaaspis breathes new life into a century old hypothesis for the evolution of paired fins, through differentiation of pectoral (arms) and pelvic (legs) fins over evolutionary time from a continuous head-to-tail fin precursor,” said corresponding author Prof. Donoghue.
This “fin-fold” hypothesis has been very popular, but it has lacked any supporting evidence until now. The discovery of Tujiaaspis resurrects the fin-fold hypothesis and reconciles it with contemporary data on genetic control of the embryonic development of fins in living vertebrates.
Tujiaaspis shows the “primitive condition” for the evolution of paired fins, according to Prof. ZHU, who said that later jawless fish showed the first evidence for the separation of this fin-fold into pectoral and pelvic fins. Prof. ZHU also noted that the vestiges of elongate fin-folds could be seen in the embryos of living jawed fishes, which could be manipulated to produce them.
Fig. 2 3D reconstruction of Tujiaaspis vividus (Image by YANG Dinghua)
Bristol’s Dr. Humberto Ferron, a co-author, used computational engineering approaches to simulate the behaviour of models of Tujiaaspis with and without the paired fins. He said,
“The paired fins of Tujiaaspis act as hydrofoils, passively generating lift for the fish without any muscular input from the fins themselves. The lateral fin-folds of Tujiaaspis allowed it to swim more efficiently.”
“Our new analyses suggest that the ancestor of jawed vertebrates likely possessed paired fin-folds, which became separated into pectoral and pelvic regions,” said co-author Dr. Joseph Keating from the University of Bristol.
He noted that the primitive fins evolved musculature and skeletal support that allowed our fish ancestor to better steer their swimming and add propulsion.
“It is amazing to think that the evolutionary innovations seen in Tujiaaspis underpin locomotion in animals as diverse as birds, whales, bats, and humans,” he said.
Fanjingshania renovata, an Ancient ‘Shark’ from China Is Humans’ Oldest Jawed Ancestor
Palaeontologists discover a 439-million-year-old ‘shark’ that forces us to rethink the timeline of vertebrate evolution
Living sharks are often portrayed as the apex predators of the marine realm. Paleontologists have been able to identify fossils of their extinct ancestors that date back hundreds of millions of years to a time known as the Palaeozoic period. These early “sharks,” known as acanthodians, bristled with spines. In contrast to modern sharks, they developed bony “armor” around their paired fins.
A recent discovery of a new species of acanthodian from China surprised scientists with its antiquity. The find predates by about 15 million years the earliest acanthodian body fossils and is the oldest undisputed jawed fish.
These findings were published in Nature on Sept. 28.
Fig. 1 Life reconstruction of Fanjingshania renovata. (Image by ZHANG Heming)
Fig. 2 Life reconstruction of Fanjingshania renovata. (Image by ZHANG Heming)
Reconstructed from thousands of tiny skeletal fragments, Fanjingshania, named after the famous UNESCO World Heritage Site Fanjingshan, is a bizarre fish with an external bony “armor” and multiple pairs of fin spines that set it apart from living jawed fish, cartilaginous sharks and rays, and bony ray- and lobe-finned fish.
Examination of Fanjingshania by a team of researchers from the Chinese Academy of Sciences, Qujing Normal University, and the University of Birmingham revealed that the species is anatomically close to groups of extinct spiny “sharks” collectively known as acanthodians. Unlike modern sharks, acanthodians have skin ossifications of the shoulder region that occur primitively in jawed fish.
Fig. 3 Life reconstruction of Fanjingshania renovata. (Image by FU Boyuan and FU Baozhong)
The fossil remains of Fanjingshania were recovered from bone bed samples of the Rongxi Formation at a site in Shiqian County of Guizhou Province, South China.
These findings present tangible evidence of a diversification of major vertebrate groups tens of millions of years before the beginning of the so called “Age of Fishes” some 420 million years ago.
Fig. 4 Fragment of the pectoral dermal skeleton (part of a pectoral spine fused to shoulder girdle plate) of Fanjingshania renovata shown in ventral view. (Image by Andreev, et al.)
