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Well, at 11:00 AM Wednesday (PST) I logged
onto the Nature website and started reading about the new eumaniraptoran, only
to see its name is- Dromaeosauridae gen. et sp. indet.!? That's rediculous
I thought, this is a complete specimen, possibly the most complete theropod
specimen ever, yet it's indeterminate. Knowing such a thing could not
possibly be true and looking forward to examining the figures and measurements
to compare it in detail with Sinornithosaurus and Microraptor, I was stunned and
annoyed to find out the authors decided not to include either line
figures or a measurement table. However, despite these drawbacks,
much data can still be examined. The following description concentrates on
comparison to both Sinornithosaurus and Microraptor, as Ji et al. believe this
may be a subadult specimen of the former and I previously noted gross
resemblence to the latter. One fact that should be noted is that Ji et al.
never went into detail about the differences between Sinornithosaurus and the
new specimen, aside from referring to an allometric analysis they
performed. It is supposedly in the supplementary information (which can be
downloaded for free), but although the supplementary information page refers to
a table 1, with graphs 1, 2 and 3, these are nowhere to be found.
cf. Sinornithosaurus (Ji, Norell, Gao, Ji and Ren,
2001)
Barremain, Early Cretaceous
Yixian Formation, Liaoning, China
Material- (NGMC 91) (69 cm) skull (~100 mm),
sclerotic rings, mandibles, ten cervical vertebrae, cervical ribs?, dorsal
series, dorsal ribs, uncinate processes, gastralia, sacrum?, caudal series,
chevrons, scapulae, coracoids?, furcula, sternum?, sternal ribs, humeri (87 mm),
ulnae (72 mm), radii, semilunate, radiale, metacarpal I (13 mm), phalanx I-1 (27
mm), manual ungual I (15 mm), metacarpal II (40 mm), phalanx II-1 (22 mm),
phalanx II-2 (24 mm), manual ungual II (16 mm), metacarpal III (40 mm), phalanx
III-1 (12 mm), phalanx III-2 (7 mm), phalanx III-3 (16 mm), manual ungual III
(11 mm), pelves?, femora (95 mm), tibiae (133 mm), fibulae, tarsi?, metatarsi
(62 mm), pedes
Diagnosis- posteriorly tapering naris; large
triangular posterior dentary process?;
Description-
The measurements above are all approximate, taken
from the scale in figure 1. The total length is 69 mm, compared to 48 cm
in Microraptor. Although the tail length of Sinornithosaurus is unknown,
the prefemoral length is estimated at 49 cm, compared to 31 cm in the new
specimen. Thus, Sinornithosaurus is 1.6 times longer than NGMC 91 and 2.3
times longer than Microraptor. Ji et al. believe this is a juvenile on the
basis of its large head. It does appear larger than Sinornithosaurus
(skull 105% of femoral length vs. 95%), but it must be noted the lack of a
clearly defined posterior skull makes the exact size uncertain. The well
ossified nature of the elements is typical of even juvenile theropods (eg.
Microvenator, Saurornitholestes mongoliensis). The skeleton, although
complete, was shattered when the slab was split, making some details hard to
observe.
The skull is roughly triangular, but not as pointed
rostrally as Archaeopteryx. Compared to Sinornithosaurus, the snout is
deeper and slighly shorter (5% shorter compared to skull length). The
external naris is elongate and posteriorly tapering, unlike other basal
eumaniraptorans. Unfortunately, this is unknown in Sinornithosaurus and
Microraptor. The premaxilla is similar to Sinornithosaurus, but deeper
subnarially. The premaxilla of Microraptor is shorter antinarially.
