Describer

Hou, 1997

Time

Cretaceous Early Barremian

Classification

Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Confuciusornithidae

Fossilsite

Chaomidianzi Formation ( previously referred to as the lower section of the Yixian Formation), Liaoning, China

Fall Under

Confuciusornis sanctus ~/~

Info

Confuciusornis sanctus ~/~ (Hou, Zhou, Gu and Zhang, 1995) > Confuciusornis chuonzhous ~/~ (Hou, 1997) > Confuciusornis sunia ~/~ (Hou, 1997) > Confuciusornis dui ~/~ (Hou, Martin, Zhou, Feduccia and Zhang, 1999) > Confuciusornis meidus ~/~ [Nomen Nudem] (Hou, 1997) > Confuciusornis shuzhi ~/~ [Nomen Nudem] (Hou, 1997)

Confuciusornis chuonzhous sp. nov.

Type

Part and counterpart of a distal portion of a left hindlimb including the distal tibiotarsus, complete tarsometatarsus, and ungual phalanges (V10919).

Age and stratigraphic position

Late Jurassic interbedded gray-green argillaceous sandstones of the basal Yixian Fm. Locality: Huangbanjigou, near the village of Shangyuanzhen, municipality of Beipiaoshi, Liaoning Province.

Etymology

For the former name of the city of Beipiao, “Chuanzhou.” This city name was established in 951 AD, resurrected in the Yuan Dynasty under the Khan Tartars, but abandoned in the Ming Dynasy. The specimen was excavated from within the political jurisdiction of Beipiao.

Diagnosis

A relatively large and robust primitive bird with a robust tibia that bears feather impressions laterally; anteroposteriorly it is relatively thick, and the distal end is unexpanded. Both an unfused astragalus and calcaneum are present, a metatarsal V is present and is isolated, except for its articulated proximal end. Ungual phalanx on digit I is exceptionally small and its degree of curvature is more than in any other Jurassic bird.

Description

In the introduction to C. sanctus it was mentioned that initially this specimen was identified as belonging to that species; however, reevaluation of specimens for the composition of this manuscript noted distinct discrepancies between this specimen and the type, justifying the erection of a new species. The fossil locality for this specimen, Huangbanjigou, is located less than one km directly north of the type locality for the genus, Jianshangou, and although there is some slight discrepancy in its lithological character, Huangbanjigou is still considered stratigraphically equivalent to the type locality.

The preserved length of the tibia is 28 mm, its maximum breadth is 4 mm, and feather impressions lie laterally although precise feather impression boundaries are obscured due to taphonomic processes. Tibia pneumatization is high, walls are relatively thick, its distal end resembles the condition on Archaeopteryx, being unexpanded laterally, and distinct condyles are absent, resembling the primitive reptilian condition by articulating with an independent astragalus and calcaneum. The metatarsals are also relatively robust with lengths in excess of 30 mm; their proximal ends have begun to fuse, and there is some lateral expansion. It is difficult to assess whether or not independent tarsals are present due to compressional distortion, but based upon the primitive morphology of this specimen, it is assumed that the tarsals were not fused to the metatarsals and perhaps were not preserved.

The most characteristic feature of the metatarsals is the presence of a relatively robust MtV that is isolated except at its proximal end, which is still not fused but in articulation with the other metatarsals. MtI is at least 8 mm shorter than the other metatarsals. MtIII is extremely long. The first phalanx of digit I is less than half the length of the other phalanges. There appears to be three phalanges on the first digit, with the medial phalanx being extremely short and in articulation with the clawed ungual phalanx.

If this interpretation is correct, then it would be exceptionally autapomorphic, as there are no reptilian ancestors with a first pedal digit with three phalanges and it is therefore possible that this condition represents an aberration. The ungual on the first digit is extremely short and thick with a slightly curved terminus and is less than half the length of the other unguals. Remaining phalanges are also exceptionally preserved, with only digit III partially obscured by digit II, and its proximal phalanx is vague. The two proximal phalanges of digit II are quite distinct, the second digit is longer than digit I, and its ungual is broader (higher) than all the others. Only the most proximal and distal phalanx is visible on digit III, with the proximal end of the first phalanx relatively long and the distal end of the second to last phalanx relatively weak. The ungual of digit III is long, exceeding 15 mm in length; its proximal end is slightly damaged, and like the ungual of digit II it has a low degree of curvature. Five phalanges are present on digit IV, the first four are relatively short, and the length of the ungual approaches that on digit III with a similar degree of curvature.

