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Saturday, November 26, 2016

Dryosaurus skeleton

Dryosaurus (meaning ‘tree lizard’) the name reflects the forested habitat, not a vague oak-leaf shape of its cheek teeth as is sometimes assumed) is a genus of an ornithopod dinosaur that lived in the Late Jurassic period. It was an iguanodont (formerly classified as a hypsilophodont). Fossils have been found in the western United States (and supposedly the Marnes de Bleville locality in Europe), and were first discovered in the late 19th century. Valdosaurus canaliculatus and Dysalotosaurus lettowvorbecki were both formerly considered to represent species of Dryosaurus.

Dryosaurus was an herbivorous ornithischian dinosaur, belonging to the Suborder Ornithopoda, and Family Hypsilophodontidae (gazelle-like, herbivorous, herding dinosaurs which included Hypsilophodon, Othnielia, Tenotosaurus, Parksosaurus, and Thescelosaurus). The type species is D. altus.

A Dryosaurus altus being pursued by a Ceratosaurus nasicornis, a new exhibit in Carnegie Museum’s refurbished Dinosaur Hall. Originally uploaded to Flickr as Dryosaurus and Ceratosaurus by Kordite.

Dryosaurus lived in lush forest during the late Jurassic Period, about 156 to 145 million years ago.

Its contemporaries were ApatosaurusDiplodocusBrachiosaurus, Coelurus, AllosaurusStegosaurus, and Ceratosaurus.
Dryosaurus laid eggs and may have cared for their young for at least a little while. This is evidenced by eggs found carefully placed in other Hypsilophodontid nests. It may have travelled in herds.
Dryosaurus was an ornithopod, whose intelligence (as measured by its relative brain to body weight, or EQ) was midway among the dinosaurs.
This horny beaked dinosaur was an herbivore (it ate only plants) that lived in lush forest habitats. Dryosaurus may have stored food in its cheeks.
Dryosaurus was named by Othniel C. Marsh in 1894. Fossils have been found in western North America and Africa.


Saturday, November 26, 2016

Artistic restoration of D. albertensis

Dromaeosaurus (“running lizard”) was a genus of theropod dinosaur which lived during the Late Cretaceous period (middle late Campanian), sometime between 76.5 and 74.8 million years ago, in the western United States and Alberta, Canada. The type species is Dromaeosaurus albertensis, which was described by William Diller Matthew and Barnum Brown in 1922.

Size (4) compared with other dromaeosaurs

One feature common to all dromaeosaurids was the extreme stiffness of their tails. The tails of most theropods were stiffened, at least for part of their length. This relative inflexibility was caused by the lengthening of the attachment processes (bony rods known as zygapophyses) on the vertebrae. In the dromaeosaurids, however, these rods became greatly elongated, each of them extending over several vertebrae and effectively bracing them together. As a result of this, the thin tails of the dromaeosaurids became completely inflexible. The stiff tail would have helped Dromaeosaurus in pursuing prey by acting as an effective counterbalance behind the hip.

Dromaeosaurus was a small carnivore, about 2 m (6.6 ft) in length and 15 kg (33 lb) in weight. Its mouth was full of sharp teeth, and it had a sharply curved “sickle claw” on each foot. It lived during the Campanian stage of the Late Cretaceous, however, some fragmentary remains such as teeth which may belong to this genus have been found from the late Maastrichtian age Lance and Hell Creek Formations, dating to 66 million years ago. Teeth have also been found in the Aguja Formation

D. albertensis skull cast

The exact relationships of Dromaeosaurus are somewhat unclear. Although its rugged build gives it a primitive appearance, it was actually a very specialized animal. In analyses of the present clade Dromaeosaurinae species as UtahraptorAchillobator and Yurgovuchia are often recovered. As of recently, the genus Dakotaraptor has been classified as the sister taxon to Dromaeosaurus.


