[D] Gansus yumenensis [~/~]
Describer
Hou & Liu Z., 1984
Time
Cretaceous Early
Classification
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Ornithurae
Fossilsite
Xiagou Formation, Gansu Province, China
Info
The description of Gansus yumenensis in 1980 constituted the first record of Mesozoic birds in China, initiated the study of paleornithology in the country, and aroused extensive attention in the academic realm. At that time, with the exception of Archaeopteryx, Gansus represented the oldest record of Aves. Furthermore, its morphology reflects adaptations for a riparian or aquatic habitat although its unguals are autapomorphic and to date workers are still perplexed by their morphology.
Specimen
A portion of a left hindlimb including a distal tibiotarsus, complete tarsometatarsus and digits with unguals. (IVPP specimen V6862).
Locality and stratigraphic age: Gray-black mudstones of the Early Cretaceous middle Xiagou Fm., northwest of the village of Chenjiawan, Changma perfecture, Yumen Municipality, Gansu Province.
Ammended Diagnosis
A small form with recurved talons maintaining well developed flexor tuberosities: digits that are slender, elongated, and composed of phalanges with inflated termini; depressions are present to facilitate the tendons of the Flexor longus digitorum; and digit IV is the longest in the series.
The tarsometatarsus is slightly shorter than the longest digit, is slightly laterally compressed incompletely fused distally, and a distal haemal foramen is well developed and placed nearly at the terminal end within the intertrochlear recess. Mt IV is shorter than MtIII. Proximally the sulcus for the extensor musculature is shallow and small, posterior to which there are no well developed calcaneal ridges or tendinal canals, but on the proximal surface there is a pronounced crest separating the anteroposteriorly elongated cotyles.
On the distal tibiotarsus a supratendinal bridge is absent, the medial condyle is particularly narrow with an acute lateral margin, the lateral condyle is particularly broadened, and the medial side of the posterior intercondylar groove is more excavated.
Summary
In August, 1983, the paleoichthyologists Fengchen Ma, Zhicheng Liu, and Shaochu Huang were conducting Mesozoic research in the Changma region of Yumen Municipality. During their excavations, Professor Liu uncovered a hindlimb, whereupon all expedition members recognized it as an extremely significant specimen of a bird and not that of a pterosaur. Professor Ma took particular care to preserve the specimen and presented it to Lanzhou University where L.H. Hou provided a diagnosis. At that time Hou was visiting the Department of Biology where he was conducting joint research with the anatomist Linyu Cong, dissecting a specimen of Alligator sinensis.
After a careful analysis of the specimen, he identified it as avian on the basis of several aspects: The distal tibia of a pterosaur has two extremely small articular condyles that are not laterally expanded; they articulate with two tarsals and lack trochlear depressions. Also, the distal hindlimb of pterosaurs maintains five fully formed tarsals, and as in other reptiles, displays five anteriorly directed metatarsals that are approximately four times the length of the digits. This is completely distinct from the avian tarsometatarsal condition.
Gansus maintains numerous autapomorphic features, among which are several characteristic of the avian condition. These include the distal tibia with two differentially sized condyles, tarsals have all become fused with the tibia and metatarsals to compose a tibiotarsus and tarsometatarsus, the metatarsals have varying lengths; MtIII trochlea is the longest, only four digits are present, and the pollex is in opposition to the other three digits.
Only the distal tibiotarsus is preserved with both condyles complete, the lateral side of which is slightly longer. In anterior perspective a supratendinal bridge is absent, the distal shaft is relatively flat, and lacks any depressed regions; thus it represents the plesiomorphic condition. The two condyles are relatively distinct from each other. The lateral condyle is distinctly larger, its dorsal margin is quite distinctly delineated from the shaft, its articular surface is reduced, its lateral margin is rather rounded, and it is not laterally expanded. The medial condyle is distally extended and medially oblique with an anteriorly projected anterior margin that composes an obtuse angle with the shaft. Its posterodorsal margin is particularly low and only reaches a point just proximal to the midline of the lateral condyle.
