Describer

Zhou, 1995

Time

Cretaceous Early Barremian

Classification

Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Enantiornithes

Fossilsite

Jiufotang Formation, Liaoning, China

Info

Partial skull, partial postcranium

Diagnosis

Presence of an acute and hooked beak, sternum with slender elongated lateral process with slight distal expansion, carina undeveloped as a low crest, torso noticeably reduced posterior to the acetabulum, proximal ischium with dorsal process, pubis distinctly posteriorly extended with a strong curvature and distal end booted, distal tibiotarsus intercondylar vacuity extremely compressed and in distal perspective the anterior margin of the medial condyle is relatively flattened. Only the proximal end of the tarsometatarsus is fused, the midsection is distinctly narrow, MtIV is relatively slender and weak, trochleae of the other three metatarsals lie nearly on the same plane, the pes unguals are longer than the phalanges and are strongly recurved with acute termini, and the pygostyle is elongated.

Type specimen

An incomplete skeleton that preserves the hindlimb, pelvic girdle, pygostyle, anterior skull, a portion of the sternum, and vertebrae. Specimen number IVPP V9770 is currently housed at the Institute of Vertebrate Paleontology and Paleoanthropology.

Locality and stratigraphic position

Light gray mudstones of the Early Cretaceous Luofotang Fm at the village of Boluochi, Chaoyang Co., Liaoning Province.

Summary

The taxonomic status of Boluochia was initially uncertain but after more in depth study and comparison it is regarded as a member of the Sinornithiformes. The main body of the premaxilla is dorsally projected with the anterior end forming an acute hooked process (Zhou, 1995). Thus the short and high morphology resembles Sinornis, only this genus is a little more autapomorphic. The femur is robust and although incomplete on the type it is determined to have a transverse diameter larger than the tibiotarsus, which is also consistent with Sinornis.

The tarsometatarsus is also similar to that of Sinornis, being rather expanded at its midsection and the trochlea for the three digits lie nearly on the same plane. A counterpart for this specimen was not recovered, prohibiting a complete comprehension of its morphology, but the premaxilla is relatively complete with a thin, flat, and linear nasal process.

The rostral process of the premaxilla is dorsally projected and its anterior end curves ventrally to compose an acute hook. These characters approach those of extant raptors. Also visible on the type is a rather moderate sized embayment located between the nasal process and the main body of the premaxilla. The maxillary process of the premaxilla is relatively short and gradually thins as it extends posteriorly. Worthy of note is that the premaxilla is edentulous, which differs not only from the reconstruction of Sinornis, but from all other avian forms produced from the same locality which, in general, bear four premaxillary teeth.

Skeletal measurements of Boluochia zhengi (mm) (from Zhou, 1995).
Pygostyle length 21.5
Pygostyle proximal breadth 4.7
Pygostyle distal breadth 4.0
Femur shaft breadth 3.0
Tibiotarsus length 37.0
Tibiotarsus shaft breadth 2.5
Tibiotarsus distal breadth 3.5
Tarsometatarsus length 17.5
Tarsometatarsus proximal breadth 4.0
Tarsometatarsus minimum breadth 1.8

The sternum of Boluochia is incomplete but slightly better preserved than in Sinornis due to the presence of its posterior portion, although the main body is missing. The carina is sill extremely undeveloped as a low crest. The posterior process is shorter than the lateral process, which is slightly expanded at its terminus. The pelvic girdle is relatively well preserved although dorsal and ventral portions are obscured; as such, only the posterior acetabulum is visible. It has a relatively flat and straight dorsal margin that is distinctly reduced. The left ischium is characteristic in possessing a proximodorsal process. Posteriorly the ischium gradually attenuates and is unlike the condition of Sinornis, which is scimitar-shaped. The pubis resembles that on Sinornis with a distal boot. The state of fusion of the three pelvic elements is unclear, although the other Early Cretaceous taxa described generally have unfused pelvic elements.

The forelimb and pectoral girdle of Boluochia are not preserved but the hindlimb is extremely well preserved. The femur is incomplete but appears to have been relatively long, distinctly robust, and very possibly similar to that of Sinornis. In lateral perspective, the distal condyles are distinctly posteriorly projected to surpass the margin of the shaft. The tibiotarsus is more slender than the femur, the cnemial crest is not well developed, and the distal medial and lateral condyles do not noticeably project anteriorly, but there is a distinctly developed medial ligamental prominence. Among the foot bones, the relatively short tarsometatarsus is the best preserved; it has a fused proximal end that is not laterally expanded.

