The discovery of the fossilized remains of three different long extinct squirrel-like creatures in northeastern China that are roughly 160 myo (Middle Jurassic) is providing evidence that mammals arose millions of years earlier than some scientists have thought.
The remains were unearthed from a rock outcropping in a cornfield that is part of the Tiaojishan Formation by private collectors and amateur paleontologists over the past few years at the Daxishan site of Linglongta, Liaoning province of China.
The discovery consists of six extremely well-preserved nearly complete fossil specimens, which included skulls, skeletons and teeth, that are from three different extinct species belonging to a poorly understood group of Mesozoic animals called Haramiyida, or haramiyids. The remains were so well preserved that not only were the remains of the bones and teeth fossilized but so were some of the soft parts including fur and the animal's guts.
The researchers say that these remains demonstrate that they clearly belong in the mammal family tree as well as show that different sort of body plans were emerging rather early on in contrast to the traditional view that all early mammals were shrew-like creatures.
Haramiyids appear to have originated during the Late Triassic and went extinct during the Late Jurassic. Their remains have predominantly been found in Europe but more recent discoveries place them in Greenland, Africa, Mongolia and China as well.
"For decades, scientists have been debating whether the extinct group, called Haramiyida, belongs within or outside of Mammalia," explained one of the authors of the paper describing the creatures, Jin Meng, a curator in the American Museum of Natural History's Division of Paleontology in New York.
Meng continued, "Previously, everything we knew about these animals was based on fragmented jaws and isolated teeth. But the new specimens we discovered are extremely well preserved. And based on these fossils, we now have a good idea of what these animals really looked like, which confirms that they are, indeed, mammals."
Prior to the discovery debate centered on whether haramiyids were a basal group of true mammals or mammaliaformes, their closest relatives. While known about for well over a century, until now, as Meng indicated, their remains were essentially limited to jaw and tooth fragments which wasn't enough to make any sort of conclusive identification.
But now the researchers, led by Shundong Bi of the Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences in Beijing and the Biology Department of Indiana University of Pennsylvania (IUP)[1], have enough material to make a much more certain identification.
Key among the finds are the skulls which reveal clear evidence of mammalian middle ear -- regarded as the defining characteristic for determining what is and what isn't a mammal. The middle ears of mammals are distinctive in that they have three bones, the malleus, incus, and stapes (also known respectively as the hammer, anvil and stirrup).
The new findings also presents evidence for their closest known relatives being rodent-like creatures known as multituberculates which existed for some 120 million years -- from 35 to 153 mya (Late Jurassic-Oligocene). Neither of these groups having any living descendants meaning that while these new discoveries may resemble modern squirrels they aren't related to them but are the result of convergent evolution.
What makes this discovery important is that by placing them firmly in the mammal group this indicates that the earliest mammals first arose some 208 mya during the Triassic.
While the majority of earlier studies had concluded that true mammals had originated somewhere between 161 and 176 mya (Middle Jurassic) several other studies along with DNA research suggested the earlier date was more likely.
When alive these animals weighed between 28 to 300 grams (1-11 oz.) or, as Meng put it, from "a house mouse to a small squirrel" in size.
The largest of the three previously unknown species is named Shenshou lui, with the genus name "shenshou" coming from combining the Mandarin Chinese words for "deity" (shen) with "animal" (shou) making "divine beast." The species name, "lui," refers to the scientist who collected the holotype specimen, Lu Jianhua. Aside from being the largest of the three it had more prominent incisors than the others.
The smallest of the group, weighing approximately 40 grams (1.4 oz.) was named Xianshou songae with the genus name essentially meaning "celestial beast" while the second or species name was named in honor of Rufeng Song, the collector of the specimen.
The last, which weighed approximately 83 grams (just under 3 oz.) was extracted from a chunk of siltstone and named Xianshou linglong with the species name being derived both from linglong, the Chinese word for "exquisite" as well as a reference to the town of Linglongta, where the specimen came from. Its sharper cusps and ridges of its upper molars allow it to be distinguished from Shenshou and Xianshou songae.