The intercondylar depression is V-shaped and distinctly medially inclined. The posterior trochlea is predominantly laterally inclined. A characteristic feature of the medial condyle is a thinly crescentic depression at its midpoint, anterodorsal to which a thick inflation extends to the dorsal margin of the condyle. The lateral condyle is slightly convex with a small thickened inflation at its midpoint and a transverse depression between the dorsal margin of the condyle and the shaft. There is an oblique truncated surface on the shaft above the condyles and the shaft is elliptical in cross-section with relatively thick walls.
Gansus yumenensis tibiotarsus measurements (mm).
Preserved length 9.5
Medial and lateral condyles lateral breadth 4.4
Medial and lateral condyles anterior breadth 4.4
Trochlea posterior breadth 4.0
Medial condyle anteroposterior length 4.5
Medial condyle anterior process height 2.7
Lateral condyle anteroposterior length 4.2
Lateral condyle maximum length 3.4
Distal breadth 3.0
Anteroposterior length 3.5
The tarsometatarsus is complete and basically fused with a straight shaft that is very weakly anteriorly concave and posteriorly convex at its midpoint. The lateral side has been subjected to compressional distortion such that it is medially depressed, and a portion of its wall is damaged. The articulation on digit I with the tarsometatarsus is not very distinct. The proximal surface of the tarsometatarsus maintains distinct medial and lateral cotyles lying at different levels. The medial cotyle is relatively large, shallow, positioned relatively high, is nearly rectangular in anteroposterior orientation, and there is an anteroposteriorly directed fissure at its midpoint. The lateral cotyle projects laterally from the shaft, is positioned slightly lower, and is relatively flattened with a linear lateral margin. A relatively pronounced rounded crest between the two cotyles represents a vestigial metatarsal boundary line.
Gansus yumenensis tarsometatarsus measurements (mm).
Length 31.6
Proximal transverse breadth 4.3
Proximal anteroposterior breadth 4.2
MtIII trochlea breadth 4.2
MtIII anteroposterior breadth 4.2
MtIV trochlea breadth 3.1
In anterior perspective the tarsometatarsus has been slightly laterally distorted but it may still be inferred that the anterior surface of the shaft is relatively flat, or slightly depressed, based upon the relatively short and shallow anterodorsal metatarsal groove and the very slightly convex distal end. Proximally, there is a distinct but small and shallow intercotylar fossa. A proximal haemal foramen is either absent or obscured due to compressional distortion, and the tuberosity for the Tibialis anticus is low and medially situated.
Approaching the distal end the shaft becomes slightly arched but lacks a conspicuous depression on the shaft proximal to the trochleae. The trochleae are not expanded. MtIII is the longest with a broad articular surface and a terminus with a relatively deep medial groove that extends to the medial and lateral sides, and there are well developed lateral fossae for ligament attachment. MtII is the shortest, posteriorly obliquely inclined, and positioned higher than in any known avian taxon. Its terminus reaches only to the base of the MtIII tochlea. A relatively small transverse groove is also present at its terminus, there is a small acute process directed posteriorly, and a pair of slightly small lateral ligament fossae are present, the lateral of which is larger. MtIV is incomplete at its distal end but is tightly fused to the lateral side of MtIII. It is slightly gracile, slender, and shorter than its lateral counterpart.
The Gansus pedal phalangeal formula is 2-3-4-5. The digits are distinctly slender and elongate with the lengths of digits III and IV exceeding that of the tarsometatarsus. The proximal phalanges are all extremely expanded and have relatively deeply excavated articular surfaces, the distal ends are dorsoventrally compressed, and there are lateral ligament fossae on all phalanges. The four unguals are slightly compressed and recurved; they have well developed flexor tuberosities at their bases, particularly on digits II and IV. Ventrally on the unguals is an acute process that is undoubtedly intimately related to habitat adaptation. Furthermore, there are relatively deep lateral ligament grooves on the talons. Among the four talons, that on digit II is relatively large while the remaining are basically consistent in size.
Gansus yumenensis digit measurements (mm).