The researchers identified features that set apart Fanjingshania from any known vertebrate. It has dermal shoulder girdle plates that fuse as a unit to a number of spines—pectoral, prepectoral and prepelvic. Additionally, it was discovered that the ventral and lateral portions of the shoulder plates extend to the posterior edge of the pectoral fin spines. The species has distinct trunk scales with crowns composed of a row of tooth-like elements (odontodes) adorned by discontinuous nodose ridges. Peculiarly, dentine development is recorded in the scales but is missing in other components of the dermal skeleton such as the fin spines.
“This is the oldest jawed fish with known anatomy,” said Prof. ZHU Min from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences. “The new data allowed us to place Fanjingshania in the phylogenetic tree of early vertebrates and gain much needed information about the evolutionary steps leading to the origin of important vertebrate adaptations such as jaws, sensory systems, and paired appendages.”
From the outset, it was clear to the scientists that Fanjingshania’s shoulder girdle, with its array of fin spines, is key to pinpointing the new species’ position in the evolutionary tree of early vertebrates. They found that a group of acanthodians, known as climatiids, possess the full complement of shoulder spines recognized in Fanjingshania. What is more, in contrast to normal dermal plate development, the pectoral ossifications of Fanjingshania and the climatiids are fused to modified trunk scales. This is seen as a specialization from the primitive condition of jawed vertebrates where the bony plates grow from a single ossification center.
Unexpectedly, the fossil bones of Fanjingshania show evidence of extensive resorption and remodelling that are typically associated with skeletal development in bony fish, including humans.
“This level of hard tissue modification is unprecedented in chondrichthyans, a group that includes modern cartilaginous fish and their extinct ancestors,” said lead author Dr. Plamen Andreev, a researcher at Qujing Normal University. “It speaks about greater than currently understood developmental plasticity of the mineralized skeleton at the onset of jawed fish diversification.”
The resorption features of Fanjingshania are most apparent in isolatedtrunk scales that show evidence of tooth-like shedding of crown elements and removal of dermal bone from the scale base. Thin-sectioned specimens and tomography slices show that this resorptive stage was followed by deposition of replacement crown elements. Surprisingly, the closest examples of this skeletal remodelling are found in the dentition and skin teeth (denticles) of extinct and living bony fish. In Fanjingshania, however, the resorption did not target individual teeth or denticles, as occurred in bony fish, but instead removed an area that included multiple scale denticles. This peculiar replacement mechanism more closely resembles skeletal repair than the typical tooth/denticle substitution of jawed vertebrates.
A phylogenetic hypothesis for Fanjingshania that uses a numeric matrix derived from observable characters confirmed the researchers’ initial hypothesis that the species represents an early evolutionary branch of primitive chondrichthyans. These results have profound implications for our understanding of when jawed fish originated since they align with morphological clock estimates for the age of the common ancestor of cartilaginous and bony fish, dating it to around 455 million years ago, during a period known as the Ordovician.
These results tell us that the absence of undisputed remains of jawed fish of Ordovician age might be explained by under sampling of sediment sequences of comparable antiquity. They also point towards a strong preservation bias against teeth, jaws, and articulated vertebrate fossils in strata coeval with Fanjingshania.
“The new discovery puts into question existing models of vertebrate evolution by significantly condensing the timeframe for the emergence of jawed fish from their closest jawless ancestors. This will have profound impact on how we assess evolutionary rates in early vertebrates and the relationship between morphological and molecular change in these groups,” said Dr. Ivan J. Sansom from the University of Birmingham.
Rare Fossil Teeth from China Overturn Long-held Views about Evolution of Vertebrates
An international team of researchers has discovered 439-million-year-old remains of a toothed fish that suggest the ancestors of modern osteichthyans (ray- and lobe-finned fish) and chondrichthyans (sharks and rays) originated much earlier than previously thought.
