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Hi there everyone! Here's some information on
a recently described pygostylian. Is it a true enantiornithine, or a
more basal form like Protopteryx?
Eoenantiornis Hou, Martin, Zhou and Feduccia
1999
E. buhleri Hou, Martin, Zhou and Feduccia
1999
Etymology- "Buhler's dawn opposite bird", after
Paul Buhler, a German funcional morphologist and paleornithologist; eo, greek
for dawn; enantiornis, opposite bird.
Barremian, Early Cretaceous
Yixian Formation, Liaoning, China
Holotype- (IVPP V11537) (135 mm) skull (22 mm),
lower jaw, eleven cervical vertebrae (~29 mm), gastralia fragments?, synsacrum,
pygostyle, coracoid (12.5 mm), furcula, sternum, humerus (29.5 mm), radius, ulna
(31 mm), semilunate carpal, metacarpal I, phalanx I-1, manual ungual I,
metacarpal II (12 mm), phalanx II-1, phalanx II-2, manual ungual II, metacarpal
III, phalanx III-1, pubis, ischium, femur (26.5 mm), tibiotarsus (31 mm),
tarsometatarsus (22.3 mm), pes
Diagnosis- sternum lacks lateral processes;
anterior sternum indented.
Description-
Scaling from Confuciusornis' femoral length,
Eoenantiornis was about 135 mm long. This taxon is well described in the
cranial, pectoral and forelimb areas, but the rest lacks both description and
illustration. It is described in part by Martin and Feduccia, so beware of
information involving manual digit identification. Also, the Sauriurae and
non-dinosaurian origin for birds are both supported.
The skull is well preserved. It is quite
short and triangular, with enormous orbits and a blunter snout than other basal
avians. The premaxilla has at least three teeth and a long subnarial
process. The maxilla lacks fenestrae anterior to the antorbital fenestra
and has a narrow dorsal process that makes up the posterior border of the
external naris. The naris itself is quite large, being larger than the
antorbital fenestra and making up 25% of the skull length. The maxilla
also shows a narrow anterior process and an elongate posterior process that
prevents the jugal from participating in the antorbital fenestra. There
are at least eight maxillary teeth, which are smaller than the premaxillary
teeth. They exhibit unserrated carinae and constricted bases. The
nasal is rather tall, but shorter than the frontal. Although the lacrimal
is missing, the antorbital fenestra was obviously short and triangular.
The jugal is also missing, but the narrow posterior maxillary process suggests
that it was rod-like. The frontal is bulbous and has a long contact with
the shorter parietal. No prefrontal, postorbital or squamosal is
shown. The quadrate has a blunt anterodorsally projected orbital
process. The dentary is straight and contains at least twelve teeth,
placed in a groove. There appears to be no external mandibular fenestra
and the surangular and angular are fused.
There are eleven cervical vertebrae.
Fragments of gastralia may be present and the synsacrum contains 6-8
vertebrae. The pygostyle is similar to Cathayornis and pointed
distally.
The coracoid is strut-like, but stouter than many
enantiornithines. It is convex ventrolaterally as in
enantiornithines. The furcula has a narrow interclavicular angle (~50
degrees) and long hypocleidium. It is grooved dorsally throughout its
length. The sternum is about as long as wide, with a ventral keel.
It has a long poteriomedial process and paired short posterior
processes. The additional long expanded lateral processes of some
other enantiornithines are lacking. It is convex laterally and has a small
indentation anteromedially.
The humerus lacks confuciusornithid specializations
and has a prominent posteriorly projecting internal tuberosity. The radius
is much thinner (~60%) than the bowed ulna. The radius is subequal to the
humerus in length. Of the carpus, the semilunate and ulnare are
preserved. There appears to be a carpometacarpus present, although distal
fusion seems absent. The first digit is shorter than metacarpal II, with
an ungual smaller than the one on digit II. Metacarpal II is slightly
expanded distally and shorter than the bowed metacarpal III. There are two
non-ungual phalanges on digit II, the proximal one longest. Only one
phalanx is present on digit III, it is short an lacks an ungual.
The pelvis is said to be opisthopubic. The
tarsometatarsus is fused proximally and the hallux is reversed.
An alula is preserved attached to the
manus.
Relationships-
Hou et al. state this species is a primitive
enantiornithine, based on the stout coracoid, lack of expanded posterolateral
sternal processes and primitive manual characters. They argue that
Eoenantiornis provides further evidence against theropod ancestry for birds
based on the following features it exhibits- unserrated teeth with basal
constrction (also in Archaeornithoides, etc.); no intermandibular joint (also in
ornithomimosaurs and oviraptorosaurs); furcula grooved (also in Bambiraptor?);
scapulocoracoid angle 90 degrees (flight-correlated); hand composed of digits
2-3-4 (man, it's a shame they didn't publish this embryological study of
Eoenantiornis ;-) ); semilunate mostly on metacarpal II (a consequence of
metacarpal II having a larger base than metacarpal I, as the latter is reduced
in avians); opisthopubic (also in dromaeosaurids, alvarezsaurids, segnosaurs,
etc.); pubis with features for pubic-breathing (this makes no sense to me,
aren't Martin, Ruben, etc. advocating the hepatic-piston system of respiration
[which requires a mobile pubis] in non-avian theropods?);
reversed hallux (also in Microraptor). Sorry for being a bit sarcastic and
this is certainly not the place to argue the birds-are-dinosaurs viewpoint, but
suffice to say, none of the features in Eoenantiornis is a "formidable
obstacle to the widely held hypothesis of a dinosaurian origin for birds",
despite Hou et al.'s claims.