Digit I phalanx I length 8.4
Digit I ungual length 4.1
Digit II phalanx I length 10.1
Digit II phalanx II length 11.5
Digit II ungual length 5.0
Digit III phalanx I length 13.0
Digit III phalanx II length 10.8
Digit III phalanx III length 8.0
Digit III ungual length 5.0
Digit IV phalanx I length 11.1
Digit IV phalanx II length 8.6
Digit IV phalanx III length 8.4
Digit IV phalanx IV length 7.5
Digit IV ungual length 4.0
Discussion
From one aspect, Gansus displays derived characters of extant forms, but it also retains several primitive characters in addition to autapomorphic characters. It represents an archaic avian evolutionary phase. Consequently, it was initially erected within a new order to represent a transitional phase within Aves. Currently, there is abundant supplemental Early Cretaceous specimens but these do not diminish the significant phylogenetic position represented by Gansus and to date supplemental taxa or specimens resembling Gansus are still absent. In 1996, correspondence was received from Richard C. Fox, from the University of Alberta, Canada, stating that he recovered a tarsometatarsus from the Early Cretaceous of northwest Alberta that may be comparable to Gansus, thereby suggesting the potential extensive distribution range of this genus 130 million years ago.
Gansus shares the following characters with extant taxa: distal tibia has become modified into a tibiotarsus, the metatarsals and portion of the tarsals have become fused to compose a tarsometatarsus with concave proximal medial and lateral cotyles, a conspicuous distal haemal foramen, and digit I lies in opposition to the other three digits. These characters are also more derived than the homologues on the abundant specimens from the Early Cretaceous Jiufotang Formation. in the Chaoyang region of Liaoning Province, on which concave proximal medial and lateral cotyles are indistinct, the distal tarsometatarsi are all unfused, and a haemal foramen is absent.
However, Gansus retains the plesiomorphic characters of the absence of a supratendinal bridge on the tibiotarsus, the presence of which is a synapomorphy shared among archaic birds; also on the proximal tarsometatarsus a haemal foramen is absent, as is a tuberosity for the Tibialis anticus, and a canal for the flexor tendons. Their symplesiomorphic characters are shared with taxa from the Chaoyang region and are more primitive than the conditions in Ichthyornis and Hesperornis.
Autapomorphic characters of Gansus include the following: (1) Tibiotarsus with thick walls, distal medial and lateral condyles are extremely distinct from each other with the lateral being broad and enlarged and the medial being narrow and medially oblique, and its anterior margin is anteriorly projected to compose a obtuse angle with the shaft.
The anterior surface of the tibiotarsus is flattened; there is a thinly crescentic trough on its medial side, and a very slightly convex lateral side with a small inflation at its midpoint. The tibiotarsus is shorter than digits III and IV, sizes of the medial and lateral cotyles are vastly distinct and its cotyles are separated by a high longitudinal crest. The anterodistal shaft of the tibiotarsus and the extended tubular-shaped Mt III trochlea differ from the anserine condition of being thin and flat or concave; nor does it resemble the arched condition of riparian taxa, but more closely approaches the morphology of the Charadriidae.
MtII is positioned particularly high and is located quite distant from MtIII. Digit I is relatively elongated and is vastly distinct from the condition in extant taxa, in which this digit generally tends to be reduced. Digit IV is the longest in the series, exceeding the length of digit III, which resembles the condition of several extant taxa including the pelicans. Gansus occupies a significant position in the evolution of archaic birds and is quite distinct not only from extant taxa but from Early Cretaceous taxa including those from Spain, Ambiortus from Mongolia, and all the taxa from the Chaoyang region of Liaoning Province. Gansus shares a tarsometatarsal character with Enaliornis from the Early Cretaceous marine deposits of the United Kingdom: MtIV is longer then its MtIII. Enaliornis is regarded as having an intimate relationship with Hesperornis, both being typical diving forms. This then indicates either a shared behavioral or habitat adaptation or a definite phylogenetic relationship among the three.
The autapomorphic characters of Gansus distinguish it from all the Liaoning taxa regardless of some of their morphology reflecting an adaptation for riparian habitats. Additional distinctions of Gansus lie in its talons, which have well enveloped flexor tuberosities, and that its digit IV is the longest in the pes. Moreover, the long bones on the Chaoyang taxa all differ by being relatively thin-walled. Worthy of note is that the talon morphology of Gansus is extremely close to the charadriid genus Scalopax, which also maintains particularly projected flexor tuberosities on the ventral aspect of its five talons.