Comparison and discussion: To date, there are only two genera and three species of Jurassic birds known globally: Archaeopteryx lithographica, A. bavarica, and Confuciusornis sanctus. This text hereby records a fourth species that was initially assigned to C. sanctus but is now erected as a new species based upon the characters of the hindlimb. With regard to tibia metatarsal proportions, the tibia is incompletely preserved on C. chuonzhous, but on C. sanctus the metatarsals are slightly longer than half the length of the tibia, which is 41 mm in length and 2.5 mm in breadth, whereas metatarsal length is 22 mm, and MtV length is 7 mm, being extremely slender and fused to MtIV. The breadth of the C. chuonzhous tibia is 4 mm, metatarsal length is 33 mm, MtV length is 10 mm; it is robust and isolated from the other metatarsals. The tibia-metatarsal proportions described above indicate the clear distinctions between the two species. C. sanctus retains only a single tarsal whereas C. chuonzhous probably possessed at least two. On C. sanctus the distal tibial condyles are conspicuous and anteriorly projected but on the new species the distal tibia is in a more primitive condition.

In extant taxa the morphology of the ungual phalanges determines the defining taxonomic characters for the orders Cursores, Natatores, Grallatores, Raptatores, Scansores, Columbiformes, and the genus Coturnix (Zheng, 1982). Feduccia (1993) compiled statistics on the claw curvature of extant avian taxa and concluded that curvature of less then 100° indicated cursorial behavior, a vast majority of taxa with curvature between 80°-140° are perching forms, and those with curvature exceeding 120° are adapted to climbing. Based upon this data, he determined that Archaeopteryx was a perching form and not a cursorial predator. The curvature of the pes claws on Confuciusornis sanctus is equivalent to those of Archaeopteryx. Moreover, the ungual on digit I is particularly elongated, indicating that the forelimb functioned as a stabilizing mechanism for the body and was a stronger element for assisting in climbing than on Archaeopteryx, being further evidence that C. sanctus was an arboreal dweller. The ungual curvature of C. chuonzhous exceeds the parameters for perching forms and the tibia and metatarsals are more robust. Consequently, this species is assigned to the cursorial group.

The fifth metatarsal of C. chuonzhous is longer than in a vast majority of the specimens of Archaeopteryx and longer than that of A. bavarica. The remaining metatarsals are all shorter than those of A. bavarica. This reflects the primitive nature of C. chuonzhous and in combination with its robust and strengthened unguals, is a further indication that it is adapted to cursorial behaviour. ibia morphology of C. chuonzhous resembles that of Archaeopteryx with indistinct medial and lateral condyles and an articulation with the calcaneum, which itself is associated with unfused tarsals that are reduced in number and articulate distally with metatarsals. This morphology is completely adequate for habitats in trees, cliffs, or on the ground. The Confuciusornis forelimb is completely modified for flight but still requires a relatively long phase of evolutionary development to match the mophology of extant taxa. Therefore, primitive birds contemporaneous with C. sanctus are considered to possess forelimbs with three independent digits for support, stability, and equilibrium of the torso. Furthermore, the primitive condition of Jurassic birds is expressed in the smaller and weaker ungual of pedal digit I, which is in contrast to the condition of later taxa and particularly extant forms. The primitive first digit unguals are smaller than the rest in the pedal sequence, prohibiting the use of the hindlimbs from acquiring prey or scaling trees. On extant species the first pedal digit is in opposition to the other three, and is large and acute.

The first ungual on arboreal dwelling taxa is the largest and most recurved. Predaceous claws, including the largest digit I ungual when seizing prey, are capable of penetrating flesh. Therefore, the function of the ungual on the first digit is regarded as extremely significant in the adaptation to a perching mode, predation, or a defensive mechanism. However, this important character has yet to be discussed by workers in the field of paleornithology, and therefore a preliminary discussion will be conducted based upon the several specimens of Archaeopteryx, two Confuciusornis specimens, and a large quantity of Early Cretaceous avian specimens.

In general, the hindlimb of archaic reptiles possessed five digits and the hindlimb claws, particularly the ungual of digit I, were not well developed. Among the Dinosauria, and particularly the ornithomimids, which basically had omnivorous feeding patterns, the first digit has been lost. In carnivorous dinosaurs, digit I has also been reduced or lost. The digital formula of Archaeopteryx is completely distinct from that of the Dinosauria, because the first ungual in Archaeopteryx and Confuciusornis has become extremely weak, although its associated phalanx shows no appreciable reduction. In contrast, the first phalanx is elongated, not shortened as on the other digits, and lies in opposition to them.

