Limnofregata hasegawai, Olson, Storrs L. & Matsuoka, Hiroshige, 2005

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 : 6-11

Limnofregata hasegawai , new species

( Figs. 5–7 )

Holotype: GMNH PV 170, skull and mandible with quadrates, lacrimals, palatines, atlas, axis, and cervical vertebrae 3–7 ( Fig. 5 ). Color brownish black. The skull is preserved in right dorso­lateral view with the mandible and right quadrate still in articulation. The left quadrate has been displaced and lies upside­down, with the mandibular articulation visible along the edge of the left orbit. The lacrimals are still nearly in articulation with the frontals, the right palatine is exposed but neither pterygoid is visible.

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

Horizon: Said to have been collected " 2–3 m " from Referred Specimen 1 of L. azygosterno n (GMNH PV 167). Presumably this distance is intended as vertical as the matrix and the coloration of the bone (blackish vs. brown) are different. Fossil Butte Member of the Green River Formation, late early Eocene.

Etymology: To our esteemed colleague and friend Yoshikazu Hasegawa of the Gunma Museum of Natural History in recognition of his many contributions to vertebrate paleontology.

Diagnosis: Much larger than L. azygosternon . Rostrum proportionately much longer, 1.6 times as long as the cranium, vs. 1.4 in L. azygosternon .

Measurements (mm) of holotype: Measurements that are comparable to those published for Limnofregata azygosternon appear in Table 2 . Additional measurements are: Cranium: width at nasofrontal hinge, 16.8. Lacrimal: length, 17.1. Quadrate: length of articulation with mandible (left), 15.8. Atlas: width 11.9. Axis: width 13.1. Cervical vertebra 3: length, 16.1. Cervical vertebra 5: length, 22.8.

Paratype 1: FMNH PA 719, complete skull and mandible with left quadrate, both lacrimals and pterygoids, assorted sclerotic plates, atlas, axis, and cervical vertebra 3 ( Fig. 6 ). The skull is preserved in dorsal and partial left lateral view with the entire left quadratojugal bar visible. The ventral surface of the mandible can be seen in its entirety.

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): The following are in addition to those in Table 2 . Cranium: width at nasofrontal hinge, ca. 22. Pterygoid: length, 23.1. Quadrate: depth, 20.6; width of mandibular articulation, 15.7; length of otic process along dorsal edge, 12.2. Lacrimal: length, 18.1; depth including descending process, 19.0; depth of descending process, 12.5; greatest depth of corpus at posterior end 7.9. Mandible: posterior width (distance between external margins of articulations), 50.7; width of articulation, 17.3. Atlas: width and depth, 12.6 X 10.9.

Paratype 2: BMS E25336 , pelvis with associated right and left femora and tibiotarsi, the first 5 free caudal vertebrae, and 10 presacral vertebrae ( Fig. 7 ). Collected by Verl and Rick Hebdon and acquired by the Buffalo Museum of Science in 1982.

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

Horizon: F­2 facies equivalent, Fossil Butte Member of the Green River Formation, late early Eocene. The specimen is from a 1.5 m thick layer of precipitated limestone overlying a 30–45 cm thick layer of bluish shale.

Measurements: See Table 1 .

Description: As in L. azygosternon the nostril is long and open, the posteriormost corner being about 1 cm anterior to the naso­frontal hinge. The anterior margin can now be clearly discerned as extending nearly to the premaxillary symphysis.

The configuration of the quadrate in ventral view in Limnofregata shows some distinctive features. In Phaethon the medial and lateral condyles are in the same transverse plane with practically no development of a posterior condyle. In Fregata , the medial condyle extends posterolaterally as a more constricted crest, which then bends at nearly a right angle to the lateral condyle, the bend forming a rather weak posterior condyle with a deep depression anterior to it. The configuration in the Sulidae is more complex, with the posterior condyle completely separate from the medial condyle. Limnofregata is most similar to Fregata , with the medial condyle forming a long rolling crest with only an incipient posterior condyle and the lateral condyle is only slightly angled off of the long axis of the articular surface. The depression seen in Fregata is shallower in Limnofregata .

The pterygoids are relatively much longer in Limnofregata than in Fregata , with slightly curved rather than straight shafts that appear to be terete, without the flattened surfaces seen in Fregata or the Sulidae . They are quite unlike the very long, straight, and slender pterygoids in Phaethon .