But on Gansus the tuberosities on digits I and II are not as well developed, which may imply that it was fully adapted to a riparian habitat. In addition to the ungual flexor tuberosity of digit three, which resembles that of Scolopax, the degree of talon curvature is also nearly identical, the hallux is shortened, and the tarsometatarsus is similar in morphology. Thus, it would appear that Scolopax is the closest relative to Gansus and that the charadriid lineage is descended from this genus.
Obviously, the hiatus of over one hundred million years between these two genera would certainly result in morphological discrepancies, as exemplified by the relatively primitive nature of the Gansus tarsometatarsus that is slightly shorter than the longest digit, digits being relatively slender and weak, presence of a relatively deep sulcus for the Flexor longus digitorum, and digit IV being the longest. These plesiomorphic characters distinguish it from later avian forms, which underwent a trend of gradual adaptive modifications.
For instance, the reason the sulcus for the Flexor longus digitorum is deeper than on Scolopax is that the archaic avian muscular system was not completely developed for fully functional flight, whereas in extant Aves are not only fully adapted for functional flight, but is also fully adapted for activities including perching, predation, terrestrial mobility, wading, swimming, and diving. Furthermore, the digits on Gansus are long, slender, and weak, whereas the charadriid forms, although they lack webbed feet, have digits that are relatively robust and fortified for mobility in regions such as beaches or paludal habitats, and thus the majority of taxa display a tendency to reduce pedal digit I.
Some species retain only three anterior digits, and the occasional charadriid evolves webbed feet. Scolopax has relatively thick ventral ‘dermal cushions’ on its pes which further facilitates floatation and terrestrial surface contact. This adaptation is absent on Gansus. Its phalangeal articular surfaces are not as well developed, its digits are slender and weak, and even though digit IV is elongated, this is not sufficient justification for the interpretation of webbing.
Among those extant taxa with an elongated digit IV none maintains webbed feet, as exemplified by Hydrophasianus. But the elongated digit IV and slender and long digits on Gansus resemble Hydrophasianus, further indicating a close descendant relationship to the Charadriidae. However, Hydrophasianus has a distinct autapomorphy in that its opposable digit I ungual is relatively elongated, whereas in Gansus the ungual of digit I is only half the length of its associated phalanx.
Source: Polyglot Paleontologist
Hou & Liu Z., 1984
Time
Cretaceous Early
Classification
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Ornithurae
Fossilsite
Xiagou Formation, Gansu Province, China
Info
The description of Gansus yumenensis in 1980 constituted the first record of Mesozoic birds in China, initiated the study of paleornithology in the country, and aroused extensive attention in the academic realm. At that time, with the exception of Archaeopteryx, Gansus represented the oldest record of Aves. Furthermore, its morphology reflects adaptations for a riparian or aquatic habitat although its unguals are autapomorphic and to date workers are still perplexed by their morphology.
Specimen
A portion of a left hindlimb including a distal tibiotarsus, complete tarsometatarsus and digits with unguals. (IVPP specimen V6862).
Locality and stratigraphic age: Gray-black mudstones of the Early Cretaceous middle Xiagou Fm., northwest of the village of Chenjiawan, Changma perfecture, Yumen Municipality, Gansu Province.
Ammended Diagnosis
A small form with recurved talons maintaining well developed flexor tuberosities: digits that are slender, elongated, and composed of phalanges with inflated termini; depressions are present to facilitate the tendons of the Flexor longus digitorum; and digit IV is the longest in the series.
The tarsometatarsus is slightly shorter than the longest digit, is slightly laterally compressed incompletely fused distally, and a distal haemal foramen is well developed and placed nearly at the terminal end within the intertrochlear recess. Mt IV is shorter than MtIII. Proximally the sulcus for the extensor musculature is shallow and small, posterior to which there are no well developed calcaneal ridges or tendinal canals, but on the proximal surface there is a pronounced crest separating the anteroposteriorly elongated cotyles.
On the distal tibiotarsus a supratendinal bridge is absent, the medial condyle is particularly narrow with an acute lateral margin, the lateral condyle is particularly broadened, and the medial side of the posterior intercondylar groove is more excavated.
