Limnofregata azygosternon Olson, 1977

Olson, Storrs L. & Matsuoka, Hiroshige, 2005, New specimens of the early Eocene frigatebird Limnofregata (Pelecaniformes: Fregatidae), with the description of a new species, Zootaxa 1046, pp. 1-15 : 2-6

Limnofregata azygosternon Olson, 1977

Figs. 1–4

This species was described from a nearly complete skeleton with feather impressions (holotype USNM 22753) and two paratypes consisting of most of a right wing and shoul­ der girdle (UWY 6919) and the proximal end of an ulna (USNM 243766). Here we identify four additional specimens that can also be referred to the species Limnofregata azygosternon .

Referred specimen 1: GMNH PV 167, postcranial skeleton with impressions of remiges and contour feathers ( Fig. 1 ). Bone color brown, feathers black. Collected by Tom Lindgren. The specimen consists of all elements of the appendicular skeleton except the proximal portion of the right humerus, plus the complete pectoral girdle and pelvis. It is lacking the skull and mandible, a number of ribs, and the entire pre­ and postsacral vertebral column except for three thoracic and a caudal vertebra. Very distinct carbonized impressions of feathers can be identified as primaries, secondaries, and wing coverts.

Locality: Smith Hollow Quarry, Lewis Ranch (Locality B of Grande and Buchheim, 1994), Lincoln County, Wyoming.

Horizon: Beds about 4 m above the “ 18 inch layer,” Fossil Butte Member of the Green River Formation, late early Eocene.

Measurements (mm): See Table 1 .

Referred specimen 2: FMNH PA 723, posterior portion of an associated skeleton consisting of the last 3 thoracic vertebrae, pelvis, caudal vertebrae and pygostyle, both hindlimbs complete with all toe bones, and assorted ribs ( Fig. 2 ).

Locality: Thomson Ranch (Locality H of Grande and Buchheim, 1994), ca. 14 km W of Kemmerer, Lincoln County, Wyoming.

Horizon: F­2 facies, Fossil Butte Member of the Green River Formation, late early Eocene.

Measurements (mm): See Table 1 .

Referred specimen 3: FMNH PA 720, complete left wing distal to the humerus ( Fig. 3 ).

Locality: Warfield Springs (NW1/4, Sec. 5, T19N, R117W; locality K of Grande and Buchheim, 1994), ca. 14 km SW of Kemmerer, Lincoln County, Wyoming, on the SE shore of Eocene Fossil Lake.

Horizon: F­2 facies equivalent, Fossil Butte Member of the Green River Formation, late early Eocene.

Measurements (mm): See Table 1 .

Referred specimen 4: USNM 447002, left side of cranium with quadrate and associated posterior portion of mandible ( Fig 4 ). The base of the bill and the occiput were preserved as a mold that has now been replaced with embedding compound.

Locality: Ca. 12 km NE of Fontanelle Reservoir Dam on N shore at autumn water level, Sweetwater County, Wyoming. This locality is in beds of the Eocene Lake Gosiute, whereas all the other specimens of Limnofregata to date have come from deposits in Fossil Lake.

Horizon: Laney Shale Member of the Green River Formation, lower middle Eocene.

Measurements (mm): Quadrate: distance from posterior margin of mandibular articulation to tip of otic process, ca. 13.4.

Description: Although USNM 447002 is very fragmentary, it is in places less crushed than other skulls of the genus. It shows a well­developed ectethmoid plate extending laterally from the mesethmoid, whereas the ectethmoid is much more reduced in Fregata . It emphasizes the large size of the orbit in Limnofregata , suggesting that the birds may have been partially nocturnal or crepuscular.

In GMNH PV 167, the toes of the right foot appear to be in their naturally maximal spread position, and shows the much better development of the foot in Limnofregata compared with Fregata . Assuming that the foot was totipalmate, as in all modern Pelecaniformes, the area of the webbing was on the order of 20 cm 2. In the largest and best preserved of the primary feathers in GMNH PV 167, the tip is much less attenuated and sharply pointed than in Fregata .

For FMNH PA 723, new information not preserved with the holotype concerns the tail. The number of free caudal vertebrae anterior to the pygostyle appears to be seven, as in Fregata . The pygostyle of Limnofregata was previously unknown, this element being entirely missing in the holotype of L. azygosternon . It is a broad plate as in Phaethon and Fregata , and completely unlike the pygostyle in the Sulidae , which is very thick and somewhat cylindrical proximally, tapering to a long, thin spine. In Limnofregata , the ventral margin of the pygostyle ( Fig. 2 ) is rectilinear, the dorsal margin is curved, and the tip is broadly rounded.

Figures

FIGURE 1. Limnofregata azygosternon, referred postcranial skeleton with feather impressions GMNH PV 167.

FIGURE 2. Limnofregata azygosternon, referred posterior portion of skeleton FMNH PA 723.

FIGURE 3. Limnofregata azygosternon, referred left wing FMNH PA 720.

FIGURE 4. Limnofregata azygosternon, referred cranium and posterior portion of mandible USNM 447002. The orbit is perhaps overemphasized by matrix remaining along the margin.

Tables

TABLE 1. Length measurements (mm) of bones in four specimens of Limnofregata.

Measurement	USNM 22753 Holotype	L. azygosternon GMNH PV UWY 167 6919 Referred Paratype	FMNH PA 723 Referred	FMNH PA 720 Referred	L. hasegawai BMS E25336 Paratype
Scapula	67.6	73.7 67.7
Coracoid (maximum)	ca. 72	71.2 68+
Humerus	ca 140	147.7 138.3
Ulna	178	176.5 166		173+
Carpometacarpus	80	80.3 79.6		79.3
Major digit phalanx 1, 2	39.8, 33.3	41.9, 35.0		40.0, 35.3
Synsacrum	61.5	61.2	66		73
Femur	~55	56.0	56.8		66
Tibiotarsus	67	67.9	67		72.6
Tarsometatarsus	26.7	27.3	26.8
Digit I total length	22.7	22.6	22.5
Digit II total length	45.8	44.4	43.1
Digit III total length	62.4 ­	63.5	63.3
Digit IV total length	51.3	52.3	52.6