A field guide to mesozoic birds and other winged dinosaurs
Basal Eumaniraptorans & Deinonychosaurians The deinonychosaurians, or “terrible claw lizards”, currently represent one of the earliest and most primitive known lineages of frond-tailed birds (along with their sister lineage, the Avialae). The fossil record of early deinonychosaurians is relatively complete, and primitive members are known from good fossil remains and feather impressions. Not surprisingly, primitive members of each group are very similar to each other, and to primitive avialans, making it clear that the deinonychosaurian lineage and the one leading to modern birds evolved from a common ancestor very much like Archaeopteryx lithogrpahica or Xiaotingia zhengi. Primitive deinonychosaurs appear to have been glissant (i.e. capable of gliding flight), with some taking steps toward powered, flapping flight. Microraptor zhaoianus had well-developed wings, and possessed an additional set of “hind wings” formed from vaned feathers on the lower legs and feet, which would have formed a biplane-like configuration when gliding or parachuting from trees. In Archaeopteryx, Microraptor, and Rahonavis, the wings were large enough and powerful enough to have allowed clumsy, level bursts of flight, though gliding was probably the preferred mode of aerial transport. In addition to the early small, glissant species, deinonychosaurians exhibited a common trend found among even modern birds: that of flightless ground birds becoming larger and more cursorial (i.e. adapted to a ground-dwelling lifestyle). These later, larger species (members of the group Eudromaeosauria) include the famous “raptors,” which did not actually resemble the reptilian monsters depicted in popular cinema, but were very large predatory ground birds, some with wings of substantial size (evidence for this comes from feather anchor points found on the wing bones of Velociraptor mongoliensis). The “raptors” did not closely resemble their carnosaurian cousins, but rather oversized Archaeopteryx. As paleontologist Mark Norell stated in an interview on the subject: “The more that we learn about these animals the more we find that there is basically no difference between birds and their closely related dinosaur ancestors like Velociraptor. Both have wishbones, brooded their nests, possess hollow bones, and were covered in feathers. If animals like Velociraptor were alive today our first impression would be that they were just 74
very unusual looking birds.” Even the characteristically enlarged “sickle claw” on the second toe of most deinonychosaurians probably did not begin as a weapon to hold and kill prey. Rather, their shape (and the shape of the forelimb claws) more closely matches the claws of climbing animals. The sickle claws and wing claws alike were also attached in a way ideal for the transfer of stress loads to the rest of the foot or wing, unlike the expected anatomy of a slicing weapon. It is likely that these claws were first used for climbing trees (like the crampons used by utility pole linemen) in small, glissant species, and were later adapted for prey capture in their larger, ground-dwelling descendants. This trend from small gliders to large ground birds is not unique to the deinonychosaurians, but their reversion to a predatory lifestyle may be. Many lineages of early maniraptorans (birds and their closest relatives) appear to have been omnivorous, herbivorous or insectivorous, and only among the eudromaeosaurians did hypercarnivory (diets including mainly large vertebrate prey items) evolve. While deinonychosaurians must have emerged at least in the Middle Jurassic, all primitive known members of this family are Late Jurassic or younger in age, and may have departed significantly from the ancestral body plan, inferred to have been small and large-winged. The first known Mesozoic bird, Archaeopteryx lithographica, was found in 1861 and named based on a single feather. The name was transferred to a more complete skeleton in 2011. Several species of archaeopterygid have been recognized in the past from the Solnhoffen limestone of Bavaria. However, most of those are probably specimens of A. lithographica at various stages of growth. Archaeopteryx is traditionally considered to be more closely related to modern birds than to deinonychosaurians, though it is possible that archaeopterygids are on the deinoychosaurian line, or basal to the avialan/deinonychosaurian split. 75
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very unusual looking <strong>birds</strong>.”<br />
Even the characteristically enlarged “sickle claw” on the second <strong>to</strong>e<br />
of most deinonychosaurians probably did not begin as a weapon <strong>to</strong> hold<br />
<strong>and</strong> kill prey. Rather, their shape (<strong>and</strong> the shape of the forelimb claws) more<br />
closely matches the claws of climbing animals. The sickle claws <strong>and</strong> wing<br />
claws alike were also attached in a way ideal for the transfer of stress loads<br />
<strong>to</strong> the rest of the foot or wing, unlike the expected ana<strong>to</strong>my of a slicing<br />
weapon. It is likely that these claws were first used for climbing trees (like<br />
the crampons used by utility pole linemen) in small, glissant species, <strong>and</strong><br />
were later adapted for prey capture in their larger, ground-dwelling descendants.<br />
This trend from small gliders <strong>to</strong> large ground <strong>birds</strong> is not unique<br />
<strong>to</strong> the deinonychosaurians, but their reversion <strong>to</strong> a preda<strong>to</strong>ry lifestyle may<br />
be. Many lineages of early manirap<strong>to</strong>rans (<strong>birds</strong> <strong>and</strong> their closest relatives)<br />
appear <strong>to</strong> have been omnivorous, herbivorous or insectivorous, <strong>and</strong> only<br />
among the eudromaeosaurians did hypercarnivory (diets including mainly<br />
large vertebrate prey items) evolve.<br />
While deinonychosaurians must have emerged at least in the Middle<br />
Jurassic, all primitive known members of this family are Late Jurassic<br />
or younger in age, <strong>and</strong> may have departed significantly from the ancestral<br />
body plan, inferred <strong>to</strong> have been small <strong>and</strong> large-<strong>winged</strong>.<br />
The first known Mesozoic bird, Archaeopteryx lithographica, was<br />
found in 1861 <strong>and</strong> named based on a single feather. The name was transferred<br />
<strong>to</strong> a more complete skele<strong>to</strong>n in 2011. Several species of archaeopterygid<br />
have been recognized in the past from the Solnhoffen limes<strong>to</strong>ne of<br />
Bavaria. However, most of those are probably specimens of A. lithographica<br />
at various stages of growth. Archaeopteryx is traditionally considered <strong>to</strong> be<br />
more closely related <strong>to</strong> modern <strong>birds</strong> than <strong>to</strong> deinonychosaurians, though it<br />
is possible that archaeopterygids are on the deinoychosaurian line, or basal<br />
<strong>to</strong> the avialan/deinonychosaurian split.<br />
75
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