Description
The subfossil left tibiotarsus (holotype, USNM 483530) described by Wetmore was of an immature but full-grown bird. Wetmore described the bone as similar to the tibiotarsus of Ara tricolor, with a larger transverse width, but more slender compared with larger macaws. The slender proportions of the bone and more elongated ridges about the proximal end distinguish the species from the Amazon parrots.
The bones found by Máiz López (USNM 448344) include the left coracoid (missing a portion of the bones head), the proximal and distal ends of the left humerus, the proximal end of the right radius, the left carpometacarpus (missing one metacarpal), the left femur (lacking the distal end), the right tibiotarsus (lacking part of the proximal articular surface), and the proximal fragment and worn distal portion of the left tibiotarsus.
Olson and Máiz López examined the bones and showed that they differed from Amazon parrot bones. The tibiotarsus has a narrower internal condyle (the round prominences at the end of a bone) and a distinctive inner cnemial crest (a ridge at the front side of the head) that is more pointed and extends further proximally. The carpometacarpus is proportionally much longer with a process on the alular metacarpal that is not curved proximally. The femur has a proportionally larger head while the ectepicondylar process (a bony elevation) and the attachment of pronator brevis (one of the two pronation muscles in the wing) on the humerus are more proximal. The elongated coracoid has a relatively narrow shaft and the ventral lip of the glenoid facet (equivalent to the glenoid fossa of mammals) is more protruded.
Olson and Máiz López showed that the Saint Croix Macaw is within the same size range as the Blue-throated Macaw and the Lear's Macaw. The length of the tibiotarsus is shorter than in the Blue-throated Macaw but longer than in the Lear's Macaw, while the lengths of the coracoid, carpometacarpus, and femur are smaller. In addition to the size, they observed that the pectoral attachment on the humerus is less excavated while the capital groove (a groove separating two parts of the head of the humerus) is wider; the head of the femur is more massive and when seen from the posterior side, more excavated under the head, neck, and trochanter. Furthermore, the more robust shaft of the femur sets it specifically apart from the Lear's Macaw while the tibiotarsus is more robust with a flared distal extremity.
Read more about this topic: Saint Croix Macaw
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