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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 |