Enantiornithes Walker, 1981
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publication ID |
https://doi.org/ 10.5281/zenodo.13651058 |
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persistent identifier |
https://treatment.plazi.org/id/6E7B87C1-4436-FFD4-FFE0-F808FB9AC5F3 |
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treatment provided by |
Felipe |
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scientific name |
Enantiornithes Walker, 1981 |
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Euenantiornithes Chiappe, 2002
Gen. et sp. indet.
Referred specimen: CAGS−IG−04−CM−007, a partially articulated appendicular skeleton consisting of a fragmentary pelvic girdle and largely complete hind limbs ( Fig. 2 View Fig ). The pelvic girdle is represented by the acetabular portion of the right ilium and ischium and nearly complete pubes; the left hind limb by the (tentatively identified) distal half of the femur, the nearly complete tibiotarsus, the proximal end of the fibula, and the complete tarsometatarsus and pes; and the right hind limb by the complete femur and the proximal half of the tibiotarsus. Possible feather impressions are preserved adjacent to the putative left femur.
Locality and horizon: Area surrounding the town of Changma in the Changma Basin of northwestern Gansu Province, China ( Fig. 1A View Fig ). More precise locality information is available to qualified researchers upon request. Lower Cretaceous (Aptian–?Albian) Xiagou Formation, lower unit of the Xinminpu Group ( Fig. 1B View Fig ).
Description
Ilium.—The acetabular portion of the right ilium is preserved and exposed in lateral view. The exposed portion of the acetabulum is subtriangular and deep. A low, dorsoventrally elongate prominence at the caudodorsal corner of the acetabulum probably represents the antitrochanter. The ventral end of the pubic peduncle possesses a small, cranially−projecting process.
Pubis.—Both pubes are preserved but broken proximally, precluding a definitive determination of their orientation relative to the remainder of the pelvis. The proximal half of the pubic shaft is straight and subcylindrical, while the distal half is mediolaterally compressed and curves caudodorsally. The distalmost ends of the pubes are slightly enlarged, but are not expanded dorsoventrally into a “boot”−like structure. The ends are closely appressed, but separated by a gap, indicating that they are not coossified.
Ischium.—A subtriangular process projecting from the caudoventral region of the acetabulum is identified as the mediolaterally thickened proximodorsal portion of the right ischium. It is fused to the ilium fragment, with no line of contact visible between these elements.
Femur.—The caudal surface of the nearly complete right femur is exposed, while an elongate fragment overlapping the left pubis is tentatively identified as the distal half of the left femur. If correctly identified, the latter is exposed in cranial view. The femoral head is spheroidal, slightly caudally projected, and bears a shallow fossa, presumably for the insertion of the capital ligament. A femoral neck (defined here as a constriction separating the femoral head from the remainder of the element) is absent. The proximolateral region of the femur bears a recess that is defined caudodistally by a prominent ridge [the caudal (= posterior) trochanter] for the insertion of the M. ischiofemoralis ( Hutchinson 2001). The caudal surface of the right femoral shaft is crushed just distal to the caudal trochanter. Distal to this, the shaft is damaged laterally, but it appears to have been slender. As preserved, it is nearly straight in mediolateral view, curving only slightly caudally both proximally and
Ilium (right)
Acetabulum, craniocaudal diameter 1.6
Femur (right)
Length 24.0
Proximal width 3.8
Distal width 3.1
Tibiotarsus (left)
Length (29.2)
Proximal width 3.4
Distal width 3.2
Tarsometatarsus (left)
Lengths
Metatarsal I 3.1*
Metatarsal II 19.6
Metatarsal III 20.3
Metatarsal IV 19.1
Proximal width 3.4
Pedal phalanges 1 (left)
I−1 4.3
I−ungual 2 (3.9)
II−1 3.6
II−2 4.9
II−ungual 2 4.4
III−1 4.8
III−2 4.0
III−3 5.0
III−ungual 2 4.4
IV−1 2.8
IV−2 2.0
IV−3 2.2
IV−4 3.7
IV−ungual 2 (4.7)
1 Nonungual phalangeal length measured from the dorsoplantarily deepest part of the proximal articular cotyle to the apex of the furthest distal condyle.
