Late Jurassic–Present, 160–0 Ma
|Fossil specimen of a Microraptor gui|
Paraves is a branch-based clade defined to include all dinosaurs which are more closely related to birds than to oviraptorosaurs. Paravians comprises two major sub-groups: Avialae, including Jeholornis and modern birds, and the Deinonychosauria, which includes the dromaeosaurids and troodontids, which may or may not form a natural group.
Like other theropods, early paravians were bipedal, walking on their two hind legs. However, whereas most theropods walked with three toes contacting the ground, fossilized footprint tracks confirm that many basal paravians, including dromaeosaurids, troodontids, and some early avialans, held the second toe off the ground in a hyperextended position, with only the third and fourth toes bearing the weight of the animal. This is called functional didactyly. The enlarged second toe bore an unusually large, curved sickle-shaped claw (held off the ground or 'retracted' when walking). This claw was especially large and flattened from side to side in the large-bodied predatory eudromaeosaurs. In these early species, the first toe (hallux) was usually small and angled inward toward the center of the body, but only became fully reversed in more specialized members of the bird lineage. One species, Balaur bondoc, possessed a first toe which was highly modified in parallel with the second. Both the first and second toes on each foot of B. bondoc were held retracted and bore enlarged, sickle-shaped claws.
An increasingly asymmetric carpal joint, a trend that can be traced back to primitive coelurosaurs, allowed the forelimbs to elongate and an elaboration of their plumage, traits that made the evolution of wings possible.
The teeth of basal paravians were curved and serrated, but not blade-like except in some specialized species such as Dromaeosaurus albertensis. The serrations on the front edge of dromaeosaurid and troodontid teeth were very small and fine, while the back edge had serrations which were very large and hooked. Paravians generally have long, winged forelimbs, though these have become smaller in flightless birds and some extinct lineages such as the troodontids. The wings usually bore three large, flexible, clawed fingers in early forms. The fingers became fused and stiffened and the claws highly reduced or lost in some advanced lineages.
Most dromaeosaurids seem to have been predatory, though some smaller species (especially among the troodontids and avialans) are known to have been at least omnivorous; and it is possible that an omnivorous diet was the ancestral state for this group, with more strict carnivory evolving in some lineages.
Downward trend in body size
The most extreme examples of miniaturization and progenesis (accelerated sexual maturation which emerge during a phase that would be considered juvenile in the predecessors, at which both growth and further anatomical development slows down or cease) are found in Paraves. The ancestors of Paraves first started to shrink in size in the early Jurassic 200 million years ago, and fossil evidence show that this theropod line evolved new adaptations four times faster than other groups of dinosaurs, and was shrinking 160 times faster than other dinosaur lineages were growing. Turner et al. (2007) suggested that extreme miniaturization was ancestral for the clade, whose common ancestor has been estimated to have been around 65 centimeters long and 600-700 grams in mass. In Eumaniraptora, both Dromaeosauridae and Troodontidae went later through four independent events of gigantism, three times in dromaeosaurids and once in troodontids, while the body mass continued to decrease in many forms within Avialae. Fossils shows that all the earliest members of Paraves found to date started out as small, while Troodontidae and Dromaeosauridae gradually increased in size during the Cretaceous period.
The ancestral paravian is a hypothetical animal: the first common ancestor of birds, dromaeosaurids, and troodontids which was not also ancestral to oviraptorosaurs. Little can be said with certainty about this animal. But the work of Xu et al. (2003), (2005) and Hu et al. (2009) provide examples of basal and early paravians with four wings, adapted to an arboreal lifestyle who would only lose their hindwings when some adapted to a life on the ground and when avialans evolved powered flight. Newer research also indicates that gliding, flapping and parachuting was another ancestral trait of Paraves, while true powered flight only evolved once, in the lineage leading to modern birds.