The maxilla has a long antorbital portion, with a large ascending process that
contacts the lacrimal and an anteriorly tapered antorbital fenestra with a
rounded anterior margin. It should be examined for autapomorphies of
Sinornithosaurus, such as the extensive antorbital fossa, sculptured lateral
surface of antorbital fossa and well-separated maxillary and promaxillary
fenestrae. Ji et al. claim to be unable to identify any accessory
fenestrae. The nasals are slightly concave, as in many eumaniraptorans,
including Sinornithosaurus. The lacrimal is typically dromaeosaur-like,
with a long posterior process. The frontal has a long rounded ventral edge
for the orbit, showing the latter was large and round, as in most
eumaniraptorans. Most of the posterior skull is difficult to interpret,
although the postorbital is visible. This would appear to be the
triradiate form typical of maniraptorans and the posterior process looks to be
directed more ventrally than Sinornithosaurus. The palatine is visible
through the antorbital fenestra, it appears similar to that of
Velociraptor. The scleral ring fills the orbit and is composed of about
fifteen ossicles. The dentary is slender and ventrally convex, like
Bambiraptor, Velociraptor and Saurornitholestes. The left dentary of
Sinornithosaurus is broken, but the right may be curved. Microraptor has a
straight dentary. There is a large triangular posterior dorsal process,
much larger than that in Sinornithosaurus. In addition, the angular
appears much longer than the latter genus. The splenial appears to be
broadly exposed laterally, like other basal eumaniraptorans. There are at
least seventeen dentary teeth, of which two are clearly visible. These
teeth are recurved, lack basal constriction and have large posterior serrations,
but no anterior serrations. Sinornithosaurus is reported to have anterior
serrations. Microraptor lacks anterior serrations, but has constricted
roots.
There are ten amphicoelous cervicals and an
indeterminate number of platycoelous dorsals. Uncinate processes, sternal
ribs, dorsal ribs and gastralia are all present. The sacrum, if preserved,
is undescribed. The tail appears to only consist of about twenty
vertebrae, the proximal two thirds of which are reported to be a fused
rod. However, I can see individual centra at least half way through the
tail. Ji et al. are uncertain if the fused section is a preservational
artifact. The tail and centra are very similar to Microraptor in length,
but only a few centra of Sinornithosaurus are known. Like both those
genera, Bambiraptor and dromaeosaurids, the prezygopophyses and chevrons are
elongate. The chevrons are shorter than Deinonychus, extending only about
two central lengths.
The scapula is short and slender and, if I'm
identifying it correctly, has an elongate acromion projecting anteriorly.
This would be longer and more pointed than the acromion of Sinornithosaurus, but
similar to Bambiraptor. The coracoid appears elongate. The furcula
is said to be very similar to Sinornithosaurus, Bambiraptor and Archaeopteryx,
but much smaller than the former.
The humerus is elongate and sigmoid, with a
deltopectoral crest that is broader proximally than Sinornithosaurus, although
the latter may be deformed in that area. The humerofemoral ratio (.92) is
extremely similar to Sinornithosaurus (.91). The radius and ulna are
similar to Microraptor, but the radius isn't as proximally expanded. The
condition in Sinornithosaurus is difficult to compare, as the radius is
partially overlapped by the ulna. The radius is slightly more than half
the width of the bowed ulna. The ulnofemoral ratio is about .76, which
makes the arms comparatively longer than Microraptor (.66), but near identical
to Sinornithosaurus (.74). There are two carpals preserved, a semilunate
and radiale. They are not fused to the unfused metacarpals. The
metacarpals and phalanges are all very close in proportions to Sinornithosaurus,
with only III-2 being more than five percent shorter or longer. Metacarpal
I is about 30% of metacarpal II in length. The proximal end is
more rounded medially than in Microraptor. Ji et al. claim the first
metacarpal is unusually bowed, but it seems similar Sinornithosaurus in this
regard. Phalanx I-1 is elongate and bowed, being two thirds the length of
metacarpal II. The first manual ungual looks to be less elongate than
Sinornithosaurus, with a smaller flexor tubercle. Although Ji et al. claim
II-1 is equal to II-2 in length (which would be a very derived character), II-2
is actually about 10% longer, as in Sinornithosaurus. II-1 is very robust,
while II-2 is slender. Phalanx II-2 has an expanded proximal end in NGMC
91, but not in Sinornithosaurus. Manual ungual II is much less recurved
than in Sinornithosaurus. Metacarpal III is about two thirds the width of
metacarpal II and is almost equal in length. Phalanx III-3 is longer than
III-1, which is longer than III-2. However, NGMC 91 lacks the extremely
short phalanx III-2 found in Sinornithosaurus and to a slightly lesser extent,
Bambiraptor. Manual ungual III is less recurved and has a smaller flexor
tubercle than Sinornithosaurus. The first and second manual unguals are
subequal, the third is smallest. Unguals II and III appear
to have proximodorsal lips, unlike Sinornithosaurus. Highly recurved
keratinous sheaths are present.
