Mancalla lucasi, Smith, Neil Adam, 2011

Smith, Neil Adam, 2011, Taxonomic revision and phylogenetic analysis of the flightless Mancallinae (Aves, Pan-Alcidae), ZooKeys 91, pp. 1-116 : 8-11

Mancalla lucasi sp. n.

Holotype.

SDSNH 25237: a partial postcranial skeleton comprising the following elements: right and left scapulae, partial sternum, right and left humeri, left femur (Fig. 3; Tables 1, 2 and 3). The holotype specimen was collected by H. M. Wagner in April, 1980.

Etymology.

This new species is named in honor of Frederic A. Lucas who described the first known remains of Mancalla .

Locality and horizon.

Late Pliocene or Early Pleistocene (Zanclean or Calabrian) Niguel Formation of Orange County, California. Latitude, longitude, and elevation data are on file at SDSNH (locality 3202). Details of the geologic setting are provided in Appendix 6.

Referred specimen.

SDSNH 59049: a complete left humerus from the Middle Pliocene to Early Pleistocene San Diego Formation (SDSNH locality 3506; Fig. 4E).

Differential diagnosis.

Scar extending into primary pneumotricipital fossa is raised in relief to the floor of the primary pneumotricipital fossa and the humeral shaft as in Mancalla cedrosensis ,rather than an excavated pit as in Mancalla vegrandis sp. n. and Mancalla californiensis Lucas 1901 (121:1; Fig. 5); dorsal and ventral edges of scar extending into primary pneumotricipital fossa taper to a point as in Mancalla vegrandis , rather than remaining parallel as in Mancalla californiensis and Mancalla cedrosensis (123:1); humerus longer than Mancalla cedrosensis , Mancalla californiensis ,and Mancalla vegrandis (Tables 2, 3).

Anatomical description.

Both scapulae are preserved (Fig. 3G, H). As in all Alcidae , the scapular shaft is mediolaterally compressed throughout its entire length. The proximal end of the scapular shaft is more rounded in other Charadriiformes. As in Mancalla vegrandis , the acromion projects farther anteriorly than that of Mancalla cedrosensis and other alcids (e.g., Uria , Aethia ). As in Mancalla cedrosensis , the coracoidal tubercle is less pronounced than in Mancalla vegrandis . As in Mancalla vegrandis and Mancalla cedrosensis , a scapulotricipital tubercle is present just distal to the glenoid process on the ventral margin of the scapular shaft. This feature is also present in other flightless wing-propelled divers such as Spheniscidae and Pinguinus , but is not known in any volant alcid. As in Mancalla vegrandis , the scapular shaft, including the caudal extremity, is slightly more robust than in other alcids (e.g., Alca , Aethia ). The caudal extremity is less dorsoventrally expanded than in Mancalla vegrandis . The caudal extremity is not known for Mancalla cedrosensis .

Fragments of the sternum preserve the sternal rostrum, coracoidal sulci, and the carinal apex (Fig. 3A, B). These features are not preserved in Miomancalla howardi and comparisons are therefore limited to extant alcids and specimens of Mancallinae that are not presently referable to species. The morphology of the sternal rostrum is consistent with that of all other Alcidae . Although no coracoid is preserved in the holotype specimen of Mancalla lucasi , the shape of the coracoidal sulci of the sternum is consistent with the ~150° angle of the sternal articulation of the coracoid in Mancalla cedrosensis and Mancalla vegrandis . The sternal articulation of the coracoid, and the coracoidal sulci of the sternum in other alcids curves more acutely (e.g., ~90° in Alca torda ; Fig. 6).

