Mancalla vegrandis, 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 : 11-14

Mancalla vegrandis sp. n.

Holotype.

SDSNH 77399: a partial postcranial skeleton comprising the following elements: two cervical vertebrae, one costal and one vertebral rib, partial furcula, scapulae, left coracoid, partial right coracoid, partial sternum, left humerus, and pelvis (Figs 7 and 8; Tables1, 2 and 3). The holotype specimen was collected by W. T. Stein in October, 1961.

Etymology.

The species name vegrandis reflects the diminutive size of this taxon compared to other known Mancalla species (vegrandis, from the Latin for small, diminutive or tiny).

Locality and horizon.

Middle Pliocene to Early Pleistocene (Zanclean-Calabrian) San Diego Formation of San Diego County, California. Latitude, longitude, and elevation data are on file at SDSNH (locality 4273). Details of the geologic setting are provided in Appendix 6.

Referred specimens.

SDSNH 42532: a complete left humerus from the Middle Pliocene to Early Pleistocene San Diego Formation of San Diego County, California (SDSNH locality 3468); SDSNH 42534: a complete right humerus from the Middle Pliocene to Early Pleistocene San Diego Formation of San Diego County, California (SDSNH locality 3468); SDSNH 28152: a complete right humerus from the Early Pliocene upper member of the San Mateo Formation of San Diego County, California (SDSNH locality 3161); SDSNH 75051: a complete right humerus from the Early Pliocene upper member of the San Mateo Formation of San Diego County, California (SDSNH locality 2643; Fig. 4 A–D).

Differential diagnosis.

Dorsal and ventral edges of the mancalline scar extending into primary pneumotricipital fossa taper to a point as in Mancalla lucasi , rather than remaining parallel as in Mancalla californiensis and Mancalla cedrosensis (123:1; Fig. 5); mancalline scar extending into primary pneumotricipital fossa is an excavated pit as in Mancalla californiensis rather than raised in relief to the floor of the primary pneumotricipital fossa and the humeral shaft as in Mancalla cedrosensis and Mancalla lucasi (121:0); humerus shorter than other known Mancalla (Tables 2 and 3).

Anatomical description.

Two cervical vertebrae are preserved (Fig. 7A and B). Comparisons with Miomancalla howardi are limited to generalities regarding shape in dorsal view, for which the morphology of Mancalla vegrandis is consistent with that of Miomancalla howardi . Only thoracic vertebrae are known for Mancalla cedrosensis . One of the vertebrae (Fig. 7A) is mediolaterally narrower than the other (Fig. 7B). Although the width of cervical vertebrae other than the axis and atlas do not vary considerably in extant Alcidae , the 3rd and 4th cervical vertebrae of some charadriiforms (e.g., Larosterna inca Lesson, 1827) are mediolaterally narrower than cervical vertebra posterior to the 4th (i.e., C5, C6, C7). The dorsal surface of the broader vertebra (Fig. 7B) is perforated by a small foramen (i.e., perforation of laminae arcocostales). In extant alcids, only the third and fourth cervical vertebrae are perforated. Typically in extant Alcidae , the third cervical vertebra is punctured by a small foramina, whereas the foramina in the fourth cervical vertebra is much larger, leaving only a thin strut of bone bordering it laterally. The morphology of the preserved vertebrae is suggestive of C3 and C4; however, definitive assignment cannot be made at this time.

One complete cervical rib and one complete costal rib (Fig. 7D and E) are preserved along with several other rib fragments (not figured). No morphological differences were evident between the ribs of Mancalla vegrandis , Mancallinae specimen SDSNH 25236,and other alcids for which the ribs are known.

All but the omal extremities of the furcula are preserved (Fig. 8D). The furcular rami are mediolaterally compressed as in all other Alcidae . The anterior surface of the furcular rami dorsal to the apophysis is rounded or convex as in Uria , rather than grooved as in Cepphus . The furcular apophysis does not bear the ventrally expanded, bladelike interclavicular process characteristic of extant Alcidae . However, the possibility that this feature was lost to damage cannot be ruled out. No additional morphological differences were evident between the preserved portions of the furcula of Mancalla vegrandis and other alcids for which the furcula is known.