Related findings were published in Nature on Sept. 28.
Rare Fossil Teeth from China Overturn Long-held Views about Evolution of Vertebrates. Fig. 1 Life reconstruction of Qianodus duplicis. (Image by ZHANG Heming)
A remote site in Guizhou Province of south China, containing sequences of sedimentary layers from the distant Silurian period (around 445 to 420 million years ago), has produced spectacular fossil finds, including isolated teeth identified as belonging to a new species (Qianodus duplicis) of primitive jawed vertebrate. Named after the ancient name for modern-day Guizhou, Qianodus possessed peculiar spiral-like dental elements carrying multiple generations of teeth that were added throughout the life of the animal.
The tooth spirals (or whorls) of Qianodus turned out to be one of the least common fossils recovered from the site. They are small elements that rarely reach 2.5 mm and as such had to be studied under magnification with visible light and X-ray radiation.
A conspicuous feature of the whorls is that they contained a pair of teeth rows set into a raised medial area of the whorl base. These so-called primary teeth show an incremental increase in size towards the inner (lingual) portion of the whorl. What makes the whorls of Qianodus unusual in comparison with those of other vertebrates is the clear offset between the two primary teeth rows. A similar arrangement of neighboring teeth rows is also seen in the dentitions of some modern sharks but has not been previously identified in the tooth whorls of fossil species.
The discovery indicates that the well-known jawed vertebrate groups from the so-called “Age of Fishes” (420 to 460 million years ago) were already established some 20 million years earlier.
“Qianodus provides us with the first tangible evidence for teeth, and by extension jaws, from this critical early period of vertebrate evolution,” said LI Qiang from Qujing Normal University.
Unlike the continuously shedding teeth of modern sharks, the researchers believe that the tooth whorls of Qianodus were kept in the mouth and increased in size as the animal grew. This interpretation explains the gradual enlargement of replacement teeth and the widening of the whorl base as a response to the continuous increase in jaw size during development.
For the researchers, the key to reconstructing the growth of the whorls was two specimens at an early stage of formation, easily identified by their noticeably smaller sizes and fewer teeth. A comparison with the more numerous mature whorls provided the palaeontologists with a rare insight into the developmental mechanics of early vertebrate dentitions. These observations suggest that primary teeth were the first to form whereas the addition of the lateral (accessory) whorl teeth occurred later in development.
Fig. 2 Volumetric reconstruction of a tooth whorl viewed from its lingual side (holotype of Qianodus duplicis). The specimen is just over 2 mm in length. (Image by Zhu, et al.)
“Despite their peculiarities, tooth whorls have, in fact, been reported in many extinct chondrichthyan and osteichthyan lineages,”said Plamen Andreev, the lead author of the study. “Some of the early chondrichthyans even built their dentition entirely from closely spaced whorls.”
The researchers claim that this was also the case for Qianodus. They made this conclusion afterexamining the small (1–2 mm long) whorls of the new species with synchrotron radiation—a CT scanning process that uses high energy X-rays from a particle accelerator.
“We were astonished to discover that the tooth rows of the whorls have a clear left or right offset, which indicates positions on opposing jaw rami,” said Prof. ZHU Min from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences.
Fig. 3 Virtual section along the length of a tooth whorl in side view (holotype of Qianodus duplicis). The specimen is just over 2 mm in length (Image by Zhu, et al.)
These observations are supported by a phylogenetic tree that identifies Qianodus as a close relative to extinct chondrichthyan groups with whorl-based dentitions.
“Our revised timeline for the origin of the major groups of jawed vertebrates agrees with the view that their initial diversification occurred in the early Silurian,” said Prof. ZHU.
The discovery of Qianodus provides tangible proof for the existence of toothed vertebrates and shark-like dentition patterning tens of millions of years earlier than previously thought. The phylogenetic analysis presented in the study identifies Qianodus as a primitive chondrichthyan, implying that jawed fish were already quite diverse in the Lower Silurian and appeared shortly after the evolution of skeletal mineralization in ancestral lineages of jawless vertebrates.