2 Ungual length measured as a chord from the extensor process of the proximal end to the distal tip of the ungual.
distally. Although much of the preserved portion of the putative left femur is badly crushed, there appears to be no patellar groove on its craniodistal surface. [Nevertheless, we deemed it most conservative to code this character (141) as indeterminate (“?”) in our phylogenetic analysis.] Caudally, a shallow popliteal fossa is developed near the distal end of the right femur. This fossa is open caudomedially, but bounded laterally by a small, poorly defined tubercle that occupies the caudolateral edge of the femur proximal to the lateral condyle. A similar tubercle is present, but better developed, in Halimornis thompsoni , a Late Cretaceous enantiornithine from Alabama, USA (Chiappe et al. 2002). Distally, the popliteal fossa is bounded by the two articular condyles and the intercondylar bridge. Both condyles project well caudally. The medial condyle is nearly twice as mediolaterally broad as the lateral, and its caudal surface is rounded. The lateral condyle extends slightly farther caudally than its medial counterpart before terminating in a distinct apex that is confluent with the intercondylar bridge. The distal end of the lateral condyle defines the lateral bor− der of a deep, broad, medially shallowing depression, possibly for the origin of the cranial cruciate ligament ( Baumel and Witmer 1993).
Tibiotarsus.—Both tibiotarsi are exposed in caudal view. Only the proximal half of the right is present, while the left is broken into two portions. No significant quantity of bone seems to be missing between the two preserved portions of the left tibiotarsus, suggesting that, when complete, it was probably only slightly longer than the femur ( Table 1). As in some other enantiornithines [e.g., Gobipteryx minuta (= “ Nanantius valifanovi ”; Kurochkin 1996: fig. 10D, F); Soroavisaurus australis ( Chiappe and Walker 2002: fig. 11.13A)], the proximal articular surface is canted slightly distolaterally. It is ovoid in proximal view, slightly broader mediolaterally than craniocaudally. The proximolateral edge of the right tibiotarsus is damaged, while that of the left cannot be observed because it is covered by the right ischium and femur and the left fibula. The proximomedial edges of both tibiotarsi bear low, mediallyprojecting longitudinal crests. That of the left tibiotarsus appears damaged medially, while that of the right is largely obscured by pedal digit I. The tibiotarsal shaft is slender and straight.
The distal end of the left tibiotarsus is clearly exposed caudally and articulated with the corresponding tarsometatarsus. The tibia is probably coossified with the proximal tarsals because there is no evidence of a suture between these elements, and because no separate tarsals are present. Both tibiotarsal condyles expand much farther cranially than caudally, and are subcircular in mediolateral view. The lateral surface of the lateral condyle is excavated by a shallow, subcircular epicondylar depression; the medial surface lacks a comparable feature.
Fibula.—The proximal end of the left fibula is articulated and closely appressed to the tibiotarsus. As articulated, it projects slightly farther proximally than does the proximolateral margin of the tibiotarsus. The fibula is slender and mediolaterally compressed, with convex lateral and concave medial surfaces. It is slightly craniocaudally expanded proximally, more so cranially than caudally. The shaft is broken immediately distal to the proximal end. Nonetheless, the morphology of the tibiotarsus indicates that the fibula was reduced distally and probably did not extend to the proximal tarsals.
Tarsometatarsus.—The complete left tarsometatarsus is preserved and prepared in plantar view. The distal tarsals are fused together, forming a solid cap that is coossified with the proximal ends of metatarsals II–IV. However, the remainder of the metatarsus appears unfused. There is no evidence of a hypotarsus.