I cannot make out pelvic details and it is not
described, but any remains should be examined closely for Sinornithosaurus
autapomorphies (pubis expanded distally, but not abruptly into a foot; obturator
process elongate, almost contacting pubis).
The femur reveals few details. The tibia is
elongate (140% femoral length) and shows a strong fibular crest.
Microraptor has a shorter tibia, at 128% of femoral length. The slender
fibula extends to the tarsus, which is undescribed. The metatarsus is
arctometatarsalian and unfused. It is reported to share non-ginglymoid
distal articulations with Sinornithosaurus and Microraptor, supposedly unlike
dromaeosaurids. Specifically, Sinornithosaurus lacks a ginglymoid
articulation on metatarsal III. This is the primitive condition and may be
assumed to be present in NGMC 91. The metatarsus is very similar to
Sinornithosaurus and Microraptor in length (65% vs. 63% vs. 62% of femoral
length). Digit I is not retroverted, possibly unlike
Microraptor. Phalanx II-2 is elongate compared to II-1, as in
dromaeosaurids and birds. Phalanx II-2 has a large proximoventral heel,
more prominent than in Sinornithosaurus and especially Microraptor. Also,
the proximodorsal articulation is not nearly as pronounced as in
Sinornithosaurus. The second pedal ungual is very elongate, but not very
curved, similar to the condition in Sinornithosaurus. Metatarsal V is
present. Impressions of tubercular scales are preserved under the
pedal digits.
Unbranched filaments, similar to those of
Sinosauropteryx, cover the head and neck, and terminate slightly anterior to the
antorbital fenestra. These filaments are from 20-32 mm long and
although appearing to form a cranial crest, are separated by sediment into
several layers. Filaments on the pectoral area, torso and hindlimbs are
attached to a single base, forming tufts or sprays. These can get quite
long on the hindlimbs (50 mm) and are similar to structures reported for
Sinornithosaurus. The remiges are located on the arms, attached to the
posterior ulnar edge. These are filaments tightly arranged in a
herring-bone pattern around a central rachis, suggesting barbules are
present. They are 52 mm long. Similar feathers have been reported
for Sinornithosaurus, although they are not as tightly arranged, perhaps
suggesting barbules are not present in the latter genus. Filament tufts
are preserved along the lateral edges of the tail, these are shorter at the tip,
forming a tapering tail fan, unlike the elongate rounded fan of Archaeopteryx or
the short distal fan of Caudipteryx.
Relationships-
NGMC 91 is obviously a dromaeosaur-grade
eumaniraptoran, as evidenced by the ventrally convex dentary, teeth with large
posterior serrations and reduced anterior serrations, elongate caudal
prezygopophyses and chevrons, boomerang-shaped furcula, radius longer than
metatarsus and modified second pedal digit (shortened phalanx II-1,
prominent proximoventral heel on phalanx II-2, enlarged ungual). It is
obviously closest to Sinornithosaurus in general morphology. Differences
from Microraptor include- premaxilla longer in front of naris; ventrally
convex dentary; teeth not constricted basally; forelimbs longer; radius not as
proximally expanded; proximal end of metacarpal I more rounded
medially; longer tibiotarsus; non-retroverted first pedal digit; much
larger proximoventral heel on phalanx II-2. Thus, NGMC 91 is unlikely to
be a specimen of that genus. Bambiraptor is also phyletically close to
Sinornithosaurus. The new specimen differs from this genus in the
following ways- snout tapered; naris posteriorly tapered and narrower;
premaxilla shallower subnarially; broader maxillary ascending process; orbit not
elongate; ventral postorbital process more prominent; teeth unserrated
anteriorly; smaller furcula; longer deltopectoral crest; longer radius and
ulna; shorter manual phalanges, except for III-2; more elongate phalanx III-2;
more elongate manual unguals with less prominent flexor tubercles; femur
straight; arctometatarsus. NGMC 91 can be distinguished from Rahonavis by-
shorter forelimbs; no ulnar quill knobs; caudal prezygopophyses and chevrons
elongate; fibula reaches tarsus; more prominent fibular crest; arctometatarsus;
longer metatarsus; proximoventral heel on phalanx II-2 fully developed.
