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This taxon was requested by Dan Bensen, so here it
is......
Liaoxiornis Hou and Chen 1999
= Lingyuanornis Ji and Ji 1999
L. delicatus Hou and Chen 1999
= Lingyuanornis parvus Ji and Ji 1999
Etymology- "small bird from Liaoxi", Liaoxi being
Pinyin for the western part of Liaoning Province.
Barremian, Early Cretaceous
Yixian Formation, Liaoning, China
Holotype- (IVPP V 130723, GMV2156 is counterpart
and holotype of Lingyuanornis) (82 mm) skull (20.3 mm), mandible (20
mm), eleven cervical vertebrae (15.3 mm), cervical rib, ten dorsal
vertebrae (15.3 mm), twenty dorsal ribs, sacrum (11 mm), six caudal
vertebrae (4.1 mm), pygostyle (16 mm), scapulae (10.1 mm), coracoids (7.5 mm),
furcula (7.6 mm), sternum (3.2 mm), humeri (15.4 mm), radii (14.9 mm), ulnae
(15.6 mm), metacarpal II, manual phalanx II-1 (4.3 mm), manual phalanx II-2 (1.3
mm), metacarpal III (7.4 mm), ilia (9-10 mm), pubes (6.6 mm), ischia, femora
(14.5 mm), tibiae (16.5, 17.1 mm), metatarsal I, metatarsal II (9.6 mm),
metatarsal III (10.4 mm), metatarsal IV (9.6 mm), pedal phalanges and
unguals
Diagnosis- anterior portion of premaxilla very
narrow; external naris placed posteriorly, extending almost to posterior border
of antorbital fenestra; very small antorbital fenestra?; retroarticular process
very elongate; very thin spike-like pygostyle; pygostyle longer than femur; no
proximodorsal ischial process?.
Description-
This specimen was discovered in the late 1990's and
described in 1999 as the smallest known Mesozoic bird. The specimen is
complete except for some distal phalanges, cervical ribs, manual digit
I and fibulae, so the total length can be measured at 82 mm. The
authors think Liaoxiornis is an adult because "the skull and postcranial
skeleton are well-developed". I feel there are several features suggesting
the holotype is a juvenile and will discuss them after the
description.
The skull is broadly similar to other basal avians,
as it is triangular, with a short pointed snout and large orbits. The
sutures between cranial elements are not well defined, which makes detailed
description difficult. The premaxilla must be very low, with an especially
narrow anterior portion. The external naris is probably placed more
posteriorly and is more narrow than either Eoenantiornis or the Spanish
hatchling. The maxilla has an elongate process extending beneath the
anterior part of the orbit, like Eoenantiornis, but not the Spanish
hatchling. Judging by the space between the naris and orbit, the
antorbital fenestra was very small, although details cannot be observed.
The nasals are slightly convex and shorter than the frontals. The jugal is
extremely slender, with what may be a small ascending process. If so, it
is more anteriorly placed than in the Spanish hatchling. The frontals are
large and bulbous, with a rim above the huge round orbits.
Several scleral ossicles are preserved, there were probably about fifteen in
life. The parietals are both exposed, one behind the other. They are
much shorter than the frontals and round in shape. The posteroventral
skull isn't preserved well, so the postorbital, squamosal, quadrate and
quadratojugal are not discernable. What may be the foramen magnum is
enlarged, being more than a fourth of the posterior skull width. Seven
teeth are present in the upper jaw, but the suture between the premaxilla and
maxilla is indistinct, so how many were in each bone is not known. These
are slightly procumbant and are said to be "similar to those of other early
birds". They extend posteriorly only to the anterior edge of the
antorbital fenestra. The dentary is very slender and straight, being more
slender than Eoenantiornis or the Spanish hatchling. There are seven
dentary teeth, less than Eoenantiornis (12) or the Spanish hatchling (8).
The surangular is slender and posteriorly expanded, while the angular extends
further posteriorly and is not as expended. Between the two is an elongate
external mandibular fenestra. There is an extremely long
retroarticular process on the articular, although the latter's attachment to the
surangular is uncertain.
The number of vertebrae is confusing, as Hou and
Chen have a different opinion than Ji and Ji. Hou and Chen estimate more
than ten cervicals, eight sacrals and four free caudals. Ji and Ji say
there are nine cervicals, eleven dorsals, seven sacrals and seven free
caudals. As there are ten dorsal ribs, I think the number of dorsal
vertebrae should be ten. This leaves eleven cervicals. Eight
vertebrae are between the ilia, Ji and Ji consider the last a caudal.