All three possessed slender light builds, long (probably prehensile) tails, limbs and elongated feet that indicate they were arboreal (tree dwellers). Meng said that "they were good climbers and probably spent more time than squirrels in trees." He pointed out that "their hands and feet were adapted for holding branches, but not good for running on the ground."
Their teeth, while differing from one another, were alike in that they had many cusps, or raised points, on the crowns. It is commonly thought that mammals evolved from a common ancestor that had three cusps but these newly discovered species possessed two parallel rows of cusps on each molar -- with up to seven cusps on each side. The researchers said that they don't know how this complex tooth pattern evolved in relation to those of other mammals.
Be that as it may, the teeth strongly indicate that these creatures omnivorous, eating insects, nuts and fruits -- much like other members of their clade (Euharamiyida) are thought to have done.
Further Reading:
Three new Jurassic euharamiyidan species reinforce early divergence of mammals Abstract
See What Our Earliest Mammal Ancestors Looked Like
Ancient Squirrel-Like Creatures Push Back Mammal Evolution
Chisel-Toothed Beasts Push Back Origin of Mammals
Mammals Originated Much Earlier than Previously Assumed
Squirrel-like Jurassic critters shed light on mammal origins
Squirrel-like critters lived alongside Jurassic age dinosaurs, study says
When did mammals evolve? Fossils of an extinct squirrel-like animal reveal the answer
Fossils of New Squirrel-like Species Support Earlier Origin of Mammals
1 The confusing sounding name comes from the fact that the university is located in Indiana County, Pennsylvania.
The remains were unearthed from a rock outcropping in a cornfield that is part of the Tiaojishan Formation by private collectors and amateur paleontologists over the past few years at the Daxishan site of Linglongta, Liaoning province of China.
The discovery consists of six extremely well-preserved nearly complete fossil specimens, which included skulls, skeletons and teeth, that are from three different extinct species belonging to a poorly understood group of Mesozoic animals called Haramiyida, or haramiyids. The remains were so well preserved that not only were the remains of the bones and teeth fossilized but so were some of the soft parts including fur and the animal's guts.
The researchers say that these remains demonstrate that they clearly belong in the mammal family tree as well as show that different sort of body plans were emerging rather early on in contrast to the traditional view that all early mammals were shrew-like creatures.
Haramiyids appear to have originated during the Late Triassic and went extinct during the Late Jurassic. Their remains have predominantly been found in Europe but more recent discoveries place them in Greenland, Africa, Mongolia and China as well.
"For decades, scientists have been debating whether the extinct group, called Haramiyida, belongs within or outside of Mammalia," explained one of the authors of the paper describing the creatures, Jin Meng, a curator in the American Museum of Natural History's Division of Paleontology in New York.
Meng continued, "Previously, everything we knew about these animals was based on fragmented jaws and isolated teeth. But the new specimens we discovered are extremely well preserved. And based on these fossils, we now have a good idea of what these animals really looked like, which confirms that they are, indeed, mammals."
Prior to the discovery debate centered on whether haramiyids were a basal group of true mammals or mammaliaformes, their closest relatives. While known about for well over a century, until now, as Meng indicated, their remains were essentially limited to jaw and tooth fragments which wasn't enough to make any sort of conclusive identification.
But now the researchers, led by Shundong Bi of the Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences in Beijing and the Biology Department of Indiana University of Pennsylvania (IUP)[1], have enough material to make a much more certain identification.
Key among the finds are the skulls which reveal clear evidence of mammalian middle ear -- regarded as the defining characteristic for determining what is and what isn't a mammal. The middle ears of mammals are distinctive in that they have three bones, the malleus, incus, and stapes (also known respectively as the hammer, anvil and stirrup).
