The St. Croix macaw (Ara autocthones ) or Puerto Rican macaw, is an extinct species of macaw whose remains have been found on the Caribbean islands of St. Croix and Puerto Rico. It was described in 1937 based on a tibiotarsus leg bone unearthed from a kitchen midden at a pre-Columbian site on St. Croix. A second specimen consisting of various bones from a similar site on Puerto Rico was described in 2008, while a coracoid from Montserrat may belong to this or another extinct species of macaw. The St. Croix macaw is one of 13 extinct macaw species that have been proposed to have lived on the Caribbean islands. Macaws were frequently transported long distances by humans in prehistoric and historical times, so it is impossible to know whether species known only from bones or accounts were native or imported.

As it is only known from bones, the St. Croix macaw's color is not known. Extant macaws can generally be grouped in either large-body or small-body size clusters. Yet, the bones of the St. Croix macaw are intermediate in size between the two, and it was slightly larger than the extinct Cuban macaw (Ara tricolor ). Only the blue-throated macaw (Ara glaucogularis ) and Lear's macaw (Anodorhynchus leari ) are similar in size. It differed from other macaws in various skeletal details and shared several features with only the genus Ara. Like other macaw species in the Caribbean, the St. Croix macaw is believed to have been driven to extinction by humans, as indicated by the fact that its remains were found in kitchen middens.

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Appearance

Since only bones are known of the St. Croix macaw, nothing can be said about its coloration. While the holotype tibiotarsus appears to belong to a fully grown individual, the fact that the bone is slightly spongy at the ends indicates it was immature. This left tibiotarsus is 77.7 mm (3.06 in) in total length, 9.4 mm (0.37 in) in breadth from side to side across the lower end, and the smallest breadth from side to side of the shaft is 3.9 mm (0.15 in). Though similar to the same bone in the Cuban macaw, it is wider from side to side (comparisons between the lower ends of their tibiotarsi indicate it was a slightly larger bird). It is slender compared to those of larger macaws. The holotype tibiotarsus is intermediate in size between those of large macaws such as the scarlet macaw (Ara macao ) and the military macaw (Ara militaris ), and the small chestnut-fronted macaw (Ara severus ). Compared to the tibiotarsi of extant macaws, the bone is more slender and has a slightly greater hindwards development of the upper end. Apart from this, its only distinguishing feature is that its dimensions do not fall within those of other known species. The slender proportions of the bone and more elongated ridges around the upper end show it is a macaw and distinguishes it from the Amazon parrots such as the large imperial amazon (Amazona imperialis ).

The tibiotarsus of the assigned specimen is essentially identical in size to the holotype. It is 74.5 mm (2.93 in) from the upper articular surface, 47.2 mm (1.86 in) from the lower end of the fibular crest to the external condyle (the round prominences at the end of a bone), 11.4 mm (0.45 in) deep through the inner cnemial crest (a ridge at the front side of the head), 5.2 by 4.0 mm (0.20 by 0.16 in) wide and deep at the mid shaft, and 10.1 mm (0.40 in) wide at the lower end. The estimated length of the coracoid is 43.5 mm (1.71 in) from its head to the internal corner of the sternal facet, 26.5 mm (1.04 in) from the base of the procoracoid process to the internal corner of the sternal facet, the glenoid facet is 10.5 by 6.1 mm (0.41 by 0.24 in) wide and deep, the shaft is 5.2 by 4.4 mm (0.20 by 0.17 in) wide and deep at midpoint, and the sternal facet is 10.5 mm (0.41 in) wide. The upper width of the humerus is 19.9 mm (0.78 in), the depth through its external tuberosity is 12.8 mm (0.50 in), the depth of its head is 6.2 mm (0.24 in), the width of the lower end is about 15.5 mm (0.61 in), and the height and width of the radial condyle is 7.4 by 5.0 mm (0.29 by 0.20 in). The carpometacarpus is 55.8 mm (2.20 in) long, its upper depth is 14.3 mm (0.56 in), the trochlea (a grooved structure where bones join) is 5.7 mm (0.22 in) wide, and the shaft is 4.5 by 5.0 mm (0.18 by 0.20 in) at midpoint. The upper part of the radius is 6.0 mm (0.24 in) at its greatest diameter. The estimated length of the femur is 51.5 mm (2.03 in), its upper width is 12.5 mm (0.49 in), the depth through its trochanter is 8.2 mm (0.32 in), the head is 6.2 mm (0.24 in) deep, and the shaft is 4.9 by 5.4 mm (0.19 by 0.21 in) wide and deep at its midpoint.

Olson and Máiz López stated that extant macaws fall into two size-clusters, representing large and small species. In contrast, the St. Croix macaw was distinct in being intermediate between the two clusters, with only the blue-throated macaw (Ara glaucogularis ) and Lear's macaw (Anodorhynchus leari ) being similar in size. They observed that the pectoral attachment on the humerus is less excavated compared to those two macaws. In contrast, the capital groove (a groove separating two parts of the humerus's head) is wider. The head of the femur is more massive, and when seen from the back, is more excavated under the head, neck, and trochanter. In contrast, the femur's more robust shaft is similar to that of Ara but dissimilar to Anodorhynchus. The tibiotarsus is more robust with a flared lower extremity. The tibiotarsus' length is shorter than in the blue-throated macaw but longer than in Lear's macaw. In contrast, the lengths of the coracoid, carpometacarpus, and femur are smaller than in either.

Olson and Máiz López ruled out the specimen from Puerto Rico belonging to the Amazon parrots by pointing out characters found only in Ara macaws. The coracoid is more elongated and has a relatively narrow shaft, and the ventral lip of the glenoid facet (equivalent to the glenoid fossa of mammals) protrudes more. The carpometacarpus is proportionally much longer with a process on the alular metacarpal that is not curved at its upper part, 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 is placed farther upwards. The femur has a proportionally larger head, and the tibiotarsus has a narrower internal condyle and a distinctive inner cnemial crest that is more pointed and extends further upwards.