[D] Chaoyangia beishanensis [~/~]
Describer
Hou & Zhang, 1993
Time
Cretaceous Early
Classification
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Ornithurae
Fossilsite
Jiufotang Formation, Chaoyang, Liaoning,China
Info
Partial skeleton
Type
An incomplete skeleton lacking the head, forelimb, and hindlimb (IVPP specimen V9934).
Locality and stratigraphic position
Middle to upper mudstones of the early Early Cretaceous Jiufotang Fm., from the Beishan quarry, Chaoyang Co., Liaoning Province.
Diagnosis
Vertebral centra are not heterocoelous, sacrals exceed eight, and are unfused. Ribs bear a proximal longitudinal groove and an ossified uncinate process. Sacral girdle is unfused, pubis is posteriorly inclined, pneumaticized, and has fused termini, ilium is reniform (kidney-shaped), and preacetabular section is longer than the unreduced postacetabular section. Long bones are thinly walled and pneumaticized. Femoral head and shaft are well developed and fourth trochanter has become lost. Cnemial crest on the tibia is enlarged and fibula is not fused to the tibia.
Description
Chaoyangia was discovered serendipitously in the fall of 1990 when an expedition was concluding its excavations in the Boluochi region of Chaoyang Co after the collection of several avian specimens. On the last day, upon closing the quarry, a brief perusal of the spoil pile was conducted for potential specimens that were missed. A massive chunk of shale was split to reveal the Chaoyangia specimen, which differed from the others collected from the quarry by being larger, and displaying an exceptionally distinct sacral girdle. The vertebral column was primitive and resembled that of Archaeopteryx, whereas the pelvic girdle resembled that of the Dinosauria, but its thoracic ribs were like thos of extant birds in having a well developed uncinate processes.
Because this was the first discovery of a specimen with such mosaic characters, an intensive search was initiated to try to find the associated portions of the skeleton, particularly the skull, pectoral girdle, and forelimb. The search ensued until nightfall without success, and the team ultimately departed the quarry with a deep sense of regret. Continuous annual excavations have been conducted at this quarry since 1990 and to date supplemental material of this genus has yet to be recovered, despite the addition of numerous typical specimens. Recently, in addition to the continuous production of Early Mesozoic birds from China, other Early Cretaceous discoveries have been made in Mongolia, Spain, Canada, South America, and Australia, but there are still no specimens resembling Chaoyangia, which is the principle justification for the erection of the new order.
The specimen is preserved in natural configuration, with the posterior three cervicals in tight association with their corresponding thoracic vertebrae. Right gastric ribs are in their natural configuration as is the pelvic girdle, which maintains enlarged ilia flanking the sacral vertebrae. The acetabulum is distinctly exposed, and the right femur is shifted anteriorly. On the left side, only three gastric ribs are preserved and have shifted in position. As in the other specimens from this quarry, feathers are not preserved. The three cervicals are tightly articulated, resembling the condition of Confuciusornis and Archaeopteryx. The centra are not heterocoelous and increase in size posteriorly. Although the centra are relatively short, lateral pleurocoels are relatively well developed, and neural spines are relatively low with their height increasing posteriorly.
Short cervical ribs are retained and become progressively elongated posteriorly. There are approximately seven thoracic vertebrae with distinctly elongated centra and high neural spines that differ from those of extant taxa by being unfused. The several posterior thoracic dorsal spines are particularly high, attaining 6 mm in height and 5.5 mm in length, whereas centra are 3.5 mm high and have a slightly concave ventromedial margin. A ventral keel is not noticeable. At least one lumbar is detached from the column; its neural spine is also relatively high, and its morphology resembles the thoracic condition, but features posterior to it are obscured by the ilium. There are at least eight sacral centra with neural spines that are particularly pronounced but are not in contact, and diapophyses are relatively elongated, contacting the dorsomedial margin of the ilium. Fusion has clearly begun in anterior caudal centra although neural spines are still unfused.
The most notable character of Chaoyangia lies in its thoracic ribs, which have well developed ossified uncinate processes which resemble the extant condition as well as that of Hesperornis and Ichthyornis, and are a unique morphology among Early Cretaceous taxa. Eight typically bicapitate ribs are represented (three on the left side and five posterior ribs on the right) as relatively complete shafts in natural position. These indicate a relatively large chest cavity in outline. Proximal ribs display the extremely autapomorphic character of longitudinal grooves 10 to 15 mm long. Ribs also resemble the extant condition: The have a longitudinal dorsal process for facilitating attachment of intercostal musculature, although the shafts are relatively slender and elongated.
This differs from the extant condition in which ribs are expanded to facilitate the stability of the thoracic cavity, or expand the surface area for facilitating intercostal musculature for functional flight (Fig. 78). Although the ribs on Chaoyangia are extremely elongated, facilitation for intercostal musculature is rather weak. Obviously, in order to fortify the thoracic chamber, Chaoyangia ossified its uncinate processes, which have become elongated to surpass the intercostal vacuities and almost attain the next anterior rib, resembling the condition of extant taxa. The uncinate processes are posteriorly oblique; they have an extremely expanded base and an acute termination, resembling an elongated cone. Two to four uncinate processes are present that are two-thirds the length of the rib shaft itself.
