[D] Liaoningornis longidigitus ~/~
Cretaceous Early Barremian
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Ornithurae
Yixian Formation, Sihetun, Shangyuanzhen, Beipiao Co., Liaoning Province, China
Genus diagnosis: Sternum possesses a well developed carina, presternum is present, ribs are robust, distolateral condyle of the humerus is enlarged, femur head is particularly well developed, and distal condyles are enlarged, tarsometatarsus is only half the length of the tibiotarsus, and length of the medial pes digit (not including the ungual) exceeds the tarsometatarsus. Liaoningornis longidigitus Hou, 1996
An incomplete skeleton (positive and negative slabs) including a portion of the forelimb, pectoral girdle with sternum, ribs, and hindlimb (IVPP specimen V11303.2).
As for genus. Sternum is goblet-shaped, distal terminus of coracoid is expanded, tarsometatarsus is robust, and unguals are as long as to the most distal phalanges, which maintain autapomorphic lateral grooves.
Locality and Stratigraphic position
Base of the Late Jurassic Yixian Fm. at the village of Sihetun, town of Shangyuanzhen, Beipiao Co., Liaoning Province.
A majority of the skeletal elements is preserved on the positive slab (V11303-1). The sternum is complete and differs morphologically from those of extant taxa by being goblet-shaped with its anterior portion laterally expanded. It gradually constricts posteriorly and becomes most slender approaching the distal end in the pelvic region, though it again expands laterally at its terminus to abruptly compose two angles. Its posterior margin is slightly anteriorly embayed. A well developed carina, approximately three mm high, extends along the midline from the anterior to posterior margins but approaching the posterior margin it bifurcates into two crests. Liaoningornis differs from all known genera by the presence of a pair of vestigial presterna, which are rhomboid with a small mammary projection on their lateral angle that resembles the anterior processes of the sternum. The anterior margin of the presternum contacts the coracoid with an enlarged and deep articular groove. Only portions of a coracoid, a scapula, and a furcular process are preserved. The distal coracoid is expanded and its length cannot be determined. Both ends of the scapula are damaged but its shaft is relatively thick with a slight curvature. Between the two presterna there is a relatively long projection resembling a furcular process. This element is incomplete and may actually represent a vestigial interclavicle because in general a furcula is not as elongated as this element.
Ribs are incomplete and dispersed within the matrix. Those that are preserved are relatively large and broad although there are two slender ribs that may represent gastric ribs. Only the distal end of the humerus is preserved which has a slight amount of curvature, although the rest of the shaft is basically straight. The lateral condyle is enlarged and transversely expanded. The ulna is relatively robust and curved with medial and lateral cotyles lying on different planes. The radius is more slender than the ulna and has a straight shaft and a humera l cotyle. Distal ends of the radius and ulna are not preserved.
The femur is long with very slight curvature. The head is particularly well developed, there is conspicuous scarring to facilitate the attachment of the Teres ligament, and the medial condyle is enlarged with a depression for facilitating ligament attachment. The tibiotarsus is slightly longer than the femur with expanded termini, a relatively planar proximal end, well developed distal condyles (particularly the projected lateral condyle), and a lateral longitudinal trough which may represent a tendinal groove, although a supratendinal bridge is absent. The fibula is slender with a length half that of the tibiotarsus and a fused proximal end. Among the metatarsals preserved, MtIII is the longest and MtIV is not as slender as MtII. The pedal digits are slender and elongated; the length of the medial digit (excluding ungual) greatly surpasses that of the tarsometatarsus. A large lateral groove on each side of the phalanges for the unguals represents an autapomorphy not documented on any other species. Talons are large and acute with well developed lateral tendinal grooves, but the unguals are not very recurved and their lengths are equivalent to the phalanges they are articulated with.
The majority of the feathers have become carbonized and thus are obscure. Those on the ulna are still recognizable and are shorter than those on Confuciusornis and Archaeopteryx. Comparison and discussion: To date, Liaoningornis represents the only Late Jurassic taxon that possesses a carina, a presternum, and a forelimb with numerous characters which allow an assignment to the Ornithurae. It also represents the smallest body size of any avian fossil. Plesiomorphic characters shared with the Reptilia include the presence of a presternum and possibly an interclavicle. Jurassic members of the Sauriurae include only Confuciusornis and Archaeopteryx, which maintain not only numerous avian characters (with Confuciusornis being a little more derived) but also numerous reptilian characters, though neither share the autapomorphic characters of Liaoningornis, and consequently it is inappropriate to assign the latter genus to the family Sauriurae. From a comprehensive perspective, Liaoningornis is dominated by apomorphic characters, particularly in its well developed sternum and carina, which are vastly more derived and modified than on the contemporaneous Confuciusornis and suggest better flight capability.