The left coracoid is complete except for a small portion of the medial margin of the sternal facet (Fig. 8E). A fragment of the right coracoid preserves the medial margin of the sternal facet and the sternal portion of the coracoidal shaft (not figured). As in Mancalla cedrosensis the furcular facet is rounded, rather than oval as in Aethia and Fratercula . The head of the coracoid is apneumatic as in all Alcidae , but the brachial tuberosity is deeply undercut as in Alca and Pinguinus . The humeral articulation is more rounded than in extant Alcidae . As in Cepphus , the scar marking the position of m. supracoracoideus is less distinct than in other Alcidae . As in Mancalla cedrosensis , Aethia , and Alle ,the procoracoidal process is not punctured by a foramen for passage of the tendon of m. supracoracoideus. The procoracoid process points dorsomedially as in all Alcidae except Aethia , in which the procoracoid points more ventromedially. As in Mancalla cedrosensis , Brachyramphus , Uria , Aethia , and Ptychoramphus Brandt, 1837, the sternal margin of the procoracoid process is concave, rather than convex as in Cerorhinca , Fratercula, and Pinguinus . As in many alcids (e.g., Alca , Brachyramphus ) a single, distinct, straight ridge, which extends from the lateral angle of the sternal facet towards the humeral facet is present. This ridge does not extend sternally in Synthliboramphus , Cepphus , Fratercula , Aethia , Ptychoramphus , and Cerorhinca . This ridge is less pronounced and positioned farther laterally in Mancalla cedrosensis . A well-developed lateral process is present. This feature is absent in Mancalla cedrosensis . The dorsal margin of the medial sternal process is notched as in most alcids (e.g., Alca torda ). As in Mancalla cedrosensis , the posterior surface of the sternal end of the coracoid is more excavated than in extant Alcidae and the sternal facet is curved ~150°.

Right and left scapulae are preserved (Fig. 8A and B). As in all Alcidae , the scapular shaft is mediolaterally compressed throughout its entire length. As in Mancalla lucasi , the acromion projects farther anteriorly than that of other alcids (e.g., Uria , Aethia ). The acromion of Mancalla cedrosensis does not project as far anteriorly as that of Mancalla vegrandis . The coracoidal tubercle is more pronounced than in Mancalla lucasi and Mancalla cedrosensis . As in Mancalla lucasi and Mancalla cedrosensis , a scapulotricipital tubercle is present just distal to the glenoid process on the ventral margin of the scapular shaft. As in Mancalla lucasi , the scapular shaft, including the caudal extremity, is slightly more robust than in other alcids (e.g., Alca , Aethia ). The caudal extremity is more dorsoventrally expanded than in Mancalla lucasi . The caudal extremity is not known for Mancalla cedrosensis .

Parts of the left distal end of the sternum including the distal end of the carina, and the left lateral process are preserved (Fig. 8C). Mancalla lucasi and Miomancalla howardi do not preserve the same portions of the sternum so comparisons cannot presently be made between the sterni of Mancallinae . As a result of the deep incisure of the lateral notches the lateral processes of Mancalla vegrandis are more elongate that any other alcids for which the sternum is known. In other Charadriiformes this condition is present only in the Glareolidae and Scolpacidae , and resembles the sternum in Spheniscidae (Fig. 9).

The left humerus is preserved (Fig. 7C). Based upon humeral proportions, Mancalla vegrandis represents the smallest known species of Mancalla (Table 2). As in other species of Mancalla , 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 . The distal end of the deltopectoral crest transitions to the shaft less abruptly than in Mancalla lucasi . As in other Mancallinae , the humeral head is rotated anteriorly and the supracoracoideus muscle scar does not broaden proximally. The 'mancalline scar’ is excavated as in Mancalla californiensis , rather than raised in relief like that of Mancalla cedrosensis and Mancalla lucasi (Fig. 5). As in Mancalla lucasi , the 'mancalline scar’ extends from a point just proximal to the junction of the bicipital crest with the humeral shaft and tapers to a point, and extends into the primary pneumotricipital fossa. The margins of this scar remain parallel in Mancalla californiensis and Mancalla cedrosensis . As in all Mancallinae , the humeral shaft is arced rather than sigmoidal or straight. As in other Mancalla , the dorsal supracondylar tubercle 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, 1966). As with all Mancallinae , the anterior surface of the ventral condyle is rounded, rather than flattened as in all other Alcidae . 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 pelvis is preserved in dorsal view (Fig. 7F). Comparisons of pelves within Mancallinae are limited to Miomancalla howardi . As in all alcids the anteroposterior length of the pelvis is greater than two times the mediolateral width across the antitrochanters. The relative length of the pelves of other charadriiforms is anteroposteriorly shorter. The proximal end of the preacetabular ilium is wide as in Miomancalla howardi and most alcids (e.g., Brachyramphus ). The distal end of the preacetabular ilium is relatively broader than in Miomancalla howardi . As in Miomancalla howardi the antitrochanteral sulcus does not extend proximally to contact the antitrochanter. As in most Alcidae (e.g., Brachyramphus ), the post-acetabular dorsal ilium narrows, rather than broadens as in Uria , Cepphus , and some Fraterculinae . The iliosynsacral suture is perforated as in Uria , Alca , Pinguinus , and Synthliboramphus, rather than fused along its entire length as in Cepphus , Brachyramphus, and Fraterculinae . The dorsal iliac spine has a pointed tip as in all alcids other than Aethia and Ptychoramphus , in which the end of the spine is blunt.

Remarks.

Mancalla vegrandis corresponds in size and humeral characteristics with some material previously referred to Mancalla milleri Howard, 1970. However, Mancalla milleri is considered Alcidae incertae sedis (see Appendix 1 for details of the taxonomic revision).