It is extremely noteworthy that Chaoyangia maintains ossified gastric ribs, and although they are neither as numerous nor as complete as the dorsal ribs, they are sufficient to indicate that the genus had a relatively derived thoracic cavity. The gastric ribs are preserved in natural configuration, with their termini in contact with the distal dorsal ribs. Both termini of the gastric ribs are not expanded as in the extant condition. Two additional gastric ribs lay isolated between the dorsal ribs.
In contrast to the derived thoracic cavity is the presence of an exceptionally primitive pelvic girdle resembling that of the Dinosauria. Elements on the right side are relatively well preserved. A basically complete reniform ilium has a relatively elongated preacetabular section that has been medially depressed due to compressional distortion. The postacetabular portion of the ilium is only slightly reduced and is extremely broadened. It has a relatively rounded terminus which differs distinctly from that of Cathayornis from the same locality. The acetabulum is small and circular and the entire dorsal wall is composed of the ilium, although its anterior portion is incomplete. The dorsal margin of the ilium is very slightly dorsally crescentic, lies lateral to the centra, and is not fused with the lumbar or sacral centra. The right pubis is complete but only the posterior portion of the left pubis is visible. Its shaft is relatively cylindrical and pneumaticized. The portion of the pubis that composes the anteroventral acetabulum, or the prepubic process, has been greatly reduced, representing only an extremely short pectineal process. The pubis extends posteriorly from the acetabulum; its distal end contacts its counterpart via a slight inflation. The distal region of contact is relatively elongated, being 17 mm in length. This condition resembles that of Archaeopteryx although it lacks a dorsally hooked process.
The right ischium is also relatively complete and forms an irregularly shaped linear element that is broader than the pubis and slightly resembles the condition in Archaeopteryx. The ischium extends posteroventrally between the two pubes and has an undulating expansion. Its distal end does not reach the medial margin of the distal pubes and terminates to become narrow and acute. The proximal end forms the posteroventral portion of the acetabulum. Its ventral margin is a dorsally arched embayment, and it differs from Cathayornis by lacking a dorsal process. All three pelvic elements are unfused and do not bear an intimate relationship with the sacral vertebrae. Thus in this respect, it closely resembles Archaeopteryx. Although the hindlimb preserves only the femur and a portion of the tibiotarsus, it appears to be relatively robust.
Both femora are preserved, with the one on the right side being nearly perfect. The right shaft is relatively straight but the left is slightly curved. The proximal end bears a relatively high trochanteric crest, and a shallow trochanteric fossa. The femoral head is relatively large and projects relatively distinctly medially, although a neck is basically absent. The fourth trochanter has become lost. The shaft is more slender than the proximal end; it has a pneumatic foramen on the proximolateral side, is highly pneumaticized, and has relatively thin walls.
Distally, the rotular groove is relatively expanded and a scar for ligament attachment is relatively distinct on the medial side of the medial condyle. Whether or not an attachment point to facilitate flexor musculature is present on the lateral side of the lateral condyle is unknown due to being obscured. The distal end is slightly concave, quite distinct from the extant condition. The distal end of the left femur is not preserved; the element has been shifted in position to lie posteroventral to the ilium, and overlies the proximal left ischium and pubis.
The right distal tibiotarsus is missing and the left side only preserves a portion of its proximal end. Like the femur, it is relatively robust, the shaft has a slight curvature, and it possesses the distinct character of a relatively well developed medial crest resembling the extant condition of projecting medially, however, its extension to the proximal surface is vague. Proximally, it is represented by a single very slightly concave articular surface that has yet to become divided into two facets; this is a direct reflection of its articular relationship with the rather planar distal femur. There is a slight proximolateral expansion but a distinct lateral cnemial crest is absent. Only a slender and short right fibula is preserved in addition to an extremely short portion of the right tarsometatarsus, but it cannot be determined whether it resembles Cathayornis. The tarsals appear to have become fused to compose a cap on the shaft of the fused tarsometatarsus.
Measurements of Chaoyangia beishanensis (mm).