As for it's relationships within Pygostylia, adding
Eoenantiornis to my Protopteryx phylogenetic analysis places Eoenantiornis
firmly in the Enantiornithines. Eoenantiornis lacks confuciusornithid
synapomorphies (toothlessness; caudal margin of sternum v-shaped; deltopectoral
crest of humerus prominent and subquadrangular; manual ungual II much smaller
than other manual unguals) and shares several synapomorphies with Jibeinia and
ornithothoracines (mobile scapulocoracoid joint; interclavicular angle 70
degrees or less; ulna subequal or longer than humerus; less than four phalanges
on manual digit III; reduced manual unguals; long hypocleidium; alula
present). It is more derived than Protopteryx (manual digit I shorter
than metacarpal II; manual phalanx II-2 shorter than phalanx II-1) and
Jibeinia (interclavicular angle of furcula about 50 degrees or
less; less than one phalanx on manual digit III; maxillary fenestra absent;
metacarpal I fused to metacarpal II). Eoenantiornis exhibits the following
enantiornithine synapomorphies- coracoid laterally convex; prominent
cranioventrally projecting bicipital crest on humerus; metacarpal III longer
than metacarpal II. Although Protopteryx also shows the last two
characters, phylogenetic analysis places it more basally.
Is there any evidence to support Hou et al.'s
belief that this taxon is basal to other enantiornithines such as Cathayornis,
Enantiornis and Sinornis? This was based on several characters.
First, they argue the coracoid is stouter than derived enantiornithines. The
width/length ratio (basal width measured along sternal articulation divided by
length perpendicular to sternal articulation) of Eoenantiornis is 54. This
compares to 21 in Neuquenornis, 30 in Enantiornis, 41 in Iberomesornis, 45 in
Concornis, 45 in Protopteryx, ~50 in Jibeinia, 55 in Longchengornis. Some
other taxa (Eoalulavis, Cathayornis? caudatus, the Spanish nestling,
etc.) don't have complete coracoids, but the partial outlines suggests they were
in the 35-45 range. Thus, some enantiornithines are comparable to
Eoenantiornis in coracoid length, but others show a range of variation that
reaches it's peak in Neuquenornis. In addition, more basal taxa like
Confusciusornis, Protopteryx and Jibeinia have more slender coracoids,
suggesting the condition in Eoenantiornis is secondarily derived. Another
supposedly plesiomorphic character is the absence of expanded lateral proccesses
on the sterna. These are present in Cathayornis, Cathayornis? caudatus,
Concornis and Largirostrornis. They are absent in Eoalulavis.
Various forms of lateral processes are also known in confuciusornithids,
Jibeinia and Protopteryx. A study of variation to identify homologies and
synapomorphies is needed before conclusions can be made.
Confuciusornithids show a single pair of lateral processes, which divide into
two short processes projecting laterally and posteriorly. Protopteryx is
similar, but its posterior processes are elongate and slightly expanded.
In Jibeinia, the lateral and posterior processes are separated further on the
sternal body, forming two processes out of the ancestral one. The lateral
pair are short and either narrow or slightly expanded (varies from right to
left), while the posterior pair are short and pointed. Most
enantiornithines have two pairs of processes as well, the posterior pair
resembling Jibeinia, the lateral pair long and expanded. Eoenantiornis
lost the lateral pair and retains the posterior pair. Eoalulavis lost
both. As the ancestral condition is to have two pairs of sternal
processes, the presence of only one pair in Eoenantiornis is probably
secondarily derived. Primitive manual characters compared to Cathayornis
include: longer first digit; distal end of phalanx II-1 not as expanded; phalanx
II-2 longer; phalanx III-1 less reduced. The first digit is
actually shorter in Jibeinia, phalanx II-1 is more expanded in
Protopteryx and phalanx II-2 is shorter in Jibeinia. This calls into
question the utility of slightly varying ratios in these elements to
indicate phylogenetic relationships. It's apparent that the evidence in
favor of Eoenantiornis being basal to other enantiornithines is less than
satisfactory. Where exactly it fits within the group is still to be
determined. It could still be a basal member, but better evidence must be
presented. I classify Eoenantiornis as an enantiornithine, but defer
further specification until a phylogenetic analysis is performed on the
group.
Whoever wants the figure of Eoenantiornis (includes
skull, coracoids, furcula, sternum and forelimb), just ask me. Fukuiraptor
is next........
Mickey Mortimer
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