Saturday, November 26, 2016

Diplodocus is a genus of diplodocid sauropod dinosaurs whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a neo-Latin term derived from Greek διπλός (diplos) “double” and δοκός (dokos) “beam”, in reference to its double-beamed chevron bones located in the underside of the tail. Chevron bones of this particular form were initially believed to be unique to Diplodocus; since then they have been discovered in other members of the diplodocid family as well as in nondiplodocid sauropods, such as Mamenchisaurus. It is now common scientific opinion that Seismosaurus hallorum is a species of Diplodocus.

Diplodocus by SameerPrehistorica

It is mainly thanks to the efforts of one man, the American industrialist Andrew Carnegie, that Diplodocus is now so well known throughout the world. Carnegie, a wealthy man, had a particular interest in dinosaurs, and in the late 1800s and early 1900s, he contributed liberally to the cause of paleontology. he financed many expeditions and excavations and filled his own museum – the Carnegie Museum of Natural History in Pittsburgh – with fossil skeletons from all over the United States. When his paleontologists discovered a complete skeleton of Diplodocus, he was so impressed with its size that he commissioned the making of 11 copies of the entire skeleton, which he gave to major museums around the world.

Like other sauropods, the manus (front “feet”) of Diplodocus were highly modified, with the finger and hand bones arranged into a vertical column, horseshoe-shaped in cross section. Diplodocus lacked claws on all but one digit of the front limb, and this claw was unusually large relative to other sauropods, flattened from side to side, and detached from the bones of the hand. The function of this unusually specialized claw is unknown.

Diplodocus’s curious name – meaning “double beam” – is derived from the bones on the underside of its tail, known as chevrons. In most other dinosaurs, these are simple V-shaped elements, but in Diplodocus they are like side-on Ts, projecting both to the front and back. Scientists used to think that, like other sauropods, Diplodocus was a lumbering beast that dragged its tail along the ground. The 1980s, however, brought a renaissance in our understanding of dinosaurs. It dawned on people that, although there was ample fossil evidence of sauropods walking across ancient landscapes, there were never any impressions of tails on the ground. The only conclusion was that the tail must have been held high. However, how an animal of Diplodocus’s weight and dimensions managed this remained a mystery.

Diplodocus is both the type genus of, and gives its name to, the Diplodocidae, the family to which it belongs. Members of this family, while still massive, are of a markedly more slender build than other sauropods, such as the titanosaurs and brachiosaurs. All are characterised by long necks and tails and a horizontal posture, with fore limbs shorter than hind limbs. Diplodocids flourished in the Late Jurassic of North America and possibly Africa.


Saturday, November 26, 2016

Restoration of Dilophosaurus in a bird-like resting pose, based on a track at the Dinosaur Discovery Site

Dilophosaurus is a genus of theropod dinosaur. It contains a single known species, Dilophosaurus wetherilli, known from fossil remains found in the Kayenta Formation of Arizona. This rock formation has been dated to the early Jurassic Period (Sinemurian age), about 193 million years ago. Dilophosaurus was among the largest carnivores of its time (about 7 meters long) and had a pair of rounded crests on its skull.

Fossils of Dilophosaurus, one of the earliest of the large theropods, were first found in 1942, in early Jurassic sediments in Arizona. More recently, remains of this animal have also been discovered in China. It was a close relative of Coelophysis, and, like this earlier dinosaur, had four fingers on each hand. The fourth finger, however, was very small and probably had no function.

Size of two specimens compared to a human, with holotype in green

As a more primitive predatory dinosaur, Dilophosaurus didn’t have forward facing eyes to give it stereo vision. It may have used scent as an integral part of its hunting technique. It had long and slender, rear-curving teeth in long jaws and strong front arms which would have been effective in grabbing prey. It was fast – probably with a top speed of about 30-mph. It also had a long tail that could have been used as a whip in a fight. Footprints attributed to Dilophosaurus appear in groups, so it may have hunted in small packs. It shares the same overall body configuration as Coelophysis even though Dilophosaurus is currently classified as a member of a different group of theropods rather than Coelophysis and its relatives.