From the proximal end to the midsection the shaft is extremely broad, resembling that on Sinornis. From the midsection distally, the shaft conspicuously constricts until the distal end, whereupon it again expands. The distal fourth metatarsal is distinctly separated from MtIII by a fissure and the trochleae of MtII, III, and IV lie nearly on the same plane. Although the phalanges are disarticulated and scattered within the matrix, they can nevertheless be identified. The unguals are large, curved, and are longer than the phalanges.

The pygostyle is particularly noteworthy in two respects. It is particularly long, or longer than the tarsometatarsus, and it has a narrow, elongated, longitudinal dorsal crest that is composed of the fused caudal neural spines. This latter character is shared with Jibeinia. There is an intimate phylogenetic relationship between Boluochia and Sinornis. Both appear to have edentulous premaxillae, although the premaxilla on Boluochia possesses a distinct hooked beak, which may be interpreted in two ways.

Primarily, it may represent a further adaptation toward carnivorous behavior, providing further capability for prey capture, because it more closely approaches the morphology of extant raptors. Secondly, it may be interpreted as an assistance for perching, correlated with the gradual loss of dentition, or like the behavior of parrots (Psittaciformes). In this manner, the short, thick, and curved beak would also facilitate ingestion of hard-shelled food items in addition to assisting pedal arrangement for perching. However, parrots have a modified pes with digits I and IV placed posteriorly and in opposition to the anterior digits II and III. Boluochia has yet to modify its pes to adapt to its environment to this extent.

Regardless of which evolutionary direction Boluochia has taken, it still represents a neomorphic condition.

And although it maintains a hooked beak, its premaxilla is still relatively low and the anterior process is extremely small. From the perspective of general morphology it resembles Sinornis and appears to be evolving in the direction of predatory behavior. Therefore, a continuous evolutionary trend is represented from Sinornis to Boluochia. The hindlimb is well preserved on both genera and is beneficial toward interpreting environmental adaptations. The femur is relatively robust on both genera and tibiotarsus is more slender and longer than the femur. But on Boluochia the distal condyles of the tibiotarsus do not conspicuously project anteriorly, which is in contrast to Sinornis.

The tarsometatarsus is also distinct in its constriction ventral to the midshaft, which is also not present on Sinornis. Moreover, the distal fourth metatarsal on Boluochia is more isolated and the trochlea for the digits more closely approach the morphology of extant raptors, as expressed by the enlarged and broadened trochlea for digit II and the relatively slender and weak trochlea for digit four, which is also closer to extant raptors than is Sinornis. However, the trochlea on Boluochia are still not perfected, because the articular surfaces are still not extremely well developed and are far from achieving the adaptive morphology of extant taxa.

Furthermore, the unguals on both genera are similar, being large, acute, and strongly recurved, but they still do not constitutute fully efficient talons. Both genera lack flexor tuberositys on their unguals, which are fundamental for both effective perching and predation. The well developed ungual morphology but absence of tuberositys is directly related to the development of ungual flexure musculature for support and the genesis of related tendons. Flexure musculature is related to the fortification of the foot for the ability of grasping with the talons which in turn is related to the alleviation of the expense of energy. Consequently, the ungual morphology indicates that each genus lies within a differential evolutionary phase of predatory behavior with Boluochia being more derived. (Zhou, Jin and Zhang, 1992)
Source: Polyglot Paleontologist

Comment by Michel Mortimer

Etymology

\\\"Professor Zheng Zuoxin\\\'s (bird) from Boluochi\\\"

Holotype

(IVPP V 9770) (~155 mm) premaxilla, nasals, partial dentary, tooth, two partial dorsal ribs, incomplete sacrum, six caudal vertebrae, pygostyle (21.5 mm), posterior sternum, two sternal ribs?, posterior ilium, pubis (23.1 mm), ischium (~16 mm), partial femora, tibiotarsi (~37 mm), metatarsal I (4.1 mm), metatarsal II (17.2 mm), metatarsal III (17.4 mm), metatarsal IV (17.5 mm), pedal phalanges and unguals

Diagnosis

Premaxilla markedly convex anterodorsally and sigmoid ventrally, forming raptorial beak; posterior premaxillary process expanded; pygostyle distally expanded; highly bowed pubic shaft; deep extensor groove on femur; cnemial crest absent; metatarsals increase in length laterally.