“This puts into question the current evolutionary models for the emergence of key vertebrate innovations such as teeth, jaws, and paired appendages,” said Ivan Sansom, a co-author of the study from the University of Birmingham.
Press release from the Chinese Academy of Sciences about the fossil teeth overturning long-held views on the evolution of vertebrates
Dawn of Fishes — Early Silurian Jawed Vertebrates Revealed Head to Tail
A newly discovered fossil “treasure hoard” dating back some 436 million years to the early Silurian period reveals, for the first time, the complete body shape and form of some of the first jawed fishes.
The discovery was published in Natureon Sept. 28 by an international team led by Prof. ZHU Min from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and Prof. Per E. Ahlberg from Uppsala University, as the cover story and one in a series of four papers in the same issue.
The Gnathostomata or jawed vertebrates, which include not only almost all the backboned animals you see in zoos and aquariums but humankind as well, have a mysterious origin. The so-called molecular clock, which deduces the age of the most recent common ancestor of two animals by evaluating the difference between the two sets of DNA, suggests that the most recent common ancestor of all modern jawed vertebrates lived 450 million years ago during the Ordovician period. As a result, the origin of jaws cannot be later than that.
However, the fossil record of jawed vertebrates only becomes abundant from the Early Devonian (~419 million years ago), i.e., the beginning of the “Age of Fishes.” Only in the past 10 years have scientists found several complete jawed fishes from the Late Silurian (~425 million years ago). Even so, these records are still more than 25 million years later than when jaws should have originated. The dearth of earlier fossils means that jawed vertebrates are a “ghost lineage” in the early Silurian.
Fig. 3 Slab containing the holotypes of Shenacanthus vermiformis and Xiushanosteus mirabilis (Image by Zhu, et al.)
The remarkable discovery of complete early Silurian jawed fishes is the result of 20 years of continuous effort by the authors searching for fossil fishes in all possible Silurian rock strata in China. The breakthrough was finally made in late 2020, when complete early Silurian fishes were found in Xiushan County, Chongqing.
LI Qiang and CHEN Yang, both co-authors and leaders of the fieldtrips, recalled their research:
“We remember it was a rainy day. We climbed a mountain ghat. At the 38th turn we found a complete Silurian fish, which initiated an explosion of discoveries in this area in the next two years.”
Fig. 1 Life reconstruction of Xiushanosteus mirabilis (Image by ZHANG Heming)
The authors reported two species. The first one and the most abundant species was named Xiushanosteus mirabilis. It is a tiny, 3-cm-long placoderm or armored jawed fish. The flat and semicircular head, along with the trunk armor, are reminiscent of its jawless ancestors, but its paired fins and powerful tail made Xiushanosteus a much more capable swimmer.
First author ZHU You’an, associate research professor at IVPP and also an Uppsala University alumnus, said,
“As a placoderm expert, I am dazzled by the early age and completeness of Xiushanosteus. It is like a dream. A lot of the anatomical features make perfect sense; it was an ‘Oh, now I know’ moment in my career.”
Fig. 2 Life reconstruction of Shenacanthus vermiformis (Image by ZHANG Heming)
The second fish reported is named Shenacanthusvermiformis. Also very small, it is an early shark relative. However, all the sharks we know are covered in tiny scales, or at most small mosaic plates. Shenacanthus instead has prominent “shoulder armor” made of several large plates that completely encircle its body. This feature, thought to be exclusive in placoderms, provides a strong hint that the first cartilaginous fishes were armored, similar to placoderms.
“Only 20 years ago it was still believed that sharks are primitive and other jawed fish evolved from a shark-like archetype. Now with the discovery of Shenacanthus, we can finally make certain that the opposite is true,” said Prof. ZHU You’an.
“Previously we could only dream of such exceptional and early fossils,” said corresponding author Prof. Ahlberg. “However, they are more than curiosities; they are first and foremost crucial data to test—and either support or confound—our long-held hypotheses regarding the rise of our lineage.”