Summary
In August, 1983, the paleoichthyologists Fengchen Ma, Zhicheng Liu, and Shaochu Huang were conducting Mesozoic research in the Changma region of Yumen Municipality. During their excavations, Professor Liu uncovered a hindlimb, whereupon all expedition members recognized it as an extremely significant specimen of a bird and not that of a pterosaur. Professor Ma took particular care to preserve the specimen and presented it to Lanzhou University where L.H. Hou provided a diagnosis. At that time Hou was visiting the Department of Biology where he was conducting joint research with the anatomist Linyu Cong, dissecting a specimen of Alligator sinensis.
After a careful analysis of the specimen, he identified it as avian on the basis of several aspects: The distal tibia of a pterosaur has two extremely small articular condyles that are not laterally expanded; they articulate with two tarsals and lack trochlear depressions. Also, the distal hindlimb of pterosaurs maintains five fully formed tarsals, and as in other reptiles, displays five anteriorly directed metatarsals that are approximately four times the length of the digits. This is completely distinct from the avian tarsometatarsal condition.
Gansus maintains numerous autapomorphic features, among which are several characteristic of the avian condition. These include the distal tibia with two differentially sized condyles, tarsals have all become fused with the tibia and metatarsals to compose a tibiotarsus and tarsometatarsus, the metatarsals have varying lengths; MtIII trochlea is the longest, only four digits are present, and the pollex is in opposition to the other three digits.
Only the distal tibiotarsus is preserved with both condyles complete, the lateral side of which is slightly longer. In anterior perspective a supratendinal bridge is absent, the distal shaft is relatively flat, and lacks any depressed regions; thus it represents the plesiomorphic condition. The two condyles are relatively distinct from each other. The lateral condyle is distinctly larger, its dorsal margin is quite distinctly delineated from the shaft, its articular surface is reduced, its lateral margin is rather rounded, and it is not laterally expanded. The medial condyle is distally extended and medially oblique with an anteriorly projected anterior margin that composes an obtuse angle with the shaft. Its posterodorsal margin is particularly low and only reaches a point just proximal to the midline of the lateral condyle.
The intercondylar depression is V-shaped and distinctly medially inclined. The posterior trochlea is predominantly laterally inclined. A characteristic feature of the medial condyle is a thinly crescentic depression at its midpoint, anterodorsal to which a thick inflation extends to the dorsal margin of the condyle. The lateral condyle is slightly convex with a small thickened inflation at its midpoint and a transverse depression between the dorsal margin of the condyle and the shaft. There is an oblique truncated surface on the shaft above the condyles and the shaft is elliptical in cross-section with relatively thick walls.
Gansus yumenensis tibiotarsus measurements (mm).
Preserved length 9.5
Medial and lateral condyles lateral breadth 4.4
Medial and lateral condyles anterior breadth 4.4
Trochlea posterior breadth 4.0
Medial condyle anteroposterior length 4.5
Medial condyle anterior process height 2.7
Lateral condyle anteroposterior length 4.2
Lateral condyle maximum length 3.4
Distal breadth 3.0
Anteroposterior length 3.5
The tarsometatarsus is complete and basically fused with a straight shaft that is very weakly anteriorly concave and posteriorly convex at its midpoint. The lateral side has been subjected to compressional distortion such that it is medially depressed, and a portion of its wall is damaged. The articulation on digit I with the tarsometatarsus is not very distinct. The proximal surface of the tarsometatarsus maintains distinct medial and lateral cotyles lying at different levels. The medial cotyle is relatively large, shallow, positioned relatively high, is nearly rectangular in anteroposterior orientation, and there is an anteroposteriorly directed fissure at its midpoint. The lateral cotyle projects laterally from the shaft, is positioned slightly lower, and is relatively flattened with a linear lateral margin. A relatively pronounced rounded crest between the two cotyles represents a vestigial metatarsal boundary line.
Gansus yumenensis tarsometatarsus measurements (mm).
Length 31.6
Proximal transverse breadth 4.3
Proximal anteroposterior breadth 4.2
MtIII trochlea breadth 4.2
MtIII anteroposterior breadth 4.2
MtIV trochlea breadth 3.1
In anterior perspective the tarsometatarsus has been slightly laterally distorted but it may still be inferred that the anterior surface of the shaft is relatively flat, or slightly depressed, based upon the relatively short and shallow anterodorsal metatarsal groove and the very slightly convex distal end. Proximally, there is a distinct but small and shallow intercotylar fossa. A proximal haemal foramen is either absent or obscured due to compressional distortion, and the tuberosity for the Tibialis anticus is low and medially situated.
