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Details on Boluochia



Boluochia Zhou 1995
B. zhengi Zhou 1995
Etymology- "Professor Zheng Zuoxin's (bird) from Boluochi"
Barremian, Early Cretaceous
Yixian Formation, Liaoning, China
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 and 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, Longchengornis(?) 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, Concornis, 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

Those who want the figures (disarticulated skeleton, metatarsus, distal view of tibiotarsus) and my skeletal reconstruction of it can contact me offlist.  Now to write "Details on Liaoningosaurus"......
 
Mickey Mortimer