Complete right and left humeri are preserved (Fig. 3C, D, E and F). Based upon humeral proportions, Mancalla lucasi represents the largest known species of Mancalla (Table 2). As in other Mancalla species, the ventral margin of the ventral tubercle is convex, and the capital groove is relatively narrower than other Alcidae . The ventral tubercle does not project as far ventrally as in Mancalla californiensis (Fig. 5). The distal end of the deltopectoral crest transitions to the shaft more abruptly than in Mancalla vegrandis . As in other Mancalla , the humeral head is rotated anteriorly, and the supracoracoideus muscle scar does not broaden proximally. Mancallinae is characterized by a scar of unknown function that is positioned adjacent to the primary pneumotricipital fossa (hereafter referred to as the 'mancalline scar’; Fig. 5). The position of the 'mancalline scar’ suggests an accessory insertion of m. humerotriceps ( Howard 1949), which can be divided into as many as four separate heads in some birds ( Baumel and Witmer 1993). Other potentially homologous muscle scars include m. coracobrachialis cranialis, which is well developed in penguins ( Ksepka et al. 2008), or m. scapulocranialis caudalis (see Matsuoka and Hasegawa 2007). However, the exact function of this feature is unknown because it is not present in any other charadriiform. The shape, position, and development of this scar is variable in Mancallinae (Fig. 5). The 'mancalline scar’ of Mancalla lucasi is raised in relief like that of Mancalla cedrosensis , rather than excavated as in Mancalla californiensis and Mancalla vegrandis (Fig. 5). As in Mancalla vegrandis , the scar extends from a point just proximal to the junction of the bicipital crest with the humeral shaft, tapers to a point, and extends into the primary pneumotricipital fossa (Fig. 5). The dorsal and ventral margins of the 'mancalline scar’ remain approximately parallel in Mancalla californiensis and Mancalla cedrosensis (Fig. 5). As in all Mancallinae , the humeral shaft is arced rather than sigmoidal or straight. As in other Mancalla , the dorsal supracondylar process is separated from the dorsal epicondyle by a small notch. A tubercle or papilla is present on the posterior side of the distal end of the humerus adjacent to the dorsal condyle ( Howard 1976). As with all Mancallinae , the anterior surface of the ventral condyle is rounded, rather than flattened as in all other alcids. Rounded fossae are present at the proximal ends of the humerotricipital and scapulotricipital grooves. The flexor process extends distal to the ventral condyle as in all Mancallinae and Pinguinus .

The left femur is preserved (Fig. 3I) and is smaller (~15%; Table 2)than in Miomancalla howardi sp. n. (Table 3), and larger (~19%) than in Mancalla cedrosensis (Howard, 1971). Extant alcids do not display statistically significant degrees of sexual dimorphism in their size, plumage, or osteological morphology ( Storer 1952; Nettleship and Birkhead 1985; Szekely et al. 2000). Thus, it can be reasonably assumed that extinct alcids were also not sexually dimorphic as the proposed sister taxon of all alcids, the Stercorariidae ( Ericson et al. 2003; Thomas et al. 2004; Baker et al. 2007; Pereira and Baker 2008), as well as the closely related Laridae are also not sexually dimorphic. This range of size between Mancalla species is greater than the range of intraspecific variation documented for other alcids (~1-5%), including the flightless Great Auk ( Moen 1991; Burness and Montevecchi 1992). As in Alle , Cepphus Mörhing, 1758, Synthliboramphus Brandt, 1837, and Brachyramphus Brandt, 1837, the femoral trochanter projects anteriorly in lateral view. The femoral trochanter in Uria , Aethia Merrem, 1788, Alca , and Pinguinus is not projected anteriorly (i.e., straight), and the trochanter is concave in lateral view in Fratercula Brisson, 1760 and Cerorhinca Bonaparte, 1828. Femora of Miocepphus are not known. No diagnostic characteristics of the femur of Mancalla lucasi were identified.

Remarks.

Mancalla lucasi corresponds in size and some humeral characteristics with material previously referred to Mancalla diegensis . However, Mancalla diegensis is considered Alcidae incertae sedis (see Appendix 1 for details of the taxonomic revision).