Approaching the distal end the shaft becomes slightly arched but lacks a conspicuous depression on the shaft proximal to the trochleae. The trochleae are not expanded. MtIII is the longest with a broad articular surface and a terminus with a relatively deep medial groove that extends to the medial and lateral sides, and there are well developed lateral fossae for ligament attachment. MtII is the shortest, posteriorly obliquely inclined, and positioned higher than in any known avian taxon. Its terminus reaches only to the base of the MtIII tochlea. A relatively small transverse groove is also present at its terminus, there is a small acute process directed posteriorly, and a pair of slightly small lateral ligament fossae are present, the lateral of which is larger. MtIV is incomplete at its distal end but is tightly fused to the lateral side of MtIII. It is slightly gracile, slender, and shorter than its lateral counterpart.
The Gansus pedal phalangeal formula is 2-3-4-5. The digits are distinctly slender and elongate with the lengths of digits III and IV exceeding that of the tarsometatarsus. The proximal phalanges are all extremely expanded and have relatively deeply excavated articular surfaces, the distal ends are dorsoventrally compressed, and there are lateral ligament fossae on all phalanges. The four unguals are slightly compressed and recurved; they have well developed flexor tuberosities at their bases, particularly on digits II and IV. Ventrally on the unguals is an acute process that is undoubtedly intimately related to habitat adaptation. Furthermore, there are relatively deep lateral ligament grooves on the talons. Among the four talons, that on digit II is relatively large while the remaining are basically consistent in size.
Gansus yumenensis digit measurements (mm).
Digit I phalanx I length 8.4
Digit I ungual length 4.1
Digit II phalanx I length 10.1
Digit II phalanx II length 11.5
Digit II ungual length 5.0
Digit III phalanx I length 13.0
Digit III phalanx II length 10.8
Digit III phalanx III length 8.0
Digit III ungual length 5.0
Digit IV phalanx I length 11.1
Digit IV phalanx II length 8.6
Digit IV phalanx III length 8.4
Digit IV phalanx IV length 7.5
Digit IV ungual length 4.0
Discussion
From one aspect, Gansus displays derived characters of extant forms, but it also retains several primitive characters in addition to autapomorphic characters. It represents an archaic avian evolutionary phase. Consequently, it was initially erected within a new order to represent a transitional phase within Aves. Currently, there is abundant supplemental Early Cretaceous specimens but these do not diminish the significant phylogenetic position represented by Gansus and to date supplemental taxa or specimens resembling Gansus are still absent. In 1996, correspondence was received from Richard C. Fox, from the University of Alberta, Canada, stating that he recovered a tarsometatarsus from the Early Cretaceous of northwest Alberta that may be comparable to Gansus, thereby suggesting the potential extensive distribution range of this genus 130 million years ago.
Gansus shares the following characters with extant taxa: distal tibia has become modified into a tibiotarsus, the metatarsals and portion of the tarsals have become fused to compose a tarsometatarsus with concave proximal medial and lateral cotyles, a conspicuous distal haemal foramen, and digit I lies in opposition to the other three digits. These characters are also more derived than the homologues on the abundant specimens from the Early Cretaceous Jiufotang Formation. in the Chaoyang region of Liaoning Province, on which concave proximal medial and lateral cotyles are indistinct, the distal tarsometatarsi are all unfused, and a haemal foramen is absent.
However, Gansus retains the plesiomorphic characters of the absence of a supratendinal bridge on the tibiotarsus, the presence of which is a synapomorphy shared among archaic birds; also on the proximal tarsometatarsus a haemal foramen is absent, as is a tuberosity for the Tibialis anticus, and a canal for the flexor tendons. Their symplesiomorphic characters are shared with taxa from the Chaoyang region and are more primitive than the conditions in Ichthyornis and Hesperornis.
Autapomorphic characters of Gansus include the following: (1) Tibiotarsus with thick walls, distal medial and lateral condyles are extremely distinct from each other with the lateral being broad and enlarged and the medial being narrow and medially oblique, and its anterior margin is anteriorly projected to compose a obtuse angle with the shaft.