Its hindlimb is also more derived and distinctly modified, providing a second justification for its axonomic assignment. Consequently, the retention of distinctly plesiomorphic characters does not exclude Liaoningornis from an assignment to the Ornithurae. The identification of a presternum is based an the following factors: It has an intimate relationship to the sternum by its sutural contact and it replaces the anterior process of the sternum with an anterolateral mammary process. At its anterior margin there is a deep transverse groove for the contact with the broad and circular coracoid. Furthermore, if this element was instead interpreted as another portion of the pectoral girdle, its location and morphology would suggest it to be the coracoid, currently recognized in association with the precoracoid or epicoracoid on extant taxa, and which would also be isolated and lie in opposition to the articular surface of the sternum. The developmental history of the presternum indicates that it becomes prominent in the early stages of the Reptilia and Mammalia. Later, what is regarded as the legitimate sternum is composed of the presternum, sternum, and xiphisternum. It is generally recognized that mature vertebrates that retain a presternum include members of the Mammalia in addition to the reptilian forms including Sphenodon, the Lacertilia, and Crocodylia, although recognition of this element is controversial as some interpret it in the Mammalia to have become modified into the manubrium while in the Lacertilia it is retained as an independent element. Its presence in the Crocodylia is also disputed, but some recognize it as the expanded anterior portion of the sternum which is in contact with the coracoid and procoracoid. On Liaoningornis it is distinct from the sternum in its lateral expansion. Thus its presence indicates an intimate relationship to the Reptilia, and within the context of avian origin and evolution represents a significant topic of discussion among ornithologists. The identification of a presternum on Liaoningornis undoubtedly represents a significant discovery in Ornithology.
Characters shared between Liaoningornis and volant birds include the presence of a well developed carina on the sternum, well developed medial and lateral cotyles on the proximal ulna, and a well developed femur head with scars facilitating the attachment of the Teres ligament. Plesiomorphic characters include the presence of a presternum, sternum as a thickened plate that lacks lateral processes, proximal tibiotarsus lacking a cnemial crest and distal end lacking a supratendinal bridge, distal tarsometatarsus is incompletely fused, and radius is relatively robust. Autapomorphic characters of Liaoningornis include the extreme lateral expansion of the distal humerus with an enlarged lateral condyle, similar diameters of radius and ulna shafts, distinct morphology of the sternum with the presence of a presternum, relatively elongated femur which is only slightly shorter than the tibiotarsus, and a tarsometatarsus shortened to one-half the length of the tibiotarsus.
Liaoningornis was excavated from the same locality that produced the new species of Confuciusornis. Both share the character of a well developed sternum, although each is morphologically distinct. Both genera are also more derived than Archaeopteryx in their well developed lateral humeral condyle and presence of medial and lateral cotyle on the ulna. It is possible that Liaoningornis is ancestral to Hesperornis and Ichthyornis because the Liaoningornis sternal morphology in addition to its tarsometatarsus-tibiotarsus length indices are consistent with Hesperornis, Ichthyornis and other aquatic birds, particularly diving birds. Liaoningornis is quite distinct from Confuciusornis in its adaptation to aquatic or riparian habitats as opposed to perching. Furthermore its morphology, particularly the pes, is similar to specimens representing Coraciiformes described from the Middle Eocene of Germany.
The large and diverse quantity of specimens from the basal Yixian Fm. of Beipiao, Liaoning, is an indication that avian diversity had already initiated by the Late Jurassic. The relatively derived genus Liaoningornis documents the earliest member of the Ornithurae and indicates that the Jurassic was the period of avian origin and diversity. Liaoningornis gave rise to the later radiation of Early Cretaceous ornithurids with Chaoyangia, Songlingornis, and Gansus representing the main lineage of avian evolution. Liaoningornis not only distinctly expresses a derived condition but also, due to its antiquity, retains reptilian characters such as a presternum and possibly an interclavicle, characters that were lost with the appearance of Jibeinia. In the Early Cretaceous, avian taxa radiate toward extant morphology as exemplified by Chaoyangia, which possesses characters shared with extant and younger fossil birds such as Ichthyornis and Hesperornis including an uncinate process on the gastric ribs, although its pelvic girdle is still extremely primitive. Not only does the Songlingornis sternum possess a carina, it also maintains posterolateral sternal foramina and its coracoid morphology approaches the complexity of extant forms. Gansus is even more derived: its tarsometatarsus is basically fused and digit IV is the longest in the series, being fully adapted for riparian habitats.