One of the best-known features of paravians is the presence of an enlarged and strongly curved "sickle claw" on a hyper-extendible second toe, modified to hold the sickle claw clear of the ground when walking, most notably developed in the dromaeosaurids and troodontids. While this characteristic claw and its associated modifications to the anatomy of the foot (such as a shortened metatarsus in eudromaeosaurs) had been known since the mid-20th century, their possible functions were the subject mainly of speculation, and few actual studies were published. Initial speculation regarded the claws as slashing implements used to disembowel large prey. In this scenario, the shortened upper foot would serve as an anchor point for powerful tendons to improve kicking ability. However, subsequent studies of the actual claw shape showed that the underside of the claw was only weakly keeled and would not have been an effective cutting instrument. Instead, it appeared to be more of a hooking implement. Manning et al. suggested in 2006 that the claws were similar to crampons and were used for climbing, and in the case of larger species or individuals, climbing up the flanks of very large prey.
A larger study of sickle-claw function, published in 2011 by Fowler and colleagues, concluded that the earlier study by Manning and colleagues was correct and that the "sickle claws" would have been ineffective as cutting weapons. They compared the claw and overall foot anatomy of various primitive species with modern birds to shed light on their actual function. Fowler and colleagues showed that many modern predatory birds also have enlarged claws on the second toes. In modern raptors, these claws are used to help grip and hold prey of sizes smaller than or equal to the predator, while the birds use their body weight to pin their prey to the ground and eat it alive. Fowler and colleagues suggested that this behavior is entirely consistent with the anatomy of advanced dromaeosaurids like Deinonychus, which had slightly opposing first toes and strong tendons in the toes and foot. This makes it likely that advanced dromaeosaurids also used their claws to puncture and grip their prey to aid in pinning it to the ground, while using shallow wing beats and tail movements to stabilize themselves. Other lines of evidence for this behavior include teeth which had large, hooked serrations only on the back edge (useful in pulling flesh upward rather than slicing it) and large claws on the wings (for greater maneuvering of prey while mantling it with the wings).
In more primitive dromaeosaurids and in troodontids, the feet were not as specialized and the claws were not as large or as hooked. Additionally, the toe joints allowed more range of motion than the simple up-down movements of advanced dromaeosaurids. This makes it likely that these species specialized in smaller prey that could be pinned using only the inner toes, not requiring the feet to be as strong or sturdy.
|Cladogram following the results of a phylogenetic study by Cau et al., 2015.|
The name Paraves was coined by Paul Sereno in 1997. The clade was defined by Sereno in 1998 as a branch-based clade containing all Maniraptora closer to Neornithes (which includes all the birds living in the world today) than to Oviraptor.
Also in 1997, a node-based clade called Eumaniraptora ("true maniraptorans") was named by Padian, Hutchinson and Holtz. They defined their clade to include only avialans and deinonychosaurs. Paraves and Eumaniraptora are generally considered to be synonyms, though a few phylogenetic studies suggest that the two groups have a similar but not identical content; Agnolín and Novas (2011) recovered scansoriopterygids and alvarezsaurids as paravians that were not eumaniraptorans, while Turner, Makovicky and Norell (2012) recovered Epidexipteryx as the only known non-eumaniraptoran paravian. A nearly identical definition, "the theropod group that includes all taxa closer to Passer than to Dromaeosaurus", was used by Agnolín and Novas (2013) for their clade Averaptora.
Since the 1960s, the dromaeosaurids and troodontids have often been classified together in a group or clade named the Deinonychosauria, initially based primarily on the presence of a retractable second toe with sickle-claw (now also known to be present in some avialans). The name Deinonychosauria was coined by Ned Colbert and Dale Russell in 1969, and defined as a clade (all theropods closer to dromaeosaurids than to birds) by Jacques Gauthier in 1986. However, several more recent studies have cast doubt on the hypothesis that dromaeosaurids and troodontids were more closely related to each other than either was to birds, instead finding that troodontids were more closely related to birds than to dromaeosaurids. Because Deinonychosauria was originally defined as all animals closer to dromaeosaurids than to birds without specific reference to troodontids, this would render Deinonychosauria a synonym of Dromaeosauridae.
In 2015 Chatterjee created Tetrapterygidae in the second edition of his book The Rise of Birds: 225 Million Years of Evolution, where he included Microraptor, Xiaotingia, Aurornis, and Anchiornis; together they were proposed to be the sister group of the Avialae.
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