Vertebral column preserved length 88.0
Second to last right rib length ~36.0
Second to last right uncinate process length 8.0
Second to last right proximal
rib to uncinate process length 25.0
Third to last right rib length 40.0
Longest right uncinate process 11.1
Fourth to last right rib length 37.5
Ilium length 32.0
Pubis length ~51.0
Distal pubis contact length 17.0
Ischium length 30.0
Femur length 45.0
Femur proximal breadth 9.5
Femur distal breadth 7.2
Tibiotarsus preserved length 5.3
Tibiotarsus proximal breadth 8.5
A cursory observation of the Chaoyangia specimen would cast doubt upon its veracity as a member of Aves because the skull, forelimb, and sternum are absent, hindlimb is incomplete, there are no feather impressions, and the pelvic girdle is extremely primitive. However, careful examination of the specimen documents fundamental avian characteristics that alleviate all suspicions. These include the relatively thin-walled and highly pneumaticized long bones, the presence of ossified uncinate processes, preacetabular region of the ilium exceeds the length of the postacetabular region, the short prepubic process is represented as the avian pectineal process, pubis is distinctly posteriorly oblique, femur has a relatively high trochanteric crest, and tibiotarsus maintains a well developed medial cnemial crest. The characters documented above indicate that Chaoyangia has a modern avian morphology in addition to retaining primitive characters, excluding its assignment to any known avian orders. Although numerous specimens are produced from the same quarry that are also small and retain primitive characters, none displays characters as apomorphic as Chaoyangia, which include the presence of uncinate processes on the ribs, resembling the condition of Hesperornis, Ichthyornis, and other extant carinate birds, in addition to the presence of ossified gastric ribs that probably indicates a stabilized thoracic cavity, thinly walled and pneumaticized long bones, sacral vertebrae exceed eight in count, femur is enlarged with the fourth trochanter lost, and distalend bears depressions for facilitating ligament attachment; and a medial cnemial crest is present on the
tibiotarsus.
Plesiomorphic characters retained by Chaoyangia include the non-heterocoelous cervical centra, unfused lumbar and sacral centra, and unfused pelvic girdle. Its autapomorphic characters include a medial longitudinal groove on the proximal dorsal ribs, apparently for the facilitation of intercostal musculature, the reniform ilium with a relatively large discrepancy between preacetabular and postacetabular length, and a rounded posterior end; and a pneumaticized pubic shaft with an elongated contact of the distal pubis.
Chaoyangia shares plesiomorphic characters with the Sauriurae including the nonheterocoelous vertebral centra, sacral girdle is not fused, and pubis is posteriorly oblique with fused termini. The most notable characters shared with extant taxa to the exclusion of the Sauriurae include the presence of uncinate processes on the dorsal ribs; ossified, elongated, and pneumaticized gastric ribs; and a well developed cnemial crest on the tibiotarsus. Thus, Chaoyangia is justifiably assigned to the Ornithurae.
Relatively contemporaneous taxa include not only the Chinese Confuciusornis, Liaoningornis, Jibeinia, Sinornis, Cathayornis, and Gansus, but also Archaeopteryx, the Mongolian Ambiortus, and the European Enaliornis, Limnornis, Noguerornis, Concornis, and Iberomesornis. The Late Jurassic Confuciusornis and Archaeopteryx represent the most primitive members of Aves to date, sharinga a similar pelvic girdle morphology of a thin and narrow postacetabular region which differs greatly from that ofChaoyangia. Also, the confuciusornis ischium is short and broad with an expansive terminus and a dorsal process which also differs markedly from that of Chaoyangia. The Confuciusornis pubis differs by being relatively thin and weak with an unexpanded terminus.
Liaoningornis is the most recently erected small genus derived from the same stratigraphic level as Confuciusornis, but it compares poorly to Chaoyangia. Although it maintains a well developed carina, it still retains particularly primitive characters such as the presence of a presternum, which differ greatly from Chaoyangia. The genus Jibeinia occupies an extremely significant position in avian phylogeny. It is produced from latest Jurassic sediments and represents a transitional phase between the morphologies of Confuciusornis-Archaeopteryx and the diverse assemblage of Early Cretaceous genera. Its most notable characters lie in its wing and sternum morphology. Obviously it could not have maintained an advanced rib morphology or derived condition of the tibiotarsus such as on Chaoyangia. Despite Sinornis and Chaoyangia being derived from different localities, they are both produced from the Early Cretaceous Jiufotang Fm. and share several characters such as an unfused pelvic girdle, although other pelvic characters are quite distinct, (e.g. Sinornis possesses a curved pubic boot) and a small cnemial crest on the tibiotarsus, it lacks uncinate processes on the ribs, and its gastric ribs lie distant from the dorsal ribs.
Cathayornis is an archaic form that is smaller than a sparrow-sized passerid, is produced from the same quarry as Chaoyangia, and differs in its absence of uncinate processes on its ribs, a smaller acetabulum, and a smaller and more narrow postacetabular region of the ilium. The Mongolian genus Ambiornis represents an archaic member of carinate birds, approaches the size of a corvid, is represented only by a forelimb, a portion of vertebrae, and sternum, and is diagnosed with a distinct carina and extremely well developed proximomedial and lateral tuberosities on its humerus. It is regrettable that the corresponding counterparts of Chaoyangia are not preserved.
Noguerornis, Concornis, and Iberomesornis are all produced from the Early Cretaceous of Spain. The former is derived from limestones at the locality of Montsec in Laerido Province, while the latter two are produced from limestones at Las Hoyas, Cayenca Province. Noguerornis is principally represented by a forelimb, partial vertebrae, furcula, and a fragmentary tibia. Thus it cannot be compared to Chaoyangia, although it represents an evolutionary stage that closely approaches that of Sinornis and Cathayornis. The two latter genera are represented by incomplete skeletons; Iberomesornis is more complete. Although it lacks a skull, the remaining skeleton is complete, resembling Cathayornis by being small and primitive but with relatively derived forelimb characters, although talons are retained. Its ribs lack uncinate processes, sacral girdle and distal tibiotarsus are not fused, a relatively long pygostyle is present, preacetabular portion of the ilium is elongated and broad, posterior portion is reduced and relatively narrow, and only five sacral centra are present. These features distinguish it distinctly from Chaoyangia.