As having eight sacrals would make six free caudals, which is identical to
Iberomesornis and Sinornis, I think that is most likely. There are
eleven cervical vertebrae, which are a bit wider than long and can be seen to
have wide diapophyses. The third to sixth centra are longer than other
others. One cervical rib is reportedly preserved. Ten dorsal
vertebrae complete the presacral column, grading from 60% wide as they are long
to 20% wider than long posteriorly. Hou and Chen state the dorsal centra
were round with high neural spines, but as they are in ventral view in their
specimen, I don't see how this could be determined. Ten pairs of dorsal
ribs are present, the fourth longest. There are no uncinate processes,
sternal ribs or gastralia preserved. Eight sacrals attach to the ilia and
are unfused. They maintain a constant width, but decrease in length
posteriorly. The centra are broad with long transverse
processes. The tail is long and six vertebrae are definitely not
fused to the pygostyle. At least eight additional vertebrae can be
seen making up the proximal third of the pygostyle. All caudals are very
wide and have transverse processes. The pygostyle is a simple spike,
tapering to a point distally. It is very long, more so than the
femur. There is no hint of any chevrons.
The scapulae are very slender, with pointed
anteriorly projected acromion processes. The coracoids are strut-like,
fairly slender and have convex lateroventral edges. The furcula is
V-shaped, with a short hypocleidium. It has an interclavicular angle of
fifty-seven degrees. The sternum is single, but very tiny. It is
roughly diamond-shaped, with rounded lateral corners, a slightly indented
anterior corner and elongate posterior corner. No additional lateral or
posterior processes are present. There is a low keel present extending
from the anterior to the posterior tip.
The humerofemoral ratio is 106 and the radiohumeral
ratio is 97. The humerus is slightly bowed posteriorly, with a prominent
internal tuberosity, low deltopecoral crest and a slightly enlarged
ectepicondyle. The ulna is slightly longer than the humerus and bowed,
while the radius is about 60% of ulnar width. The proximal ends of
metacarpals II and III are said to be fused to the semilunate by Hou and Chen,
but reported to be separate by Ji and Ji. I cannot identify metacarpal I,
but metacarpals II and III are subequally robust. Metacarpal III is bowed
and extends past metacarpal II. There are two phalanges on digit II, II-2
much shorter than II-1.
The ilium has a greatly expanded and rounded
preacetabular process, with a ventral projection. The postacetabular
process is narrow and pointed, without a brevis shelf. It is shorter than
the preacetabular process. There appears to be no supracetabular
crest. The pubes are fused at the symphysis and are said to lack a
foot. The ischium is reported to lack a proximodorsal
process.
The femur is gently curved, but further details are
lacking. The tibia is also nondescript, but has a fibular crest. It
is 115-118 percent of femoral length, while the tarsometatarsus is 72
percent. The fibula is not preserved. A tarsal element is present,
though I'm unsure which one. A tarsometatarsus is present (fused
proximally) and fairly slender, with parallel sides except for a small proximal
expansion. Metatarsal II is shortest, III the longest. Metatarsal IV
is said to be slender by Ji and Ji. The pedal phalanges are disarticulated
and poorly preserved, but there was a hallux with a large curved
ungual.
Hou and Chen don't mention feathers, but Ji and Ji
state that primaries are present and at least 2.5 times ulnar
length.
Age-
As stated above, Hou and Chen consider Liaoxiornis
adult because "the skull and postcranial skeleton are well-developed". I
disagree based on several features-
- very small size
Not a good indicator of age by itself, but
certainly makes young age more probable. Liaoxiornis was 82 mm
long, compared to 100 in Longchengornis, 115 in Jibeinia, 119 in Cathayornis?
caudatus, 135 in Eoenantiornis, 138 in Sinornis, 140 in Cuspirostrisornis, 145
in Largirostornis and 190-280 in Confuciusornis. Incidentally,
Longchengornis and Cathayornis? caudatus both have only partially fused
pygostylia as well.
- very large skull
Craniohumeral ratios of enantiornithines are-
Liaoxiornis (139), Cathayornis yandica (113), Cathayornis? caudatus (106),
Largirostrisornis (106), Eoenantiornis (75), Cuspirostrisornis (~66).
Juvenile theropods are known to have relatively large skulls (eg. Scipionyx) and
Liaoxiornis' skull is at least 30% larger than other
enantiornithines.
- poorly ossified elements
Most of the skull elements are indistinct, there
are no sternal ribs, the distal manual and pedal phalanges are not completely
preserved and the fibula isn't preserved. The extremely small sternum is
also likely caused by incomplete ossification. There are long ventral
edges of the coracoids that would normally attach to the front of the sternum,
but the sternum is much too small. I think there was cartilage present
around the ossified section in life. The poor ossification also means that
the absence of uncinate processes cannot be used to prove enantiornithines lack
such structures. This may also explain the short hypocleidium and lack of
a proximodorsal ischial process.