Description

This specimen was found by Zhou in 1990 along with the holotype of Cathayornis and an unidentified sternal impression (IVPP V 9941). The specimen is mostly preserved as a natural mold. It was about 155 mm long, estimated from my skeletal reconstruction. The completely fused synsacrum and pygostyle indicate it was an adult.

The premaxilla is very distinctive. It has an elongate expanded nasal process probably extending posteriorly to the orbit. The subnarial process is much shorter and upcurved, suggesting the maxilla articulated beneath it. The premaxillary body itself is reminiscent of predatory birds, having a hooked tip created by the concave ventral edge and convex anteroventral edge. No teeth are present in the premaxilla, which seems to lack alveoli. The pointed distal tip of the anterior nasal process is preserved below the nasal premaxillary process. An anterior dentary fragment is fairly robust and bears one tooth. The tooth is nearly conical and constricted at the base.

Two dorsal rib fragments are present. A fragment of the synsacrum is completely fused, with the neural spines fused into a dorsal lamina. There seem to be six caudal vertebrae preserved, although details are lacking. A well-fused pygostyle has a flared proximal end, gently tapers distally and ends in a bulbous expansion. There is a dorsal crest starting slightly posterior to the proximal end.

The posterior edge of the sternum is preserved. It consists of elongate posterolateral processes with slightly expanded tips, short pointed posteromedial processes and an elongate pointed posteromedian process. The keel is present only posteriorly. Two sternal rib fragments are probably present by the sternum.

The posterior half of the ilium is present, showing a fairly long, decurved and tapered postacetabular process. The pubis and ischium are probably fused. The former is slender and strongly bowed anteriorly, with a distal pubic foot only developed posteriorly. The ischium is about 70% of pubic length and strongly bent ventrally almost one third down the shaft. There is no obturator process, the distal end is pointed and the proximodorsal process was elongate and may have contacted the ilium.

The proximal end of the right femur and distal end of the left are present, both poorly preserved. The head is declined and a trochanteric crest present. On the distal end, a deep extensor groove can be seen. There is no cnemial crest on the proximal tibiotarsus. Distally, the condyles \\\"are less produced anteriorly\\\" and are subequal in size. The medial condyle is flat anteriorly in distal view and the intercondylar groove is narrow.

No supratendinal bridge is developed, unlike ornithurines. The tarsometatarsus is fused proximally to the distal tarsals. Proximally, the metatarsus is sharply flared, while it flares more gradually distally. Metatarsal II is straight until bending medially at the distal fifth, but metatarsal IV is bowed medially its entire length. The metatarsals decrease in length medially, as in Longipteryx. Metatarsal II has the widest trochlea, metatarsal IV has the narrowest and is reduced in width overall. There are no posterior projections on the trochleae. Metatarsal I is J-shaped. The pedal phalanges are all preserved, but disarticulated. I can assign the short robust one to II-1 and the long robust one to II-2. Three elongate phalanges are probably from digit III, while the three shortest are from digit IV. One is almost articulated with metatarsal I, so undoubtedly belongs there.

The unguals are also preserved, seemingly in their proper places. II and III are subequal in length and about 7% longer than I and IV, but II is more robust than III. The unguals are fairly straight, but keratinous sheaths on all of them show the claws were well curved in life.

Relationships

Zhou assigns this taxon to the Enantiornithes based on- narrow intercondylar groove on tibiotarsus; medial condyle of tibiotarsus anteriorly flat in distal view; metatarsal IV reduced; slender posterolateral sternal processes with expanded distal ends; proximodorsal ischial process. The first is also found in Apsaravis (Norell & Clarke, 2001); the fourth is also found in Protopteryx, Longipteryx, Yanornis and probably Jibeinia; the fifth is also seen in Bambiraptor, Sinornithosaurus, Unenlagia, Rahonavis, Archaeopteryx and other pygostylians. The second is suggested for the first time in this paper. Sazavis, Vorona and Nanantius have it, but so does Apsaravis. This leaves the reduced fourth metatarsal. This character really needs to be quantified, as Confuciusornis and Protopteryx also have reduced fourth metatarsals. That of Boluochia doesn\\\'t look more reduced than Confuciusornis. Other \\\"enantiornithine characters\\\" present, like the caudally restricted sternal keel, wide trochlea of metatarsal II and J-shaped metatarsal I are present in more basal pygostylians as well.