The genus Limnornis is produced from the Early Cretaceous of Romania and is represented only by a right distal humerus, several right metacarpals, and a portion of the left femur which has a projected distolateral condyle, a rounded medial condyle, a deep popliteal fossa, and a narrow intercondylar fossa, indicating a more derived condition than Chaoyangia. It is regrettable that its ribs are not preserved for a more complete comparison. It is appropriate here to conduct a comparison of several morphological aspects of Archaeopteryx, Confuciusornis, and Chaoyangia. There is a relatively large size discrepancy between the current specimens of Archaeopteryx. Chaoyangia approaches the size of the Berlin specimen and further resembles it by having amphicoelous vertebral centra and relatively elongated lateral centra depressions or pleurocoels. It is noteworthy that on the cast of the London specimen presented to IVPP, there is a rib that resembles the condition on Chaoyangia, where a distinctly elongated groove lies on the dorsal surface, although the ribs are relatively short and no gastric ribs are noted, indicating its more primitive condition. The presence of this groove, however, indicates that Archaeopteryx possessed a rather stabilized intercostal musculature system. The dorsal ribs of Chaoyangia are elongated with relatively long uncinate processes and distal ends that are in contact with the gastric ribs. The interconnection of intercostal uncinate processes is a further adaptation for flight and reinforces the rigid thoracic lattice structure, representing the most conspicuous apomorphy of the genus. The thoracic condition of Confuciusornis resembles Archaeopteryx but its ribs are more slender and longitudinal grooves are shallower.
Chaoyangia represents the most archaic avian form with uncinate processes on the ribs. Within the evolution of the avian thoracic cavity, the development of the uncinate process is extremely significant. Thus a comparison of uncinate processes is conducted between Mesozoic and extant genera to attempt to decipher an evolutionary pattern. Figure 81A, B, and C illustrates Mesozoic forms, in which it may be noted that the uncinate processes are all located distally on the ribs. On the extant forms the anterior uncinate processes are positioned slightly distal but do not exceed the point two-thirds the length of the rib. Posteriorly along the rib cage the uncinate processes migrate dorsally; the one on the terminal rib is dorsal to the midpoint of the shaft. Rib morphology is based upon differential habitat adaptations. Hesperornis and other extant natatorial birds maintain relatively robust ribs, with uncinate processes that are particularly broadened at their termini. However, other extant taxa display uncinate processes in which the base is the broadest. Wading birds possess ribs that are narrow and elongated with proximoventral pneumatic foramina; thus Ichthyornis rib morphology more closely resembles that of wading birds. Raptor rib morphology is broad and thin with numerous pneumatic foramina from the proximal end to the midportion of the rib. Dense forest taxa generally maintain a fortified rib that is moderate in thickness but is particularly strong in morphology with only a small quantity or a single pneumatic foramen at the proximal end. Chaoyangia most closely approaches Ichthyornis in morphology and thus resembles a wading bird, although no extant taxa have the longitudinal groove on the ribs such as on Chaoyangia.
Several reptiles tat maintain uncinate processes on their ribs, including the genus Sphenodon and members of the Crocodylia, although these are cartilaginous and not fully ossified. Ostrom (1960) states that Dromaeosaurus also possesses uncinate processes but these are probably a homoplasy. The pelvic girdle on the fossil specimens indicates the following evolutionary trends:
(1) The preacetabular portion of the ilium becomes extended concurrent with the increase of sacral fusion, while the postacetabular portion becomes reduced as an adaptation for the shortened caudal series and to reduce weight for the flight load.
(2) The ischium becomes an irregularly shaped thin plate that extends and expands posteriorly to gradually approach the ilium and as in the extant condition, to finally fuse with the ilium. The ischium of Chaoyangia is longer and thinner than Confuciusornis or Archaeopteryx in addition to lacking a proximal ischial fenestra.
(3) The prepubic process becomes reduced with the entire pubis becoming posteriorly elongated to form the longest and thinnest element on the pelvic girdle as in the extant condition.The distal pubes eventually diverge whereas the distal ischia become fused. The Chaoyangia pubis is not only longer than Confuciusornis and Archaeopteryx, it is pneumaticized, which is a highly autapomorphic condition. The pubes on Archaeopteryx and Confuciusornis are distinct.
(4) The hindlimb of Chaoyangia is more derived than its predecessors and distinctly closer to the morphology of later forms; its femur resembles those of ground dwellers. The femoral head on Confuciusornis and Archaeopteryx is undeveloped and the tibia retains the reptilian condition. Early Cretaceous avian specimens from China and other regions of the world share numerous characters with their Late Jurassic predecessors in addition to maintaining more derived characters. Chaoyangia is the most characteristic and derived of these genera due to its thoracic construction, which provides the most effective adaptability for flight among all the Early Cretaceous species.