Dilophosaurus had long, sharp, pointed teeth. However, it probably did not use its teeth to kill its victims; some scientists have suggested that its jaws were not strong enough to enable it to have fed on live prey. It probably used its clawed hands and feet to kill its victims and then fed on their carcasses. It may also have scavenged animals killed by other predators.

In the movie Jurassic Park, Dilophosaurus is depicted with an extendable frill – rather like that of an Australian frillnecked lizard – and also as spitting poison. However, imagination has ruled the day here for there is no evidence for either of these features. The idea that it spat venom may have resulted from suggestions that, as it seemed unable to attack live animals with its teeth and jaws, it killed them with poison. However, as no living crocodylian or bird is known to use venom in this way, there can be no reason to suppose that Dilophosaurus did.

The first three dimensional, standing skeleton of Dilophosaurus, Museum of Northern Arizona[19]

The most distinctive characteristic of Dilophosaurus is the pair of rounded crests on its skull, made up of extensions of the nasal and lacrimal bones. These are considered to be too delicate for anything but display purposes. Dodson (1997) noted that cranial crests first appeared in Dilophosaurus and were later retained, in one form or another, by other theropods.

The function of the crests on the skull of Dilophosaurus have been the subject of speculation among scientists ever since they were discovered. Traditionally, these bizarre cranial structures (and similar structures and post-cranial armor in other dinosaurs) were thought to be variously for attracting mates, intimidating/fighting rivals in the group, and intimidating potential predators of other species. However, Padian, Horner and Dhaliwal (2004) argued that phylogenetic, histological, and functional evidence indicates that these bizarre structures were most likely used for intra-species recognition.


Saturday, November 26, 2016

Deinonychus antirrhopus by Carlo-Arellano

Deinonychus  is a genus of carnivorous dromaeosaurid coelurosaurian dinosaurs, with one described species, Deinonychus antirrhopus. This species, which could grow up to 3.4 metres (11 ft) long, lived during the early Cretaceous Period, about 115–108 million years ago (from the mid-Aptian to early Albian stages). Fossils have been recovered from the U.S. states of Montana, Utah, Wyoming, and Oklahoma, in rocks of the Cloverly Formation, Cedar Mountain Formation and Antlers Formation, though teeth that may belong to Deinonychus have been found much farther east in Maryland.

Size compared with a human

Paleontologist John Ostrom’s study of Deinonychus in the late 1960s revolutionized the way scientists thought about dinosaurs, leading to the “dinosaur renaissance” and igniting the debate on whether dinosaurs were warm-blooded or cold blooded. Before this, the popular conception of dinosaurs had been one of plodding, reptilian giants. Ostrom noted the small body, sleek, horizontal posture, ratite-like spine, and especially the enlarged raptorial claws on the feet, which suggested an active, agile predator.

“Terrible claw” refers to the unusually large, sickle-shaped talon on the second toe of each hind foot. The fossil YPM 5205 preserves a large, strongly curved ungual. In life, archosaurs have a horny sheath over this bone, which extends the length. Ostrom looked at crocodile and bird claws and reconstructed the claw for YPM 5205 as over 120 millimetres (4.7 in) long. The species name antirrhopus means “counter balance”, which refers to Ostrom’s idea about the function of the tail. As in other dromaeosaurids, the tail vertebrae have a series of ossified tendons and super-elongated bone processes. These features seemed to make the tail into a stiff counterbalance, but a fossil of the very closely related Velociraptor mongoliensis (IGM100/986) has an articulated tail skeleton that is curved laterally in a long S-shape. This suggests that, in life, the tail could bend to the sides with a high degree of flexibility. In both the Cloverly and Antlers formations, Deinonychus remains have been found closely associated with those of the ornithopod Tenontosaurus. Teeth discovered associated with Tenontosaurusspecimens imply they were hunted, or at least scavenged upon, by Deinonychus.