I entered Boluochia into my Pygostylia matrix and found it comes out as a member of the Jibeinia-ornithothoracine clade. It is a pygostylian based on- large pygostyle; prominent posterolateral sternal processes; ossified sternal keel; obturator process absent. It is more derived than confuciusornithids based on- elongate posteromedian sternal process; slender posterolateral sternal processes. It is more derived than Protopteryx based on- prominent posteromedial sternal processes; fifth metatarsal absent. It is more derived than Longipteryx based on- posteromedial sternal processes pointed. Although Longipteryx and Boluochia share metatarsals increasing in length laterally, I can\\\'t find other characters that may unite them. There are several characters in Boluochia that can be compared to other enantiornithines and related taxa to help determine its relationships.

- The toothless premaxilla is found in Gobipteryx and Nanantius? valifanovi, but not Longipteryx, Jibeinia, Cathayornis, Cuspirostrisornis, Eoenantiornis, Largirostrisornis, Liaoxiornis or the Spanish nestling.

- The elongate nasal process of the premaxilla is seen in Confuciusornis, Yanornis and to a lesser extent Gobipteryx, but not Cathayornis, Cuspirostrisornis, Eoenantiornis or Largirostrisornis.

- The toothed dentary is more primitive than Gobipteryx or Nanantius? valifanovi, but similar to Longipteryx, Jibeinia, Cathayornis, Cuspirostrisornis, Eoenantiornis, Largirostrornis, Liaoxiornis and the Spanish nestling.

- The posterolateral sternal processes are not as expanded as Longipteryx, Concornis, Cathayornis, Iberomesornis, Largirostrornis, Yanornis or Songlingornis. Eoenantiornis, Eoalulavis, Liaoxiornis and Liaoningornis lack posterolateral sternal processes, while that of Longchengornis is unexpanded.

- The pubic foot is present, as in Longipteryx, Jibeinia, Cathayornis, Cuspirostrisornis and Sinornis. Liaoxiornis, Liaoningornis(?) and euornithines lack pubic feet.

- Among basal birds, only Protopteryx lacks a cnemial crest, to the best of my knowledge.

- If Zhou means the distal condyles are not projected much anteriorly by saying they are \\\"produced less anteriorly\\\", this is more primitive than Lectavis, Nanantius, N? valifanovi and Vorona .

- The subequal lateral and medial distal tibial condyles are more primitive than Concornis, Lectavis, Nanantius, N? valifanovi, Sazavis and Vorona. More basal birds (Confuciusornis) and euornithines (Patagopteryx, Apsaravis) have the primitive condition.

- Avisaurus, Nanantius? valifanovi, Neuquenornis and Soroavisaurus have a strong plantar projection on the trochlea of metatarsal III that is lacking in Boluochia, Vorona and Yungavolucris.

-Avisaurus, Lectavis, Nanantius? valifanovi, Soroavisaurus and Yungavolucris have a dorsal tubercle on metatarsal II lacking in Boluochia and Vorona .

The above comparison indicates that a couple characters (toothless premaxilla; elongate nasal process of premaxilla) are shared with Gobipteryx , although these are also developed in other lineages. In addition, one character each is shared with Longipteryx (metatarsals increase in length laterally) and Protopteryx (no cnemial crest), although it would take one more step to place it with Longipteryx and three more to place it with Protopteryx .

Several characters (subequal distal tibial condyles; distal tibial condyles not projected anteriorly?; no strong plantar projection on metatarsal III; no dorsal tubercle on metatarsal II) suggest Boluochia is more basal than an enantiornithine clade containing Avisaurus, Coniornis, Lectavis, Nanantius, N? valifanovi, Neuquenornis, Sazavis, Soroavisaurus, Vorona and Yungavolucris. Other Yixian enantiornithines are poorly described and illustrated, so cannot be compared. Boluochia lacks several euornithine characters (sternal keel extending to anterior rim; pubic foot absent; metatarsals completely fused). In addition, the wide trochlea of metatarsal II is unknown in euornithines.

This suggests Boluochia was not a member of this group. In conclusion, Boluochia seems to be a non-euornithine member of the Longipteryx-Jibeinia-ornithothoracine clade. If it is an enantiornithine, it is more basal than several members. Further phylogenetic analyses of basal pygostylians may pin down its relationships further.

Reference- Zhou, 1995. Discovery of a New Enantiornithine Bird from the Early Cretaceous of Liaoning, China. Vertebrata PalAsiatica 33(2): 99-113

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