The shaft of metatarsal I is splint−shaped and appressed to the distalmost end of the plantaromedial surface of metatarsal II. This may represent its natural position; however, we suspect that it has been displaced plantarily, such that its condyle would have been situated more medially in life. Although the proximal end of metatarsal I is broken away, the element appears to have been very short. As in Sinornis ( Sereno et al. 2002: fig. 8.5), angular dorsal and plantar processes occur immediately proximal to its bulbous and dorsoplantarily elliptical distal condyle.
Metatarsals II–IV are mediolaterally compressed, coplanar, and elongate, with the longest (metatarsal III) measuring 85% the length of the femur ( Tables 1, 2). The three metatarsals are slightly plantarily expanded proximally; at least metatarsals II and IV are dorsally expanded as well. The proximal ends of the latter two elements are also moderately mediolaterally expanded, “pinching” metatarsal III between them. Both the proximomedial margin of metatarsal II and the proximolateral margin of metatarsal IV are attenuated into plantarily−projecting ridges (the medial and lateral plantar crests of Baumel and Witmer 1993) that persist for the proximal three−fourths of the metatarsus, tapering distally. Metatarsal II is only slightly shorter than metatarsal III, and appears dorsoplantarily compressed. Metatarsal IV, marginally the shortest of the three, is also the narrowest, especially at midlength, where its mediolateral diameter is approximately half that of the third metatarsal. Distally, a notch that probably corresponds to the distal vascular foramen ( Baumel and Witmer 1993) is present between metatarsals III and IV. Because these metatarsals are distally unfused, we consider the distal vascular foramen to be only partially enclosed in CAGS−IG−04−CM−007.
Metatarsals II–IV are slightly mediolaterally expanded distally. The distal trochleae of metatarsals II and III possess defined medial and lateral rims separated by a shallow sulcus. The medial rim of the metatarsal II trochlea is partially covered by metatarsal I and phalanges I−1 and II−1. Both distal rims of the metatarsal III trochlea project plantarily roughly the same distance. The lateral rim is slightly wider than the medial. The trochlea of metatarsal IV, although partially obscured by phalanx IV−1, appears to be relatively small and plantarily undivided. Metatarsal V is absent.
Pes.—The left pes is complete, well preserved, and except for digit I and the ungual of digit III, exposed in plantar view. The pedal phalangeal formula is 2−3−4−5−x. All nonungual phalanges are expanded mediolaterally and dorsoplantarily at their proximal and distal ends. Digit I is exposed in lateral view, having rotated medially from a probably retroverted in vivo orientation. Its proximal phalanx is approximately the same length as its ungual ( Table 1). The proximal phalanx of digit II is slightly shorter than both its penultimate phalanx and ungual. In digit III, the proximal phalanx is the most robust and about the same length as the penultimate phalanx, the second is the shortest, and the ungual is intermediate in length. The first three phalanges of digit IV are much shorter than the penultimate phalanx, which is in turn shorter than the claw. The proximoplantar ends of phalanges II−1 and III−1 bear shallow sagittal sulci that are absent in phalanx IV−1. The nonungual phalanges of digits I and III and at least the distalmost three phalanges of digit IV bear collateral ligament pits; the relevant regions of digit II cannot be observed. The collateral ligament pits are best developed on the penultimate phalanges, where they are dorsally situated.
Pedal unguals II–IV are approximately the same length; ungual I is marginally shorter. None of the unguals are sharply recurved. Those of digits I–III exhibit grooves on their exposed medial and/or lateral surfaces; damage prevents a definitive assessment of this character in the digit IV ungual. Ungual I possesses a weak flexor tuberculum that is barely distinct from the proximoventral corner of the articular cotyle. In contrast, ungual III displays a better−developed, plantarily rounded flexor tuberculum that is situated distal to, and is distinctly separate from, the proximoventral end of the articular cotyle. All unguals are generally laterally compressed, but at least ungual III is slightly wider plantarily than dorsally. No keratinous ungual sheaths are preserved.
Integument.—Possible feather impressions are preserved adjacent to the putative left femur, but are too poorly preserved to be definitively identified.
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