The anterior surface of the tibiotarsus is flattened; there is a thinly crescentic trough on its medial side, and a very slightly convex lateral side with a small inflation at its midpoint. The tibiotarsus is shorter than digits III and IV, sizes of the medial and lateral cotyles are vastly distinct and its cotyles are separated by a high longitudinal crest. The anterodistal shaft of the tibiotarsus and the extended tubular-shaped Mt III trochlea differ from the anserine condition of being thin and flat or concave; nor does it resemble the arched condition of riparian taxa, but more closely approaches the morphology of the Charadriidae.
MtII is positioned particularly high and is located quite distant from MtIII. Digit I is relatively elongated and is vastly distinct from the condition in extant taxa, in which this digit generally tends to be reduced. Digit IV is the longest in the series, exceeding the length of digit III, which resembles the condition of several extant taxa including the pelicans. Gansus occupies a significant position in the evolution of archaic birds and is quite distinct not only from extant taxa but from Early Cretaceous taxa including those from Spain, Ambiortus from Mongolia, and all the taxa from the Chaoyang region of Liaoning Province. Gansus shares a tarsometatarsal character with Enaliornis from the Early Cretaceous marine deposits of the United Kingdom: MtIV is longer then its MtIII. Enaliornis is regarded as having an intimate relationship with Hesperornis, both being typical diving forms. This then indicates either a shared behavioral or habitat adaptation or a definite phylogenetic relationship among the three.
The autapomorphic characters of Gansus distinguish it from all the Liaoning taxa regardless of some of their morphology reflecting an adaptation for riparian habitats. Additional distinctions of Gansus lie in its talons, which have well enveloped flexor tuberosities, and that its digit IV is the longest in the pes. Moreover, the long bones on the Chaoyang taxa all differ by being relatively thin-walled. Worthy of note is that the talon morphology of Gansus is extremely close to the charadriid genus Scalopax, which also maintains particularly projected flexor tuberosities on the ventral aspect of its five talons.
But on Gansus the tuberosities on digits I and II are not as well developed, which may imply that it was fully adapted to a riparian habitat. In addition to the ungual flexor tuberosity of digit three, which resembles that of Scolopax, the degree of talon curvature is also nearly identical, the hallux is shortened, and the tarsometatarsus is similar in morphology. Thus, it would appear that Scolopax is the closest relative to Gansus and that the charadriid lineage is descended from this genus.
Obviously, the hiatus of over one hundred million years between these two genera would certainly result in morphological discrepancies, as exemplified by the relatively primitive nature of the Gansus tarsometatarsus that is slightly shorter than the longest digit, digits being relatively slender and weak, presence of a relatively deep sulcus for the Flexor longus digitorum, and digit IV being the longest. These plesiomorphic characters distinguish it from later avian forms, which underwent a trend of gradual adaptive modifications.
For instance, the reason the sulcus for the Flexor longus digitorum is deeper than on Scolopax is that the archaic avian muscular system was not completely developed for fully functional flight, whereas in extant Aves are not only fully adapted for functional flight, but is also fully adapted for activities including perching, predation, terrestrial mobility, wading, swimming, and diving. Furthermore, the digits on Gansus are long, slender, and weak, whereas the charadriid forms, although they lack webbed feet, have digits that are relatively robust and fortified for mobility in regions such as beaches or paludal habitats, and thus the majority of taxa display a tendency to reduce pedal digit I.
Some species retain only three anterior digits, and the occasional charadriid evolves webbed feet. Scolopax has relatively thick ventral ‘dermal cushions’ on its pes which further facilitates floatation and terrestrial surface contact. This adaptation is absent on Gansus. Its phalangeal articular surfaces are not as well developed, its digits are slender and weak, and even though digit IV is elongated, this is not sufficient justification for the interpretation of webbing.
Among those extant taxa with an elongated digit IV none maintains webbed feet, as exemplified by Hydrophasianus. But the elongated digit IV and slender and long digits on Gansus resemble Hydrophasianus, further indicating a close descendant relationship to the Charadriidae. However, Hydrophasianus has a distinct autapomorphy in that its opposable digit I ungual is relatively elongated, whereas in Gansus the ungual of digit I is only half the length of its associated phalanx.
Source: Polyglot Paleontologist