The new findings also presents evidence for their closest known relatives being rodent-like creatures known as multituberculates which existed for some 120 million years -- from 35 to 153 mya (Late Jurassic-Oligocene). Neither of these groups having any living descendants meaning that while these new discoveries may resemble modern squirrels they aren't related to them but are the result of convergent evolution.
What makes this discovery important is that by placing them firmly in the mammal group this indicates that the earliest mammals first arose some 208 mya during the Triassic.
While the majority of earlier studies had concluded that true mammals had originated somewhere between 161 and 176 mya (Middle Jurassic) several other studies along with DNA research suggested the earlier date was more likely.
When alive these animals weighed between 28 to 300 grams (1-11 oz.) or, as Meng put it, from "a house mouse to a small squirrel" in size.
The largest of the three previously unknown species is named Shenshou lui, with the genus name "shenshou" coming from combining the Mandarin Chinese words for "deity" (shen) with "animal" (shou) making "divine beast." The species name, "lui," refers to the scientist who collected the holotype specimen, Lu Jianhua. Aside from being the largest of the three it had more prominent incisors than the others.
The smallest of the group, weighing approximately 40 grams (1.4 oz.) was named Xianshou songae with the genus name essentially meaning "celestial beast" while the second or species name was named in honor of Rufeng Song, the collector of the specimen.
The last, which weighed approximately 83 grams (just under 3 oz.) was extracted from a chunk of siltstone and named Xianshou linglong with the species name being derived both from linglong, the Chinese word for "exquisite" as well as a reference to the town of Linglongta, where the specimen came from. Its sharper cusps and ridges of its upper molars allow it to be distinguished from Shenshou and Xianshou songae.
All three possessed slender light builds, long (probably prehensile) tails, limbs and elongated feet that indicate they were arboreal (tree dwellers). Meng said that "they were good climbers and probably spent more time than squirrels in trees." He pointed out that "their hands and feet were adapted for holding branches, but not good for running on the ground."
Their teeth, while differing from one another, were alike in that they had many cusps, or raised points, on the crowns. It is commonly thought that mammals evolved from a common ancestor that had three cusps but these newly discovered species possessed two parallel rows of cusps on each molar -- with up to seven cusps on each side. The researchers said that they don't know how this complex tooth pattern evolved in relation to those of other mammals.
Be that as it may, the teeth strongly indicate that these creatures omnivorous, eating insects, nuts and fruits -- much like other members of their clade (Euharamiyida) are thought to have done.
Jurassic_mammals1.jpg
Left: holotype specimen of Senshou lui Top right: specimen of Xianshou linglong
Bottom right: specimen of [/i]Xianshou songae[/i]
Jurassic_mammals3.jpg
Paratypes 1 and 2 of Shenshou lui
Jurassic_mammals2.jpg
Artist's reconstruction of Xianshou songae
Left: holotype specimen of Senshou lui Top right: specimen of Xianshou linglong
Bottom right: specimen of [/i]Xianshou songae[/i]
Jurassic_mammals3.jpg
Paratypes 1 and 2 of Shenshou lui
Jurassic_mammals2.jpg
Artist's reconstruction of Xianshou songae
Further Reading:
Three new Jurassic euharamiyidan species reinforce early divergence of mammals Abstract
See What Our Earliest Mammal Ancestors Looked Like
Ancient Squirrel-Like Creatures Push Back Mammal Evolution
Chisel-Toothed Beasts Push Back Origin of Mammals
Mammals Originated Much Earlier than Previously Assumed
Squirrel-like Jurassic critters shed light on mammal origins
Squirrel-like critters lived alongside Jurassic age dinosaurs, study says
When did mammals evolve? Fossils of an extinct squirrel-like animal reveal the answer
Fossils of New Squirrel-like Species Support Earlier Origin of Mammals
1 The confusing sounding name comes from the fact that the university is located in Indiana County, Pennsylvania.
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