- lack of fusion
Although the metatarsals, distal pygostyle and
possibly carpometacarpus are fused, nothing else is. The sacrals are
unfused and vertebrae are still visible for the first third of the
pygostyle. Additionally, the tarsal is large enough to be an unfused
astragalus.
So, I think the evidence is pretty conclusive that
Liaoxiornis is juvenile and not the smallest Mesozoic bird, as has been
claimed. An interesting idea would be to compare it with the Spanish
hatchling (Sanz et al., 1997) and the new juveniles found in the regurgitated
pellet (Sanz et al., 2001).
Liaoxiornis vs. Lingyuanornis-
One month after Liaoxiornis delicatus was
described by Hou and Chen, Ji and Ji described Lingyuanornis parvus. Over
two years later, Harris discovered they were based on part and counterpart of
the same specimen (pers. comm. 2001). Creisler determined Liaoxiornis has
priority (http://www.cmnh.org/fun/dinosaur-archive/2001Jun/msg00051.html),
but is their synonymy proven? They are determined to be the same specimen
based on the following.
- They were both discovered in Lingyuan, Liaoning
Province.
- They are both in the exact same position,
anterior back and neck curved to the right so that the snout intercepts the
right elbow. The long bones are all also at the same angles.
Liaoxiornis is is ventral view, and Lingyuanornis is in dorsal.
- They are very similar in size. Humerus-
15.5 vs. 15 mm; ulna- 15.6 vs. 17 mm; metacarpal III 7.4 mm vs. 8 mm; femur-
14.5 vs. 16 mm; tibia- 17.1 vs. 18 mm; tarsometatarsus- 10.4 vs. 11
mm.
- They are very similar in morphology, the
differences present probably due to the more schematic illustration of Hou and
Chen or the differences in perspective. For instance, Lingyuanornis isn't
nearly so perfect looking or symmetrical and the mandible is hidden. A few
bones at the extremities are missing in Lingyuanornis (phalanges, distal
pygostyle), suggesting either these were restored in Liaoxiornis, or more of the
specimen got stuck on the Liaoxiornis side.
Thus, the two are synonyms and should be referred
to as Liaoxiornis delecatus.
Relationships-
Hou and Chen refer Liaoxiornis to the Sauriurae
based on unfused skull bones, teeth and "sauriurine-like shoulder girdle and
pelvis". As the Sauriurae has been universally dismissed in phylogenetic
analyses as being a paraphyletic grade of basal avians, Liaoxiornis'
relationships must be looked for elsewhere. Ji and Ji refer Lingyuanornis
to the Enantiornithes, though the Chinese text prevents me from examining
why. This is probable, though it should be noted there is a distinct
possibility this clade is paraphyletic. Caution must be used when
examining the presence of characters in Liaoxiornis due to its juvenile
status. Remember, I use the following phylogeny for basal
pygostylians-
_____Confuciusornithidae
|_____Protopteryx
|_____Longipteryx?
|_____Jibeinia
|_____Enantiornithes
|___Euornithes
Liaoxiornis is more derived than confuciusornithids
based on- less than twelve dorsal vertebrae; less than seven free caudals;
V-shaped furcula with narrow interclavicular angle (<70 degrees);
strut-like coracoid; ulna subequal or longer than humerus; metacarpal III longer
than metacarpal II; reduced manual unguals. These characters easily
override the plesiomorphically long tail and absent proximodorsal ischial
process, to firmly establish Liaoxiornis as a derived pygostylian. It's
less derived than Protopteryx based on- short hypocleidium. It's more
derived than Protopteryx based on- manual phalanx II-2 shorter than II-1; more
than seven sacral vertebrae. It is more derived than Jibeinia based
on- less than three phalanges in manual digit II. It is less derived than
ornithothoracines based on- interclavicular angle >50 degrees.
Potential enantiornithine characters are difficult to identify, as the
description is short and so many former enantiornithine characters are now known
in more basal pygostylians, as well as basal euornithines (eg. Apsaravis).
The coracoid is laterally convex at its ventral end and the fourth metatarsal is
reduced in width. This makes it most parsimoniously an enantiornithine,
although the large interclavicular angle makes this conclusion somewhat
tenuous. Possible interrelationships within that clade are difficult to
establish without an accepted phylogeny of enantiornithines. The juvenile
status of the holotype also makes this difficult. There is a possibility
Liaoxiornis is a juvenile of one of the many better known Liaoning
enantiornithines (which are probably oversplit), but such comparisons are beyond
the scope of this post. There are several characters distinguishing it
from other enantiornithines, although these could change in ontogeny. For
now I recommend classifying Liaoxiornis as a provsionally valid, probable
enantiornithine.
Anyone who wants a scan of Liaoxiornis' skeleton or
a pdf of Lingyuanornis' description, contact me offlist.
Mickey
Mortimer |