Differences between NGMC 91 and Sinornithosaurus are more subtle and must be
examined closely, as they could be ontogenetic. The observable differences
are as follows-
- 63% as long. This could easily be
ontogenetic and is one of the main reasons this specimen could be a subadult
Sinornithosaurus.
- skull 10% larger relative to femur. This
measurement is only approximate, as the posterior edge of the skull is difficult
to define. Another less important source of error is the skull length of
Sinornithosaurus, as the skull was found in many pieces (my length estimate is
several millimeters longer than Xu et al.'s). This difference is
consistant with the possibility NGMC 91 is a young specimen, as relative skull
length decreases with age.
- snout shorter and deeper. This difference
could also be due to ontogenetic factors, as juvenile theropods are
characterised by short snouts (eg. Scipionyx, Byronosaurus).
- premaxilla deeper subnarially.
Sinornithosaurus differs from other dromaeosaurids and Bambiraptor (but not
Archaeopteryx) in that it has a very shallow premaxilla ventral to the external
naris. This is not seen in the new specimen, where the depth is similar to
Microraptor. I see no reason to presume this a juvenile
character.
- posterior postorbital process directed more
caudally. This is a risky difference, as the postorbitals of both
specimens are disarticulated. However, they would appear to coincide best
if the anterodorsally pointing surface of NGMC 91 and the dorsally pointing
surface of Sinornithosaurus are interpreted as being the orbital surfaces.
When interpreted in this way, the posterior process projects distinctly more
caudally than in Sinornithosaurus, where it is projected further
ventrally. Is this due to individual variation or age?
- much larger posterior dentary process. The
posterior dentary morphology of dromaeosaurs is poorly known, but
Sinornithosaurus has at least two posterior processes. The first is
ventral, while the smaller one is further dorsal. Presumedly, the
mandibular fenestra fit between them. NGMC 91 is difficult to interpret
due to photographic quality, but there is a large posterior dentary process
lying halfway through the depth of the jaw (visible directly below the
lacrimal). Below it is the splenial and behind and above, the
surangular. This does not correspond to either process in
Sinornithosaurus. One issue to consider is whether the mandible of NGMC 91
is exposed medially, in which case different processes might be
expected.
- longer angular. The angular of NGMC 91
seems to be much longer than that of Sinornithosaurus. This is complicated
by the fact I may be misinterpreting the photo and that the mandible may be
exposing its medial surface.
- teeth without anterior serrations. The few
dentary teeth observable are said to lack anterior serrations.
Sinornithosaurus was reported to have anterior serrations much smaller than
posterior serrations, implying their presence. While the presence of
serrations typically doesn't vary ontogenetically in theropods, it does vary
with position in the jaw. In some theropods (eg. Troodon) the
posterior dentary teeth are known to lack serrations when the other teeth have
them. As the two observable teeth in NGMC 91 are from the posterior
dentary, perhaps that explains their morphology, and perhaps Xu et al. weren't
explicitely talking about the posterior dentary teeth when commenting on
Sinornithosaurus' morphology. Thus, there are so many variables, I'm
hesitant to claim this is a definite difference between the
specimens.