Source: Polyglot Paleontologist
Hou & Zhang, 1993
Time
Cretaceous Early
Classification
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Ornithurae
Fossilsite
Jiufotang Formation, Chaoyang, Liaoning,China
Info
Partial skeleton
Type
An incomplete skeleton lacking the head, forelimb, and hindlimb (IVPP specimen V9934).
Locality and stratigraphic position
Middle to upper mudstones of the early Early Cretaceous Jiufotang Fm., from the Beishan quarry, Chaoyang Co., Liaoning Province.
Diagnosis
Vertebral centra are not heterocoelous, sacrals exceed eight, and are unfused. Ribs bear a proximal longitudinal groove and an ossified uncinate process. Sacral girdle is unfused, pubis is posteriorly inclined, pneumaticized, and has fused termini, ilium is reniform (kidney-shaped), and preacetabular section is longer than the unreduced postacetabular section. Long bones are thinly walled and pneumaticized. Femoral head and shaft are well developed and fourth trochanter has become lost. Cnemial crest on the tibia is enlarged and fibula is not fused to the tibia.
Description
Chaoyangia was discovered serendipitously in the fall of 1990 when an expedition was concluding its excavations in the Boluochi region of Chaoyang Co after the collection of several avian specimens. On the last day, upon closing the quarry, a brief perusal of the spoil pile was conducted for potential specimens that were missed. A massive chunk of shale was split to reveal the Chaoyangia specimen, which differed from the others collected from the quarry by being larger, and displaying an exceptionally distinct sacral girdle. The vertebral column was primitive and resembled that of Archaeopteryx, whereas the pelvic girdle resembled that of the Dinosauria, but its thoracic ribs were like thos of extant birds in having a well developed uncinate processes.
Because this was the first discovery of a specimen with such mosaic characters, an intensive search was initiated to try to find the associated portions of the skeleton, particularly the skull, pectoral girdle, and forelimb. The search ensued until nightfall without success, and the team ultimately departed the quarry with a deep sense of regret. Continuous annual excavations have been conducted at this quarry since 1990 and to date supplemental material of this genus has yet to be recovered, despite the addition of numerous typical specimens. Recently, in addition to the continuous production of Early Mesozoic birds from China, other Early Cretaceous discoveries have been made in Mongolia, Spain, Canada, South America, and Australia, but there are still no specimens resembling Chaoyangia, which is the principle justification for the erection of the new order.
The specimen is preserved in natural configuration, with the posterior three cervicals in tight association with their corresponding thoracic vertebrae. Right gastric ribs are in their natural configuration as is the pelvic girdle, which maintains enlarged ilia flanking the sacral vertebrae. The acetabulum is distinctly exposed, and the right femur is shifted anteriorly. On the left side, only three gastric ribs are preserved and have shifted in position. As in the other specimens from this quarry, feathers are not preserved. The three cervicals are tightly articulated, resembling the condition of Confuciusornis and Archaeopteryx. The centra are not heterocoelous and increase in size posteriorly. Although the centra are relatively short, lateral pleurocoels are relatively well developed, and neural spines are relatively low with their height increasing posteriorly.
Short cervical ribs are retained and become progressively elongated posteriorly. There are approximately seven thoracic vertebrae with distinctly elongated centra and high neural spines that differ from those of extant taxa by being unfused. The several posterior thoracic dorsal spines are particularly high, attaining 6 mm in height and 5.5 mm in length, whereas centra are 3.5 mm high and have a slightly concave ventromedial margin. A ventral keel is not noticeable. At least one lumbar is detached from the column; its neural spine is also relatively high, and its morphology resembles the thoracic condition, but features posterior to it are obscured by the ilium. There are at least eight sacral centra with neural spines that are particularly pronounced but are not in contact, and diapophyses are relatively elongated, contacting the dorsomedial margin of the ilium. Fusion has clearly begun in anterior caudal centra although neural spines are still unfused.
The most notable character of Chaoyangia lies in its thoracic ribs, which have well developed ossified uncinate processes which resemble the extant condition as well as that of Hesperornis and Ichthyornis, and are a unique morphology among Early Cretaceous taxa. Eight typically bicapitate ribs are represented (three on the left side and five posterior ribs on the right) as relatively complete shafts in natural position. These indicate a relatively large chest cavity in outline. Proximal ribs display the extremely autapomorphic character of longitudinal grooves 10 to 15 mm long. Ribs also resemble the extant condition: The have a longitudinal dorsal process for facilitating attachment of intercostal musculature, although the shafts are relatively slender and elongated.
This differs from the extant condition in which ribs are expanded to facilitate the stability of the thoracic cavity, or expand the surface area for facilitating intercostal musculature for functional flight (Fig. 78). Although the ribs on Chaoyangia are extremely elongated, facilitation for intercostal musculature is rather weak. Obviously, in order to fortify the thoracic chamber, Chaoyangia ossified its uncinate processes, which have become elongated to surpass the intercostal vacuities and almost attain the next anterior rib, resembling the condition of extant taxa. The uncinate processes are posteriorly oblique; they have an extremely expanded base and an acute termination, resembling an elongated cone. Two to four uncinate processes are present that are two-thirds the length of the rib shaft itself.