After the initial radiation of Aves from the Reptilia, in order to flee the surface of the ground to avoid predatory enemies and develop the ability to scale trees, the ancestral bird modified its reptilian extremities, particularly the first pedal digit, which was modified from an anterior placement to a posterior position to lie in opposition to the other digits for the function of scaling trees and perching on branches. Extant lacertilians with the capability of scaling trees and walls include forms such as the African Chamaeleo and Gekko, although one these taxa relies on friction of the digital skin pads, while the other relies on the inflation of the digital scales and not their digital morphology.

Obviously, the preliminary evolutionary modification of avian forms included the contemporaneous changes in the fore- and hindlimbs. A discussion of forelimb modification and the evolution of flight will be provided later in the text. Here we turn to avian digital modification, noting that fore- and hindlimb modifications were coordinated. After the completion of the posterior rotation of digit I in the most archaic avian lineage (which required a relatively long period of time), there was continuous evolution of the adaptation to the aerial realm and particularly for the ability to fly between arboreal habitats, which required the evolution of pedal digit I. The presence of Archaeopteryx and Confuciusornis in the Jurassic suggests that perhaps the majority of primitive arboreal birds did not deviate from utilizing their forelimb unguals, and thus the forelimbs, manus, and digits of these taxa were unfused with the presence of three digits bearing acute clawed unguals. The first manus ungual on Confuciusornis is extremely large and intensely recurved, which is an indication of its strength.

However, evolutionary modification is not in equilibrium because there are more derived avian forms that are contemporaneous with Archaeopteryx and Confuciusornis. This condition is similar in extant avian taxa, which contain birds that are adapted for highly specialized habitats in addition to birds that are morphologically relatively primitive. Extinct families were unable to adapt to habitats. On the latest avian specimen excavated from the Jurassic of Fengning Co., Hebei Province, there is a large, relatively well developed, and acute pedal ungual on digit I, and although it is slightly smaller than the other unguals it is quite distinct. Its modification is associated with additional derived characters, including the absence of a fifth metatarsal; tarsals have all been lost, and caudal vertebrae have become fused to compose a pygostyle.

Within the large collection of Early Cretaceous avian specimens from Chaoyang, Liaoning, the first pedal ungual has become distinctly modified to match the morphology of extant forms; its size and degree of acuity is in complete contrast to those of the other unguals, which indicates that the facility for grasping with the pes has been advanced, and that it has become modified for adaptation to a new habitat. The modification of the first pedal ungual accompanied the appearance and modification of characteristic digit musculature and ligaments. The hallux particularly displays several significant well developed muscles and tendons. First, the long hallucal flexor muscle initiates with a tendon on the fibular side of the femoral popliteal area, where it is unified with the long digital flexor for the extension of digits; it concurrently facilitates extension and lateral rotation of the foot. Additionally, the transverse metatarsal ligament initiates on the first metatarsal at the base of the first digit, intimately connecting the first digit to the other three digits.

The rectal hallical ligament and oblique hallucal ligament are directly connected to the phalanx to strengthen the hallux (digit I) and other digits for the ability to grasp. Another muscular complex for strengthening pedal mobility is the perforate digital flexor (flexor digitorum perforans), which on modern birds traverses muscular and tendinal complexes to directly contact the termini of each of the digits. Surficially, it has extremely specialized digital fibrous sheaths (vagina fibrosa) within which musculature is mobilized and thus the tendons and sheaths are closely interconnected. In this manner, the muscles are allowed to relax while retaining pedal immobility and perching with a firm grasp. This avian autapomorphy is extremely significant behaviorly, as it allows birds to pass the night sleeping while perched in a tree, facilitates laying eggs into the nest, and use as defense against daily predators. With this capability, although the individual is in a tree, it expends no energy, and there is no fatigue, allowing extensive periods of relaxation.

There is still an enormous amount of functional development associated with the avian foot, such that here it is only possible to raise several aspects of it. Descriptions of muscle complexes including the short digit flexors, digital extensors, dorsal complexes, and posterior complexes will not be reiterated here. Archaeopteryx and Confuciusornis probably only possessed rudimentary pedal musculature for true avian functions. Their feet were in an evolutionary phase that is extremely distant from extant avian pedal morphology, which allow a free range of avian mobility.

_{Source: Polyglot Paleontologist}

• < Prev
• Next >