“The excavation continues to yield remarkable materials,” said Prof. ZHU Min, who led the project and is also a CAS academician. “The Chongqing Lagerstätte, like the Chengjiang and Jehol biotas, will become a world-famous paleontological heritage and will provide key evidence for how the extraordinary diversity of the jawed vertebrates we see today arose.”
Harvard researchers find rapid evolution of reptiles was triggered by nearly 60 million years of global warming and climate change
Artistic reconstruction of the reptile adaptive radiation in a terrestrial ecosystem during the warmest period in Earth’s history. Image depicts a massive, big-headed, carnivorous erythrosuchid (close relative to crocodiles and dinosaurs) and a tiny gliding reptile at about 240 million years ago. The erythrosuchid is chasing the gliding reptile and it is propelling itself using a fossilized skull of the extinct Dimetrodon (early mammalian ancestor) in a hot and dry river valley. Credits: Image created by Henry Sharpe
Studying climate change-induced mass extinctions in the deep geological past allows researchers to explore the impact of environmental crises on organismal evolution. One principal example is the Permian-Triassic climatic crises, a series of climatic shifts driven by global warming that occurred between the Middle Permian (265 million years ago) and Middle Triassic (230 million years ago). These climatic shifts caused two of the largest mass extinctions in the history of life at the end of the Permian, the first at 261myo and the other at 252myo, the latter eliminating 86% of all animal species worldwide.
The end-Permian extinctions are important not only because of their magnitude, but also because they mark the onset of a new era in the history of the planet when reptiles became the dominant group of vertebrate animals living on land. During the Permian, vertebrate faunas on land were dominated by synapsids, the ancestors of mammals. After the Permian extinctions, in the Triassic Period (252-200 million years ago), reptiles evolved at rapid rates, creating an explosion of reptile diversity. This expansion was key to the construction of modern ecosystems and many extinct ecosystems. These rapid rates of evolution and diversification were believed by most paleontologists to be due to the extinction of competitors allowing reptiles to take over new habitats and food resources that several synapsid groups had dominated before their extinction.
Evolutionary response from reptiles to global warming and fast climatic changes. Rates of evolution (adaptive anatomical changes) in reptiles start increasing early in the Permian (at about 294 million years ago), which also marks the onset of the longest period of successive fast climatic shifts in the geological record. From 261 until 235 million years ago, increased global warming from massive volcanic eruption contributed to further climate change and led to the hottest period in Earth’s history. This resulted in two mass extinctions and the demise of reptile competitors on land (mammalian ancestors). The most intensive period of global warming coincided with the fastest rates of evolution in reptiles, marking the diversification of reptile body plans and the origin of modern reptile groups. Credits: Figure by Tiago Simões
However, in a new study in Sciences Advancesresearchers in the Department of Organismic and Evolutionary Biology and the Museum of Comparative Zoology at Harvard University and collaborators reveal the rapid evolution and radiation of reptiles began much earlier, before the end of the Permian, in connection to the steadily increasing global temperatures through a long series of climatic changes that spanned almost 60 million years in the geological record.
“We found that these periods of rapid evolution of reptiles were intimately connected to increasing temperatures. Some groups changed really fast and some less fast, but nearly all reptiles were evolving much faster than they ever had before,” said lead author postdoctoral fellow Tiago R Simões.
Previous studies on the impacts of these changes have often neglected terrestrial vertebrates due to limited data availability, focusing mostly on the response from marine animals
In this study, Simões and senior author Professor Stephanie E. Pierce (both at Harvard) worked alongside collaborators Professor Michael Caldwell (University of Alberta, Canada) and Dr. Christian Kammerer (North Carolina Museum of Natural Sciences) to examine early amniotes, which represent the forerunners of all modern mammals, reptiles, birds, and their closest extinct relatives, at the initial phase of their evolution. At this point in time the first groups of reptiles and mammal ancestors were splitting from each other and evolving along their own separate evolutionary paths.