It is extremely noteworthy that Chaoyangia maintains ossified gastric ribs, and although they are neither as numerous nor as complete as the dorsal ribs, they are sufficient to indicate that the genus had a relatively derived thoracic cavity. The gastric ribs are preserved in natural configuration, with their termini in contact with the distal dorsal ribs. Both termini of the gastric ribs are not expanded as in the extant condition. Two additional gastric ribs lay isolated between the dorsal ribs.
In contrast to the derived thoracic cavity is the presence of an exceptionally primitive pelvic girdle resembling that of the Dinosauria. Elements on the right side are relatively well preserved. A basically complete reniform ilium has a relatively elongated preacetabular section that has been medially depressed due to compressional distortion. The postacetabular portion of the ilium is only slightly reduced and is extremely broadened. It has a relatively rounded terminus which differs distinctly from that of Cathayornis from the same locality. The acetabulum is small and circular and the entire dorsal wall is composed of the ilium, although its anterior portion is incomplete. The dorsal margin of the ilium is very slightly dorsally crescentic, lies lateral to the centra, and is not fused with the lumbar or sacral centra. The right pubis is complete but only the posterior portion of the left pubis is visible. Its shaft is relatively cylindrical and pneumaticized. The portion of the pubis that composes the anteroventral acetabulum, or the prepubic process, has been greatly reduced, representing only an extremely short pectineal process. The pubis extends posteriorly from the acetabulum; its distal end contacts its counterpart via a slight inflation. The distal region of contact is relatively elongated, being 17 mm in length. This condition resembles that of Archaeopteryx although it lacks a dorsally hooked process.
The right ischium is also relatively complete and forms an irregularly shaped linear element that is broader than the pubis and slightly resembles the condition in Archaeopteryx. The ischium extends posteroventrally between the two pubes and has an undulating expansion. Its distal end does not reach the medial margin of the distal pubes and terminates to become narrow and acute. The proximal end forms the posteroventral portion of the acetabulum. Its ventral margin is a dorsally arched embayment, and it differs from Cathayornis by lacking a dorsal process. All three pelvic elements are unfused and do not bear an intimate relationship with the sacral vertebrae. Thus in this respect, it closely resembles Archaeopteryx. Although the hindlimb preserves only the femur and a portion of the tibiotarsus, it appears to be relatively robust.
Both femora are preserved, with the one on the right side being nearly perfect. The right shaft is relatively straight but the left is slightly curved. The proximal end bears a relatively high trochanteric crest, and a shallow trochanteric fossa. The femoral head is relatively large and projects relatively distinctly medially, although a neck is basically absent. The fourth trochanter has become lost. The shaft is more slender than the proximal end; it has a pneumatic foramen on the proximolateral side, is highly pneumaticized, and has relatively thin walls.
Distally, the rotular groove is relatively expanded and a scar for ligament attachment is relatively distinct on the medial side of the medial condyle. Whether or not an attachment point to facilitate flexor musculature is present on the lateral side of the lateral condyle is unknown due to being obscured. The distal end is slightly concave, quite distinct from the extant condition. The distal end of the left femur is not preserved; the element has been shifted in position to lie posteroventral to the ilium, and overlies the proximal left ischium and pubis.
The right distal tibiotarsus is missing and the left side only preserves a portion of its proximal end. Like the femur, it is relatively robust, the shaft has a slight curvature, and it possesses the distinct character of a relatively well developed medial crest resembling the extant condition of projecting medially, however, its extension to the proximal surface is vague. Proximally, it is represented by a single very slightly concave articular surface that has yet to become divided into two facets; this is a direct reflection of its articular relationship with the rather planar distal femur. There is a slight proximolateral expansion but a distinct lateral cnemial crest is absent. Only a slender and short right fibula is preserved in addition to an extremely short portion of the right tarsometatarsus, but it cannot be determined whether it resembles Cathayornis. The tarsals appear to have become fused to compose a cap on the shaft of the fused tarsometatarsus.
Measurements of Chaoyangia beishanensis (mm).
