[D] Halimornis thompsoni [~/~]
Chiappe, Lamb & Ericson, 2002
Cretaceous Late Santonian Campanian
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Enantiornithes
AGr-4, approximately 11 km west of Clinton, Greene County, Unnamed lower member, Moorville Chalk, Alabama, US
Vertebrae and limb elements.
Discovered in 1992, during a field party led by Storrs L. Olson (National Museum of Natural History)
Enantiornithine bird with the bicipital crest of the humerus approaching the level of the humeral head and thus more proximally located than in any other enantiornithine, and an inflated area projecting laterally in the distal end of the femur. These characteristics are regarded as autapomorphies.
Halimornis thompsoni from the Greek ‘‘halimos’’ (meaning ‘‘belonging to the sea,’’ in allusion to the marine deposits in which the specimen was found), and ‘‘ornis’’ (meaning bird); the specific name ‘‘thompsoni’’ is after Mrs. W. Thompson, the landlord of the area in which the specimen was found, in recognition of her many years of support of fossil collecting on her property.
The new specimen is housed in two institutions. The first discovered elements are catalogued as D2K 035 (Discovery 2000, Birmingham). These elements include the proximal end of the right humerus, the distal end of the right femur, two trunk vertebrae, one caudal vertebra, and the pygostyle. Subsequent excavation of the site produced a few other elements, which were catalogued as UAMNH PV996.1.1 (Alabama Museum of Natural History, Tuscaloosa). These include the shoulder half of the left scapula, a thoracic vertebral centrum, and a thoracic neural arch. All of these elements were found as a fan-shaped scatter on a slightly sloping surface, within an area smaller than 400 cm2. Our understanding of the taphonomic conditions of the Moorville Chalk, and the fact that the greatest distance between any two elements was 32 cm, leads us to the conclusion that the bones unquestionably belong to a single individual. The two different collection numbers are a consequence of the inactivity of the former Red Mountain Museum collections (now Discovery 2000).
Halimornis thompsoni shares several synapomorphies with Euenantiornithes, a subgroup of Enantiornithes defined as all taxa closer to Sinornis santensis than to Iberomesornis romerali. These synapomorphies include the presence of: (1) a prominent longitudinal furrow on the medial surface of the scapular blade; (2) a cranioventrally projected bicipital crest of the humerus; (3) a median concavity of the superior margin of the humeral head; and (4) the median position of the parapophyses of the thoracic vertebrae. The presence of the prominent craniocaudally concavity-convexity of the humeral head, a fossa for muscular attachment on the ventral face of the bicipital crest of the humerus, wide lateral excavations on the thoracic centra, and a cranial fork in the pygostyle may also be synapomorphies of this clade (if so, some would be convergent to other birds). The fragmentary nature of the single known specimen of Halimornis thompsoni, along with the incomplete knowledge of the interrelationships among enantiornithines complicates the assessment of its phylogenetic position beyond Euenantiornithes.