Skeleton of the dromaeosaurid dinosaur Deinonychus at Field Museum of Natural History. At the bottom is the skeleton of Buitreraptor.

Fossilized remains of Deinonychus have been recovered from the Cloverly Formation of Montana and Wyoming and in the roughly contemporary Antlers Formation of Oklahoma, in North America. The Cloverly formation has been dated to the late Aptian through early Albian stages of the early Cretaceous, about 115 to 108 Ma. Additionally, teeth found in the Arundel Clay Facies (mid-Aptian), of the Potomac Formation on the Atlantic Coastal Plain of Maryland may be assigned to the genus.

The first remains were uncovered in 1931 in southern Montana near the town of Billings. The team leader, paleontologist Barnum Brown, was primarily concerned with excavating and preparing the remains of the ornithopod dinosaur Tenontosaurus, but in his field report from the dig site to the American Museum of Natural History, he reported the discovery of a small carnivorous dinosaur close to a Tenontosaurus skeleton, “but encased in lime difficult to prepare.” He informally called the animal “Daptosaurus agilis” and made preparations for describing it and having the skeleton, specimen AMNH 3015, put on display, but never finished this work. Brown brought back from the Cloverly Formation the skeleton of a smaller theropod with seemingly oversized teeth that he informally named “Megadontosaurus”. John Ostrom, reviewing this material decades later, realized that the teeth came from Deinonychus, but the skeleton came from a completely different animal. He named this skeleton Microvenator.


Saturday, November 26, 2016

Corythosaurus in color by Ahrkeath on DeviantArt

Corythosaurus is a genus of hadrosaurid “duck-billed” dinosaur from the Upper Cretaceous Period, about 77–75.7 million years ago. It lived in what is now North America. Its name means “helmet lizard”, derived from Greek κόρυς. It was named and described in 1914 by Barnum Brown. Corythosaurus is now thought to be a lambeosaurine, related to NipponosaurusVelafronsHypacrosaurus, and OlorotitanCorythosaurus has an estimated length of 9 metres (30 ft), and has a skull, including the crest, that is 70.8 centimetres (27.9 in) tall.

Corythosaurus is known from many complete specimens, including the nearly complete holotype found by Brown in 1911. The holotype skeleton is only missing the last section of the tail, and part of the forelimbs, but was preserved with impressions of polygonal scales. Corythosaurus is known from many skulls with tall crests. The crests resemble the crests of the cassowary and a Corinthian helmet. The most likely function of the crest is thought to be vocalization. As in a trombone, sound waves would travel through many chambers in the crest, and then get amplified when Corythosaurus exhaled. A Corythosaurus specimen has been preserved with its last meal in its chest cavity. Inside the cavity were remains of conifer needles, seeds, twigs, and fruits: Corythosaurus probably fed on all of these.

The two species of Corythosaurus are both present in slightly different levels of the Dinosaur Park Formation. Both still co-existed with theropods and other ornithischians, like DaspletosaurusBrachylophosaurusParasaurolophusScolosaurus, and Chasmosaurus.

Excavation of the holotype specimen of Corythosaurus casuarius by the Red Deer River.

Benson et al. (2012) estimated that Corythosaurus has an average length of 9 metres (30 ft). Richard Swann Lull’s earlier length estimate, published in 1942, found a slightly longer total length of 9.4 m (31 ft), a size similar to Lambeosaurus lambei, another Canadian lambeosaurine. In 1962, Edwin H. Colbert used models of specific dinosaurs, including Corythosaurus, to estimate their weight. The Corythosaurus model used, was modelled by Vincent Fusco after a mounted skeleton, and supervised by Barnum Brown. After testing, it was concluded that the average weight of Corythosaurus was 3.82 tonnes (3.76 long tons; 4.21 short tons). More recent size estimates of Corythosaurus, published in 2001, find the genus to be among the largest hadrosaurids, only smaller than Shantungosaurus and Parasaurolophus. The total length of Corythosaurusspecimen AMNH 5240 was found to be 8.1 m (27 ft), with a weight of 3.0785 tonnes (3.0299 long tons; 3.3935 short tons).