Vertebral column preserved length 88.0
Second to last right rib length ~36.0
Second to last right uncinate process length 8.0
Second to last right proximal
rib to uncinate process length 25.0
Third to last right rib length 40.0
Longest right uncinate process 11.1
Fourth to last right rib length 37.5
Ilium length 32.0
Pubis length ~51.0
Distal pubis contact length 17.0
Ischium length 30.0
Femur length 45.0
Femur proximal breadth 9.5
Femur distal breadth 7.2
Tibiotarsus preserved length 5.3
Tibiotarsus proximal breadth 8.5
A cursory observation of the Chaoyangia specimen would cast doubt upon its veracity as a member of Aves because the skull, forelimb, and sternum are absent, hindlimb is incomplete, there are no feather impressions, and the pelvic girdle is extremely primitive. However, careful examination of the specimen documents fundamental avian characteristics that alleviate all suspicions. These include the relatively thin-walled and highly pneumaticized long bones, the presence of ossified uncinate processes, preacetabular region of the ilium exceeds the length of the postacetabular region, the short prepubic process is represented as the avian pectineal process, pubis is distinctly posteriorly oblique, femur has a relatively high trochanteric crest, and tibiotarsus maintains a well developed medial cnemial crest. The characters documented above indicate that Chaoyangia has a modern avian morphology in addition to retaining primitive characters, excluding its assignment to any known avian orders. Although numerous specimens are produced from the same quarry that are also small and retain primitive characters, none displays characters as apomorphic as Chaoyangia, which include the presence of uncinate processes on the ribs, resembling the condition of Hesperornis, Ichthyornis, and other extant carinate birds, in addition to the presence of ossified gastric ribs that probably indicates a stabilized thoracic cavity, thinly walled and pneumaticized long bones, sacral vertebrae exceed eight in count, femur is enlarged with the fourth trochanter lost, and distalend bears depressions for facilitating ligament attachment; and a medial cnemial crest is present on the
tibiotarsus.
Plesiomorphic characters retained by Chaoyangia include the non-heterocoelous cervical centra, unfused lumbar and sacral centra, and unfused pelvic girdle. Its autapomorphic characters include a medial longitudinal groove on the proximal dorsal ribs, apparently for the facilitation of intercostal musculature, the reniform ilium with a relatively large discrepancy between preacetabular and postacetabular length, and a rounded posterior end; and a pneumaticized pubic shaft with an elongated contact of the distal pubis.
Chaoyangia shares plesiomorphic characters with the Sauriurae including the nonheterocoelous vertebral centra, sacral girdle is not fused, and pubis is posteriorly oblique with fused termini. The most notable characters shared with extant taxa to the exclusion of the Sauriurae include the presence of uncinate processes on the dorsal ribs; ossified, elongated, and pneumaticized gastric ribs; and a well developed cnemial crest on the tibiotarsus. Thus, Chaoyangia is justifiably assigned to the Ornithurae.
Relatively contemporaneous taxa include not only the Chinese Confuciusornis, Liaoningornis, Jibeinia, Sinornis, Cathayornis, and Gansus, but also Archaeopteryx, the Mongolian Ambiortus, and the European Enaliornis, Limnornis, Noguerornis, Concornis, and Iberomesornis. The Late Jurassic Confuciusornis and Archaeopteryx represent the most primitive members of Aves to date, sharinga a similar pelvic girdle morphology of a thin and narrow postacetabular region which differs greatly from that ofChaoyangia. Also, the confuciusornis ischium is short and broad with an expansive terminus and a dorsal process which also differs markedly from that of Chaoyangia. The Confuciusornis pubis differs by being relatively thin and weak with an unexpanded terminus.
Liaoningornis is the most recently erected small genus derived from the same stratigraphic level as Confuciusornis, but it compares poorly to Chaoyangia. Although it maintains a well developed carina, it still retains particularly primitive characters such as the presence of a presternum, which differ greatly from Chaoyangia. The genus Jibeinia occupies an extremely significant position in avian phylogeny. It is produced from latest Jurassic sediments and represents a transitional phase between the morphologies of Confuciusornis-Archaeopteryx and the diverse assemblage of Early Cretaceous genera. Its most notable characters lie in its wing and sternum morphology. Obviously it could not have maintained an advanced rib morphology or derived condition of the tibiotarsus such as on Chaoyangia. Despite Sinornis and Chaoyangia being derived from different localities, they are both produced from the Early Cretaceous Jiufotang Fm. and share several characters such as an unfused pelvic girdle, although other pelvic characters are quite distinct, (e.g. Sinornis possesses a curved pubic boot) and a small cnemial crest on the tibiotarsus, it lacks uncinate processes on the ribs, and its gastric ribs lie distant from the dorsal ribs.
Cathayornis is an archaic form that is smaller than a sparrow-sized passerid, is produced from the same quarry as Chaoyangia, and differs in its absence of uncinate processes on its ribs, a smaller acetabulum, and a smaller and more narrow postacetabular region of the ilium. The Mongolian genus Ambiornis represents an archaic member of carinate birds, approaches the size of a corvid, is represented only by a forelimb, a portion of vertebrae, and sternum, and is diagnosed with a distinct carina and extremely well developed proximomedial and lateral tuberosities on its humerus. It is regrettable that the corresponding counterparts of Chaoyangia are not preserved.
Noguerornis, Concornis, and Iberomesornis are all produced from the Early Cretaceous of Spain. The former is derived from limestones at the locality of Montsec in Laerido Province, while the latter two are produced from limestones at Las Hoyas, Cayenca Province. Noguerornis is principally represented by a forelimb, partial vertebrae, furcula, and a fragmentary tibia. Thus it cannot be compared to Chaoyangia, although it represents an evolutionary stage that closely approaches that of Sinornis and Cathayornis. The two latter genera are represented by incomplete skeletons; Iberomesornis is more complete. Although it lacks a skull, the remaining skeleton is complete, resembling Cathayornis by being small and primitive but with relatively derived forelimb characters, although talons are retained. Its ribs lack uncinate processes, sacral girdle and distal tibiotarsus are not fused, a relatively long pygostyle is present, preacetabular portion of the ilium is elongated and broad, posterior portion is reduced and relatively narrow, and only five sacral centra are present. These features distinguish it distinctly from Chaoyangia.