“Reptiles represent an ideal and rare terrestrial system to study this question as they have a relatively good fossil record and survived a series of climatic crises including the ones leading up to the largest extinction in the history of complex life, the Permian-Triassic mass extinction,” said Simões.
Reptiles were relatively rare during the Permian compared to mammalian ancestors. However, things took a major shift during the Triassic when reptiles underwent a massive explosion in the number of species and morphological variety. This lead to the appearance of most of the major living groups of reptiles (crocodiles, lizards, turtles) and several groups that are now entirely extinct.
The researchers created a dataset based on extensive first-hand data collection of more than 1,000 fossil specimens from 125 species of reptiles, synapsids, and their closest relatives during approximately 140 million years before and after the Permian-Triassic extinction. They then analyzed the data to detect when these species first originated and how fast they were evolving using state-of-the-art analytical techniques such as Bayesian evolutionary analysis, which is also used to understand the evolution of viruses such as SARS-COVID 19. The researchers then combined the new dataset with global temperature data spanning several million years in the geological record to provide a broad overview of the animals’ major adaptive response towards climatic shifts.
“Our results reveal that periods of fast climatic shifts and global warming are associated with exceptionally high rates of anatomical change in most groups of reptiles as they adapted to new environmental conditions,” said Pierce, “and this process started long before the Permian-Triassic extinction, since at least 270 million years ago, indicating that the diversification of reptile body plans was not triggered by the P-T extinction event as previously thought, but in fact started tens of million years before that.”
“One reptile lineage, the lepidosaurs, which gave rise to the first lizards and tuataras, veered in the opposite direction of most reptile groups and underwent a phase of very slow rates of change to their overall anatomy,” said Simões, “essentially, their body plans were constrained by natural selection, instead of going rogue and radically changing like most other reptiles at the time.” The researchers suggest this is due to pre-adaptations on their body size to better cope with high temperatures.
“The physiology of organisms is really dependent on their body size,” said Simões, “small-bodied reptiles can better exchange heat with their surrounding environment. The first lizards and tuataras were much smaller than other groups of reptiles, not that different from their modern relatives, and so they were better adapted to cope with drastic temperature changes. The much larger ancestors of crocodiles, turtles, and dinosaurs could not lose heat as easily and had to quickly change their bodies in order to adapt to the new environmental conditions.”
Simões, Pierce, and collaborators also mapped out how body size changed across geographical regions during this timeframe. They revealed that climatic pressures on body size were so high there was a maximum body size for reptiles to survive in tropical regions during the lethally hot periods of this time.
“Large-sized reptiles basically took two routes to deal with these climate shifts,” said Pierce, “they either migrated closer to temperate regions or invaded the aquatic world where they didn’t need to worry about overheating because water can absorb heat and maintain its temperature much better than air.”
“This strong association between rising temperatures in the geological past and a biological response by dramatically different groups of reptiles suggests climate change was a key factor in explaining the origin and the explosion of new reptile body plans during the latest Permian and Triassic,” said Simões.
The researchers would like to thank the Museum of Comparative Zoology (MCZ), Harvard University, vertebrate paleontology staff and the curators across 50+ natural history collections worldwide for their help with specimen access. Funding was provided by: Alexander Agassiz Postdoctoral Fellowship, MCZ; National Sciences and Engineering Research Council of Canada (NSERC) postdoctoral fellowship; Grant KA 4133/1-1 from the Deutsche Forschungsgemeinschaft; NSERC Discovery Grant #23458 and NSERC Accelerator Grant; Faculty of Science, Chairs Research Allowance, University of Alberta; Lemann Brazil Research Fund; Funds made available through Harvard University.
Successive climate crises in the deep past drove the early evolution and radiation of reptiles, Science Advances (19-Aug-2022), DOI: 10.1126/sciadv.abq1898
Press release from Harvard University, Department of Organismic and Evolutionary Biology.