Size of the two species compared to a human.

Proportionally, the skull is much shorter and smaller than that of Edmontosaurus (formerly Trachodon), Kritosaurus, or Saurolophus, but when including its crest, its superficial area is almost as large.


Saturday, November 26, 2016

Compsognathus longipes, a coelurosaur from the Late Jurassic of Europe, pencil drawing by Nobu Tamura

Compsognathus is a genus of small, bipedal, carnivorous theropod dinosaurs. Members of its single species Compsognathus longipes could grow to the size of a turkey. They lived about 150 million years ago, the Tithonian age of the late Jurassic period, in what is now Europe. Paleontologists have found two well-preserved fossils, one in Germany in the 1850s and the second in France more than a century later. Today, C. longipes is the only recognized species, although the larger specimen discovered in France in the 1970s was once thought to belong to a separate species and named C. corallestris.

Size comparison of the French (orange) and German (green) specimens, with a human

Many presentations still describe Compsognathus as “chicken-sized” dinosaurs because of the small size of the German specimen, which is now believed to be a juvenile. Compsognathus longipes is one of the few dinosaur species for which diet is known with certainty: the remains of small, agile lizards are preserved in the bellies of both specimens. Teeth discovered in Portugal may be further fossil remains of the genus.

Although not recognized as such at the time of its discovery, Compsognathus is the first theropod dinosaur known from a reasonably complete fossil skeleton. Until the 1990s, it was the smallest known non-avialan dinosaur; earlier it had been incorrectly thought to be the closest relative of Archaeopteryx.

Compsognathus also holds the distinction of being the first dinosaur genus to be portrayed with feathers, by Thomas Henry Huxley in 1876.

Reconstruction of a skeleton, Museum of Ancient Life – Thanksgiving Point

The genus Compsognathus gives its name to the family Compsognathidae, a group composed mostly of small dinosaurs from the late Jurassic and early Cretaceous periods of China, Europe and South America. For many years it was the only member known; however in recent decades paleontologists have discovered several related genera. The clade includes AristosuchusHuaxiagnathusMirischiaSinosauropteryx, and perhaps Juravenator and Scipionyx. At one time, Mononykus was proposed as a member of the family, but this was rejected by Chen and coauthors in a 1998 paper; they considered the similarities between Mononykus and the compsognathids to be an example of convergent evolution. The position of Compsognathus and its relatives within the coelurosaur group is uncertain. Some, such as theropod expert Thomas Holtz Jr. and co-authors Ralph Molnar and Phil Currie in the landmark 2004 text Dinosauria, hold the family as the most basal of the coelurosaurs, while others as part of the Maniraptora.


Saturday, November 26, 2016

Coelophysis by Typothorax on DeviantArt

Coelophysis is an genus of coelophysid theropod dinosaur that lived approximately 203 to 196 million years ago during the latter part of the Triassic Period in what is now the southwestern United States. It was a small, slenderly-built, ground-dwelling, bipedal carnivore, that could grow up to 3 m (9.8 ft) long. Coelophysis is one of the earliest known dinosaur genera. Scattered material representing similar animals has been found worldwide in some Late Triassic and Early Jurassic formations. The type species C. bauri, originally given to the genus Coelurus by Edward Drinker Cope in 1887, was described by the latter in 1889. The names Longosaurus and Rioarribasaurus are synonymous with Coelophysis. Another dinosaur genus, Megapnosaurus, has also been considered to be a synonym. This primitive theropod is notable for being one of the most specimen-rich dinosaur genera.