- acromion process of scapula more elongate and
tapered. This is dependent on my identification of the scapula from figure
1 in Ji et al. I'm fairly certain I'm correct, as it is too elongate for a
coracoid and is shaped correctly. Is the difference ontogenetic
though? The similarity to the acromion of subadult Bambiraptor might have
implications for the age of this specimen.
- smaller furcula. This could be ontogenetic,
but this is difficult to determine.
- deltopectoral crest broader proximally. The
difficulty here is that the left humerus of Sinornithosaurus could be deformed,
as it is not smoothly sigmoid, while the right humerus is obscured
proximally.
- manual ungual I less elongate, II and III
less recurved, I and III with smaller flexor tubercles. Variation in
curvature (Deinonychus pedal ungual II) and flexor tubercle size (Velociraptor
manual ungual III) is known in dromaeosaurids, so this may be intraspecific
variation. I would assume ontogeny could affect this as well.
- manual phalanx II-2 expands gradually in
transverse width proximally. This differs from Sinornithosaurus, where the
phalanx has parallel sides for its entire length. This could be due to
misinterpretation because the phalanx in NGMC 91 is slightly tilted, resulting
in a ventromedial view (it is a direct ventral view in
Sinornithosaurus).
- manual phalanx III-2 more elongate. The
only manual element that is significantly different in length from
Sinornithosaurus, this element is not nearly as reduced as in that genus or
Bambiraptor. Is this ontogenetic?
- manual unguals II and III with proximodorsal
lips. Known in a variety of dromaeosaurs, this seems to be the case in
NGMC 91, but not Sinornithosaurus. I'm fairly certain my identification of
this is correct in NGMC 91. Is it probable this is ontogenetic as
well?
- pedal phalanx II-2 with larger proximoventral
heel and more rounded posterodorsal corner. These elements are quite
different in the specimens. The phalanx of NGMC 91 has a much smaller
distal articulation relative to total length, has a much larger heel and a
blunter proximodorsal corner than Sinornithosaurus (based on a close-up
from Ji et al., and Xu and Wang's description of the pes of
Sinornithosaurus, respectively). You might expect the second digit to be
more powerful in the adult, not the juvenile, which makes this difference
odd.
- near-identical limb ratios. This
"difference" may sound odd, but most theropods have varying limb ratios through
ontogeny. For instance, Saurornitholestes, Gorgosaurus and Allosaurus all
have much shorter distal hindlimbs as adults than as juveniles. Thus, the
near identical ratios seen in NGMC 91 and Sinornithosaurus might suggest the
first is not a juvenile example of the second.
The question that must now be asked is "are these
differences enough to suggest ontogeny and individual variation cannot be
responsible?". Also, can we identify autapomorphies of Sinornithosaurus in
this specimen? I think, if most of the differences listed above are real,
it's probably another genus or a new species of Sinornithosaurus. The
complications detailed above make me hesitant to put my full support behind this
opinion though. However, although I cannot identify any autapomorphies of
Sinornithosaurus in NGMC 91, their presence could easily be determined from
closer inspection of the material. So for now, I'll be referring to it as
cf. Sinornithosaurus, a much more appropriate designation than Dromaeosauridae
gen. et sp. indet.. By the way, the characters that can be scored in my
matrix are identical to Sinornithosaurus, although a few are unknown in the
latter genus. Including both makes NGMC 91 in a polytomy with
Bambiraptor+Dromaeosauridae and Sinornithosaurus. Adding the newly codable
characters of NGMC 91 to the entry of Sinornithosaurus does not alter the
latter's position, currently as the sister group to
Bambiraptor+Dromaeosauridae. Eumaniraptoran relationships are very
uncertain however, so I would say that both Sinornithosaurus and NGMC 91 are
eumaniraptorans neither belonging to the Dromaeosauridae nor the
Pygostylia. Their closest relatives are Bambiraptor, Microraptor,
Unenlagia, Rahonavis and Archaeopteryx.
Anyone who wants the three figures not available on
the AMNH website, feel free to contact me offlist. They are of the
complete manus(pleural), caudal vertebra and pedes.
Mickey Mortimer
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