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Alca minor
Taxonomy
Alca minor was named by Smith and Clarke (2011). Its type specimen is USNM 302324, a limb element (left humerus), and it is a 3D body fossil. Its type locality is Lee Creek Mine, Yorktown Formation, which is in a Zanclean estuary/bay sandstone in the Yorktown Formation of North Carolina.
Synonymy list
| Year | Name and author |
|---|---|
| 2011 | Alca minor Smith and Clarke p. 27 figs. Fig. 14 |
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If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.
†Alca minor Smith and Clarke 2011
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Diagnosis
| Reference | Diagnosis | |
|---|---|---|
| N. A. Smith and J. A. Clarke 2011 | Although this smallest known species of Alca (Table 4) can be morphologically differentiated from other known species of Alca, there are no autapomorphic characters preserved in the three specimens referred to this species. Unlike all other Alca, and similar to the condition observed in Uria, the coracobrachial nerve appears to be transmitted in a sulcus rather than in a closed canal; however, the possibility that the coracobrachial nerve passage was exposed owing to weathering cannot be excluded. The holotype (USNM 302324) and referred specimens (USNM 192879 and USNM 495600) of A. minor were previously referred to “Miocepphus undescribed species” (Olson and Rasmussen 2001:270). Alca minor is differentiated from Miocepphus by the equal width of the tricipital sulci (82:1), and by the relative robustness of the humerus. In A. minor the average dorsoventral height of the proximal humerus is 2.33× wider than the shaft and the average dorsoventral height of distal humerus is 1.57× wider than the shaft. The tricipital sulci of Miocepphus are of different widths, the humeral shaft is thicker, and the proximal and distal ends of the humerus are more dorsoventrally expanded. In Miocepphus the average dorsoventral height of the proximal humerus is 2.68× wider than the shaft (ratio derived from measurements of M. blowi and M. mergulellus), and the average dorsoventral height of distal humerus is 1.75× wider than the shaft (ratio derived from measurements of Miocepphus holotype specimens). Alca minor is differentiated from Alle alle, the smallest member of the Alcini, by its larger size, equal width of the tricipital sulci (82:1), and dorsally curving ventral margin of the distal humerus (Alle characterized by ~52% shorter greatest length, scapulotricipital sulcus wider than humerotricipital sulcus; ventrally flared ventral margin of distal humerus). Alca minor is differentiated from similarly sized Synthliboramphus and Brachyramphus by the rounded shape (67:0) and shallow depth of the primary pneumotricipital fossa (66:0). Because of the fragmentary nature of the holotype specimen of A. ausonia (IGF 14875; Fig. 2), discernable differences between that specimen and A. minor are limited to size. The greatest width at the midpoint of the humeral shaft of A. minor is ~32% smaller than that of Alca ausonia (Table 4). This is significantly outside the range of intraspecific variation documented for other alcids (Moen 1991, Burness and Montevecchi 1992, Table 3). Alca minor is further differentiated from A. grandis by the lack of a posterodorsally projecting tubercle on the posterior margin of the ventral condyle (84:0; Fig. 10). In contrast to other Alca in which the primary pneumotricipital fossa is deeper and more ovoid, the primary pneumotricipital fossa is relatively shallow and more rounded. As in specimens referred to A. ausonia, the distal margin of the primary pneumotricipital fossa is concave (70:0) rather than straight as in other Alca. Alca minor is differentiated from all other species of Alca by its overall smaller size (Table 4; greatest length of humerus: ~75%< A. stewarti; ~63%< A. olsoni; ~61%< A. carolinensis; ~53%< A. grandis; ~27%< A. torda). |
Measurements
No measurements are available
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| Source: subo = suborder, o = order | |||||
| References: Marsh 1875, Benton 1983, Bush and Bambach 2015 | |||||