Size of C. bauri compared to a human

Coelophysis is known from a number of complete fossil skeletons of the species C. bauri, which was a lightly built dinosaur which measured up to 3 metres (9.8 ft) in length and which was more than a meter tall at the hips. Paul (1988) estimated the weight of the gracile form at 15 kg (33 lb), and the weight of the robust form at 20 kg (44 lb). Coelophysis was a bipedal, carnivorous, theropod dinosaur that was a fast and agile runner. Despite being an early dinosaur, the evolution of the theropod body form had already advanced greatly from creatures like Herrerasaurus and Eoraptor. The torso of Coelophysisconforms to the basic theropod body shape, but the pectoral girdle displays some interesting special characteristics: C. baurihad a furcula (wishbone), the earliest known example in a dinosaur. Coelophysis also preserves the ancestral condition of possessing four digits on the hand (manus). It had only three functional digits, the fourth embedded in the flesh of the hand.

Coelophysis is a distinct taxonomic unit (genus), composed of two species; C. bauri and C. rhodesiensis (the latter formerly classified as the genus Megapnosaurus). Two additional originally described species, C. longicollis and C. willistoni, are now considered synonymous with C. bauriC. rhodesiensis is probably part of this generic complex, and is known from the Jurassic of southern Africa. A third possible species is Coelophysis kayentakatae, previously referred to the genus Megapnosaurus. There is not a clear consensus at this point.

Evolution of the Horse

Thursday, November 24, 2016

The Evolution of Horses, from Eohippus to the American Zebra

The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transforming the small, dog-sized, forest-dwelling Eohippus into the modern horse. Paleozoologists have been able to piece together a more complete outline of the evolutionary lineage of the modern horse than of any other animal.

The horse belongs to the order Perissodactyla (odd-toed ungulates), the members of which all share hooved feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure. This means that horses share a common ancestry with tapirs and rhinoceroses. The perissodactyls arose in the late Paleocene, less than 10 million years after the Cretaceous–Paleogene extinction event. This group of animals appears to have been originally specialized for life in tropical forests, but whereas tapirs and, to some extent, rhinoceroses, retained their jungle specializations, modern horses are adapted to life on drier land, in the much harsher climatic conditions of the steppes. Other species of Equus are adapted to a variety of intermediate conditions.

This image shows a representative sequence, but should not be construed to represent a “straight-line” evolution of the horse. Reconstruction, left forefoot skeleton (third digit emphasized yellow) and longitudinal section of molars of selected prehistoric horses  The Earliest Horses – Hyracotherium and Mesohippus

The Earliest Horses – Hyracotherium and Mesohippus

Until an even earlier candidate is found, paleontologists agree that the ultimate ancestor of all modern horses was Eohippus, the “dawn horse,” a tiny (no more than 50 pounds), deer-like herbivore with four toes on its front feet and three toes on its back feet. (Eohippus was for many years known as Hyracotherium, a subtle paleontological difference of which the less you know, the better!) The giveaway to Eohippus’ status is its posture: this perissodactyl put most of its weight on a single toe of each foot, anticipating later equine developments. Eohippus was closely related to another early ungulate, Palaeotherium, which occupied a distant side branch of the horse evolutionary tree.

Toward True Horses – Epihippus, Parahippus and Merychippus

During the Miocene epoch, North America saw the evolution of “intermediate” horses, bigger than Hyracotherium and its ilk but smaller than the equines that followed. One of the most important of these was Epihippus (“marginal horse”), which was slightly heavier (possibly weighing a few hundred pounds) and equipped with more robust grinding teeth than its ancestors. As you might have guessed, Epihippus also continued the trend toward enlarged middle toes, and it seems to have been the first prehistoric horse to spend more time feeding in meadows than in forests.

Following Epihippus were two more “hippi,” Parahippus and Merychippus. Parahippus (“almost horse”) can be considered a next-model Miohippus, slightly bigger than its ancestor and (like Epihippus) sporting long legs, robust teeth, and enlarged middle toes. Merychippus (“ruminant horse”) was the largest of all these intermediate equines, about the size of a modern horse (1,000 pounds) and blessed with an especially fast gait.

