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Details on Eoenantiornis



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