A field guide to mesozoic birds and other winged dinosaurs
fur-like filaments called pycnofibres. It is possible that these were present in the bird/pteorsaur concestor and represent the earliest stages (Stage I, described below) of feather evolution. However, we will need a more complete record of skin coverings in ornithodirans before we can test this idea. Around the base of the ornithodiran lineage and that of its immediate sub-group Dinosauromorpha, the ancestors of birds began to walk upright. This probably began as a mode of fast locomotion, similar to the way crocodylians adopt a more upright stance when walking quickly. The early ornithodirans, while still primarily quadrupeds, evolved very long hind legs, and were probably bipedal runners. In the slightly more advanced Dinosauriformes, all four limbs were probably held beneath the body, and by the advent of Dinosauria proper, a specialized hole had opened in the hip socket (a perforate acetabulum), locking the hind legs into an upright stance permanently, making sprawling the hind limbs impossible for all dinosaurs, except those advanced species which modified this arrangement for swimming and climbing. The first dinosaur, the concestor of birds and ornithischians (another highly inappropriate and misleading name meaning “bird hips”), was probably an omnivorous biped with five fingers (three of which bore claws) and five toes. It likely existed in the early part of the Late Triassic. Soon after the split between the ancestors of birds and ornithischians, the eusaurischian concestor (the common ancestor of birds and sauropod dinosaurs) evolved the beginnings of the avian lung system, which involves extensions of the lung in the form of air sacs invading many of the bones in the skeleton, making them ‘hollow’ and air-filled. With the advent of the group Theropoda, bird ancestors became more fully dedicated to a bipedal, running lifestyle, and shortly thereafter the foot became modified to have three primary toes, with a fourth toe small and high on the ankle, like a dewclaw, and the fifth toe was lost completely. That small dewclaw, or hallux, would be retained and later enlarged and modified in several theropod groups to assist in walking, in prey capture, and in perching. The two claw-free fingers of the hand were slower to be lost, but by the time the group Avetheropoda appeared, only three fingers remained. At some point, probably after the avetheropods had evolved, members of the bird lineage developed one of their most important characteristics: feathers. The first feathers were probably simple, hair-like filaments made of beta-keratin, and the earliest examples have been tentatively identified in the possible primitive coelurosaurian Sciurumimus (that is, if these aren’t the same structures seen in pterosaurs and/or some ornithischians). The earliest down feathers appear in the group Tyrannoraptora, in both primitive members of the group (the deinodontoids and compsognathids), 16
Above: Sequence of aquisition of major avian characteristics within Dinosauria. so it’s likely that their actual origin is somewhat earlier than their Middle Jurassic concestor. Evidence from specimens of the ornithomimosaurs Dromiceiomimus, a group which probably branched after compsognathids but before maniraptorans in the Middle Jurassic, suggests that these may have had pennaceous, if not closed-vaned, feathers on the arm forming the earliest wings. The slightly more advanced group Segnosauria also preserves evidence of relatively long wing feathers, but these appear filamentous. More evidence and better-preserved specimens of these groups will be needed before the exact nature of their feathers can be determined. Truly modern, definitively vaned feathers appeared soon afterwards, in the Middle Jurassic concestor of birds and caenagnathiformes. 17
- Page 1 and 2: A Field Guide to Mesozoic Birds and
- Page 3 and 4: To my wife Rashida, For always bugg
- Page 5: Enantiornithiformes................
- Page 8 and 9: Introduction
- Page 10 and 11: up of remains from various reptiles
- Page 12 and 13: enough information about their plum
- Page 14 and 15: Bird Origins & Evolution
- Page 18 and 19: This same group (Chuniaoae) is wher
- Page 20 and 21: 20
- Page 22 and 23: erately long rachides with long bar
- Page 24 and 25: istles, the opposite of filoplumes,
- Page 26 and 27: ers are anchored to a single plough
- Page 28 and 29: using their wings to passively exte
- Page 30 and 31: modern downstroke where the arm mov
- Page 32 and 33: While Mesozoic birds and other anci
- Page 34 and 35: Feathers & Wings Feathers are class
- Page 36 and 37: longest digit is called the major d
- Page 38 and 39: which aids in aerial maneuverabilit
- Page 40 and 41: Hesperornis, the premaxillary nail
- Page 42 and 43: carnivorous. However, many deinonyc
- Page 44 and 45: Feather Color Recent work by Jakob
- Page 46 and 47: Above: Hypothetical restoration of
- Page 48 and 49: 48
- Page 51 and 52: Guide to Mesozoic Birds
- Page 53 and 54: like in life based on fossil and ot
- Page 55 and 56: Above: Relationships of basal caena
- Page 57 and 58: 57
- Page 59 and 60: 59
- Page 61 and 62: 61
- Page 63 and 64: Above: Relationships of caenagnatho
- Page 65 and 66: 65
Above: Sequence of aquisition of major avian characteristics within Dinosauria.<br />
so it’s likely that their actual origin is somewhat earlier than their Middle<br />
Jurassic conces<strong>to</strong>r. Evidence from specimens of the ornithomimosaurs<br />
Dromiceiomimus, a group which probably branched after compsognathids<br />
but before manirap<strong>to</strong>rans in the Middle Jurassic, suggests that these may<br />
have had pennaceous, if not closed-vaned, feathers on the arm forming<br />
the earliest wings. The slightly more advanced group Segnosauria also preserves<br />
evidence of relatively long wing feathers, but these appear filamen<strong>to</strong>us.<br />
More evidence <strong>and</strong> better-preserved specimens of these groups will be<br />
needed before the exact nature of their feathers can be determined.<br />
Truly modern, definitively vaned feathers appeared soon afterwards,<br />
in the Middle Jurassic conces<strong>to</strong>r of <strong>birds</strong> <strong>and</strong> caenagnathiformes.<br />
17
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