This image shows a representative sequence, but should not be construed to represent a “straight-line” evolution of the horse. Reconstruction, left forefoot skeleton (third digit emphasized yellow) and longitudinal section of molars of selected prehistoric horses. Author: H. Zell

Next Step, Equus – Hipparion and Hippidion

Following the success of intermediate horses like Parahippus and Merychippus, the stage was set for the emergence of bigger, more robust, more “horsey” horses. Chief among these were the similarly named Hipparion (“like a horse”) and Hippidion (“like a pony”). Hipparion was the most successful horse of its day, radiating out from its North American habitat (by way of the Siberian land bridge) to Africa and Eurasia. Hipparion was about the size of a modern horse; only a trained eye would have noticed the two vestigial toes surrounding its single hooves.

Lesser known than Hipparion, but perhaps more interesting, was Hippidion, one of the few prehistoric horses to have colonized South America (where it persisted until historical times). The donkey-sized Hippidion was distinguished by its prominent nasal bones, a clue that it had a highly developed sense of smell. Hippidion may well turn out to have been a species of Equus, making it more closely related to modern horses than Hipparion was.

Some of the most notable prehistoric horses

American Zebra Also known as the Hagerman horse.

Anchitherium A long-lived “side branch” on the equine tree of life.

Dinohippus This prehistoric horse wasn’t quite as fearsome as its name.

Epihippus This tiny, prehistoric horse lived about 30 million years ago.

Eurohippus Scientists have discovered a pregnant specimen of this ancient horse.

Hipparion One of the most successful horses of the Miocene epoch.

Hippidion This donkey-sized horse had a prominent snout.

Hypohippus This Miocene horse had unusually short legs.

Hyracotherium The horse formerly known as Eohippus.

Merychippus An important intermediate step in equine evolution.

Mesohippus This “middle horse” was about the size of a deer.

Miohippus This “Miocene horse” actually lived much earlier.

Orohippus This prehistoric horse was a close relative of Hyracotherium.

Palaeotherium This tapir-like beast was remotely related to modern horses.

Parahippus This “almost horse” had noticeably enlarged middle toes.

Pliohippus This prehistoric horse was built for speed.

Quagga This South African zebra went extinct in 1883.

Tarpan The immediate predecessor of the modern horse.

Did Sex Drive Mammal Evolution?

Sunday, November 20, 2016

How new species are created is at the core of the theory of evolution. Mammals may be a good example of how sex chromosome change drove major groups apart.

How new species are created is at the very core of the theory of evolution. The reigning theory is that physically separated populations of one species drift apart gradually.

But changes in chromosomes, particularly sex chromosomes, can interpose drastic barriers to reproduction. Mammals may be a good example. Comparisons of the sex chromosomes of the three major mammal groups show that there were two upheavals of sex chromosomes during mammal evolution.

The first corresponded to the divergence of monotreme mammals (platypus and echidna) from the rest, and the second to the divergence of marsupials from placental mammals (including humans).

In a paper published in BioEssays, I propose that drastic sex chromosome changes could have played a direct role in separating our lineage (placental mammals), first from the egg-laying monotremes, then from marsupials.

In humans and other placental mammals, such as mice, dogs and elephants, sex is determined by a pair of chromosomes. Females have two copies of the X while males have a single copy of the X and a small Y that contains the male-determining gene SRY.

Other vertebrate animals also have sex chromosomes, but they are different. Birds have an unrelated sex chromosome pair called ZW, and a different sex determining gene called DMRT1.

Snakes also have a ZW system, but again it is a different chromosome with different genes. Lizards and turtles, frogs and fish have all sorts of sex chromosomes that are different from the mammal system and from each other.
The rise and fall of sex chromosomes

Sex chromosomes are really weird because of the way they evolved. They start off as ordinary chromosomes, known as autosomes. A new sex gene arises on one member of the pair, defining either a male-determining Y as in humans or a female-determining W as in birds.