New research questions hypotheses about climate-controlled ecosystem change during the origin of dinosaurs in Argentina
A group of researchers from CONICET and the University of Utah demonstrated that during the time of the first dinosaurs, variations in the diversity and abundance of the plant and vertebrate animal species cannot be related to the climatic changes recorded throughout its deposition, in contrast with previous hypotheses.
Artist’s reconstruction of the Triassic ecosystem preserved in the Ischigualasto Formation. Animals include amphibians (bottom center-left underwater), rhynchosaurian reptiles (left mid-ground on riverbank), early crocodilian relatives (far left mid-ground and center far background), early mammal relatives (center mid-ground in river and along riverbank, and far right foreground), and early dinosaurs (far left foreground, center right foreground, and far right mid-ground). Credits: Jorge Gonzalez/Natural History Museum of Utah
In the new study, published in the open access journal Frontiers in Earth Science, the team of scientists investigated multiple independent lines of evidence (sedimentology, clay mineralogy, and geochemistry) to elucidate changes paleoclimatic conditions (such as mean annual precipitation and mean annual temperature) within the Ischigualasto Formation. These fossil-rich sedimentary rocks were deposited by rivers and streams between ~231 and 226 million years ago during the Late Triassic Period in what is now northwestern Argentina (La Rioja and San Juan provinces). In the middle of the formation, the researchers observed a clear change in conditions approximately from warmer, drier conditions to more temperate humid conditions, but no concurrent major changes could be identified in the fossil record.
An overview of extensive Ischigulasto Formation outcrops in the study area, located in La Rioja Province, northwestern Argentina. Credits: Randall Irmis/Natural History Museum of Utah
“We conclude that variations in the abundance and diversity of species, as recorded by their first and last appearances in the fossil record, are better explained by preservation and sampling biases biases than by changes in climate,” said Adriana Mancuso, lead author and CONICET independent researcher at the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales in Mendoza, Argentina.
“What we see is that how many specimens collected from each interval of the sequence, and the chemical & physical characteristics that allow greater or lesser preservation of the remains of animals and plants, were significant factors. These two factors, collection and preservation, have more influence on the increase or decrease of abundance and diversity than the climate changes recorded,” explained Mancuso.
However, although the evolution of the ecosystem does not generally show a biotic response associated with climate change, the research group did observe a relationship between climatic variations and two groups of reptiles, rhynchosaurs (herbivorous early archosauromorphs) and pseudosuchians (crocodilian-line archosaurs).
“We did find that the abundance of rhynchosaurs and extinction of a few pseudosuchian species appear to coincide with a climate shift,” said Randall Irmis, co-author from the U and the Natural History Museum of Utah.
New research questions hypotheses about climate-controlled ecosystem change during the origin of dinosaurs in Argentina: a team member exposes fresh rock to obtain a geologic sample for geochemical lab analysis to reconstruct the paleoclimate record of the Ischigualasto Formation. Credits: Adriana Mancuso
Beyond conclusions about this specific fossil and paleoclimate record from Argentina, the new research emphasizes the importance of an explicit framework for testing hypotheses about the link between climatic changes and the fossil record.
“In addition to the contribution on the relationship of biotic and climatic events in the Ischigualasto Formation, the work provides a methodological framework to test climate-biota associations, highlighting the data gaps that must be filled, and makes new testable predictions that can be tested in future studies,” concludes Mancuso.
Other authors include Tomás Pedernera and Cecilia Benavente of the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (CONICET), Leandro Gaetano from the Instituto de Estudios Andinos (CONICET) and Departamento de Ciencias Geológicas of the Universidad de Buenos Aires, and Benjamin Breeden of the University of Utah.
Bibliographical information:
Paleoenvironmental and biotic changes in the Late Triassic of Argentina: testing hypotheses of abiotic forcing at the basin scale, Frontiers in Earth Science (13-Jun-2022), DOI: 10.3389/feart.2022.883788