The genus Limnornis is produced from the Early Cretaceous of Romania and is represented only by a right distal humerus, several right metacarpals, and a portion of the left femur which has a projected distolateral condyle, a rounded medial condyle, a deep popliteal fossa, and a narrow intercondylar fossa, indicating a more derived condition than Chaoyangia. It is regrettable that its ribs are not preserved for a more complete comparison. It is appropriate here to conduct a comparison of several morphological aspects of Archaeopteryx, Confuciusornis, and Chaoyangia. There is a relatively large size discrepancy between the current specimens of Archaeopteryx. Chaoyangia approaches the size of the Berlin specimen and further resembles it by having amphicoelous vertebral centra and relatively elongated lateral centra depressions or pleurocoels. It is noteworthy that on the cast of the London specimen presented to IVPP, there is a rib that resembles the condition on Chaoyangia, where a distinctly elongated groove lies on the dorsal surface, although the ribs are relatively short and no gastric ribs are noted, indicating its more primitive condition. The presence of this groove, however, indicates that Archaeopteryx possessed a rather stabilized intercostal musculature system. The dorsal ribs of Chaoyangia are elongated with relatively long uncinate processes and distal ends that are in contact with the gastric ribs. The interconnection of intercostal uncinate processes is a further adaptation for flight and reinforces the rigid thoracic lattice structure, representing the most conspicuous apomorphy of the genus. The thoracic condition of Confuciusornis resembles Archaeopteryx but its ribs are more slender and longitudinal grooves are shallower.
Chaoyangia represents the most archaic avian form with uncinate processes on the ribs. Within the evolution of the avian thoracic cavity, the development of the uncinate process is extremely significant. Thus a comparison of uncinate processes is conducted between Mesozoic and extant genera to attempt to decipher an evolutionary pattern. Figure 81A, B, and C illustrates Mesozoic forms, in which it may be noted that the uncinate processes are all located distally on the ribs. On the extant forms the anterior uncinate processes are positioned slightly distal but do not exceed the point two-thirds the length of the rib. Posteriorly along the rib cage the uncinate processes migrate dorsally; the one on the terminal rib is dorsal to the midpoint of the shaft. Rib morphology is based upon differential habitat adaptations. Hesperornis and other extant natatorial birds maintain relatively robust ribs, with uncinate processes that are particularly broadened at their termini. However, other extant taxa display uncinate processes in which the base is the broadest. Wading birds possess ribs that are narrow and elongated with proximoventral pneumatic foramina; thus Ichthyornis rib morphology more closely resembles that of wading birds. Raptor rib morphology is broad and thin with numerous pneumatic foramina from the proximal end to the midportion of the rib. Dense forest taxa generally maintain a fortified rib that is moderate in thickness but is particularly strong in morphology with only a small quantity or a single pneumatic foramen at the proximal end. Chaoyangia most closely approaches Ichthyornis in morphology and thus resembles a wading bird, although no extant taxa have the longitudinal groove on the ribs such as on Chaoyangia.
Several reptiles tat maintain uncinate processes on their ribs, including the genus Sphenodon and members of the Crocodylia, although these are cartilaginous and not fully ossified. Ostrom (1960) states that Dromaeosaurus also possesses uncinate processes but these are probably a homoplasy. The pelvic girdle on the fossil specimens indicates the following evolutionary trends:
(1) The preacetabular portion of the ilium becomes extended concurrent with the increase of sacral fusion, while the postacetabular portion becomes reduced as an adaptation for the shortened caudal series and to reduce weight for the flight load.
(2) The ischium becomes an irregularly shaped thin plate that extends and expands posteriorly to gradually approach the ilium and as in the extant condition, to finally fuse with the ilium. The ischium of Chaoyangia is longer and thinner than Confuciusornis or Archaeopteryx in addition to lacking a proximal ischial fenestra.
(3) The prepubic process becomes reduced with the entire pubis becoming posteriorly elongated to form the longest and thinnest element on the pelvic girdle as in the extant condition.The distal pubes eventually diverge whereas the distal ischia become fused. The Chaoyangia pubis is not only longer than Confuciusornis and Archaeopteryx, it is pneumaticized, which is a highly autapomorphic condition. The pubes on Archaeopteryx and Confuciusornis are distinct.
(4) The hindlimb of Chaoyangia is more derived than its predecessors and distinctly closer to the morphology of later forms; its femur resembles those of ground dwellers. The femoral head on Confuciusornis and Archaeopteryx is undeveloped and the tibia retains the reptilian condition. Early Cretaceous avian specimens from China and other regions of the world share numerous characters with their Late Jurassic predecessors in addition to maintaining more derived characters. Chaoyangia is the most characteristic and derived of these genera due to its thoracic construction, which provides the most effective adaptability for flight among all the Early Cretaceous species.
Source: Polyglot Paleontologist