The acquisition of a sex factor on one member of the pair is the kiss of death for that chromosome, and it degrades quickly. This explains why only a few active genes remain on the human Y and the bird W.

When old sex chromosomes self-destruct, a new sex gene and sex chromosomes may take over. This is fraught with peril because the interaction of old and new systems of sex determination is likely to cause severe infertility in hybrids.

Rival sex genes may be at war with each other, causing intersexual development, or at least infertility. For instance, what will be the sex of a hybrid that has both a male-determining Y and a female-determining W?

Added to this are problems with gene dosage because the degenerate Y and the W have few genes. If an XY male mates with a ZW female, most of the progeny will be short of genes. There may also be problems with gene dosage because genes on the X and the Z are used to working harder to compensate for their single dosage.

Rearrangement of sex chromosomes with autosomes also causes severe infertility because half the reproductive cells of a hybrid will have too many, or too few, copies of the fused chromosome.

Such hybrid infertility poses a reproductive barrier between populations with the new and the old sex system. So could such barriers drive apart populations to form distinct species?

Reproductive barriers and new species

The idea that chromosome change could drive the formation of new species was popular 50 years ago.

But it was thoroughly dismissed by evolutionary geneticists in favour of the idea that speciation, the formation of new and distinct species, must occur in populations already separated by a physical barrier such as a river or mountains, or behaviour such as mating time, and occupied different environments.

Small mutations would accumulate slowly and the two populations would be selected for different traits. Eventually they would become so different that they could no longer mate with each other and would form two species. This allopatric speciation relied on external factors.

The alternate view, that sympatric speciation can happen within a population because of intrinsic genome changes, fell out of favour. Partly this was because it is hard to demonstrate speciation of populations sharing the same environment, the argument always being that the environment could be subtly different.

The other problem was imagining how a major chromosome change that occurred in one animal could spread to a whole population. Sex chromosome change is especially drastic because it directly affects reproduction. But our comparisons show that sex chromosomes have undergone dramatic changes throughout vertebrate evolution.

It is important to examine closely examples of evolutionary divergence that were accompanied by drastic sex chromosome change. Strangely, mammals may offer us a window into this evolutionary past. Their sex chromosomes are extremely stable, yet they have undergone rare dramatic changes, each of which lines up near when one lineage became two.
Sex chromosome change and mammal divergence

Placental mammals all share essentially the same XY. Marsupials, too, have XY chromosomes, but they are smaller; genes on the top bit of human X are on autosomes in marsupials.

Comparisons outside mammals shows that this bit was fused to ancient marsupial-like X and Y chromosomes before the different lines of placental mammals separated 105-million years ago.

Monotreme mammals (platypus and echidna) have bizarre multiple X and Y chromosomes. Surprisingly, comparing the genes they bear showed that they are completely unrelated to the XY of humans and marsupials. In fact, platypus sex chromosomes are related to bird sex chromosomes.

The human XY pair is represented by an ordinary chromosome in platypus. So our XY and SRY are quite young because they must have evolved after monotremes diverged from our lineage 190-million years ago.

Sex chromosome change has occurred very rarely in mammals, so it seems significant that each change corresponds to a major divergence. That’s why I propose that sex chromosome turnover separated monotremes from the rest of the mammals, and sex chromosome fusion occurred later to separate our lineage from marsupials.

Strengthening the argument that sex chromosome turnover begets speciation is evidence of a new round of sex chromosome change and speciation.

In Japan and eastern Europe, species in two rodent lineages have completely eliminated the Y chromosome and replaced SRYwith a different gene on a different chromosome. In each lineage the Y-less rodents have recently diverged into three species.

What does this mean for our own lineage? The primate Y seems to be more stable than the rodent Y. But if it continues to degrade at the same rate, it will disappear in about 4.6 million years.

Will it be replaced by some different gene and chromosome? And if so, will this unleash a new round of hominid speciation? We may have to wait another 4.6 million years to find out.

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