The mandibular articulations are much heavier in Limnofregata than in Fregata , the rami much thicker and deeper, and the mandibular symphysis shorter. In the ventral view afforded by the paratypical skull of L. hasegawai , the internal processes of the articulations are shorter, thicker, and much less distinctly set off from the heavy proximal portion of the rami than in Fregata .

The one visible lacrimal in the holotype of L. azygosternon is partially obscured and was misinterpreted in the original description. The entire structure is beautifully revealed in paratype FMNH PA 719 of L. hasegawai in which both lacrimals are preserved with the entire external surface exposed ( Fig 6 ). The corpus of the bone that articulates with the cranium is much larger than in Fregata , with the posterior portion broad and truncated. There are two small pneumatic foramina in the corpus, just anterior to the descending process. The descending process is very long and slender, with a very small expansion on the posterior margin about 2/3 the distance to the ventral extremity. The lacrimal in Fregata is very different, with a small, nonpneumatic corpus and a greatly expanded, inflated descending process that bears a single pneumatic foramen.

The postcranial specimen of L. hasegawai is not in particularly good condition, but has the pelvis preserved in dorsal view and the thoracic vertebrae are better preserved than in the holotype of Limnofregata azygosternon . These do not differ greatly from those in Fregata . The specimen confirms that the distinctive notch in the posterior margin of the innominate bone in the area of fusion of the ischim and ilium ( Olson, 1977:15) is the normal condition. As in the holotype of L. azygosternon , the pygostyle is missing, suggesting that it may have floated away with the presumably enlarged rectrices.

Size in Limnofregata . The seven available specimens now assigned to L. azygosternon are remarkably homogeneous in size and those for which comparable length measurements are available are practically identical ( Table 1 ), especially given the variation induced by crushing and other vicissitudes of preservation. In the two skulls of L. hasegawai such factors of preservation have doubtless contributed to inaccuracies in the measurements of the cranium and rostrum. For example, in the paratype FMNH PA 719, the skull is broken across the nasofrontal hinge and the anterior margin of the cranium has been shoved under the posterior margin of the rostrum perhaps as much as 6 mm. On the other hand, the length measurements of the two known mandibles of L. hasegawai and in the holotype of L. azygosternon are unequivocal. The two mandibles of L hasegawai are identical in length and are 20% larger than in the holotype of L. azygosternon . The postcranial paratype of L hasegawai basically agrees in size, as the femur is 17% larger and the pelvis 20% larger than in L. azygosternon . The tibiotarsus seems disproportionately small, however, as it is only 7% larger. The same appears to apply within modern frigatebirds, however, as the tibiotarsus of the largest species, F. m i n o r, is proportionately smaller than in F. a r i e l, the smallest species (Table 3).

The differences in size between L. azygosternon and L. hasegawi are as great as between the smallest and largest individuals of modern Fregata , which comprises 5 sexually dimorphic species (females larger). Thus, on size alone, Limnofregata would have to be divided into two species­level taxa, as the differences could not be due to differences between sexes, especially as the species of Limnofregata are unlikely to have been sexually dimorphic (see discussion below). The differences in bill proportions, with L. azygosternon having proportionately a much shorter bill, would also not be expected within a single species.

Figures

FIGURE 5. Limnofregata hasegawai, new species, holotype GMNH PV 170 (above), compared with skull and mandible of the holotype of L. azygosternon USNM 22753 (below).

FIGURE 6. Limnofregata hasegawai, new species, paratype, FMNH PA 719, skull and mandible. Scale = 2 cm. Inset: right and left lacrimal bones showing pneumatic foramina in the corpus.

FIGURE 7. Limnofregata hasegawai, paratype, BMS E 25336 associated pelvis and hindlimbs without feet.

Tables

TABLE 2. Cranial measurements of two species of Limnofregata compared

Measurement	L. azygosternon USNM 22753 Holotype	L. hasegawai GMNH PV 170 Holotype	L. hasegawai FMNH PA 719 Paratype 1
Total length of skull and rostrum	132.1	158.4	156.1+
Length of cranium to nasofrontal hinge	55.3	60.1	ca. 63
Length of rostrum from nasofrontal hinge	76.8	98.3	100.6
Length from posterior margin of postorbital process to tip of premaxillary	118.8	141.2	136+
Total length of mandible	120.6	149.4	149.4
Length of quadratojugal bar	50.0	66.2	65.8
Greeatest depth of mandible	9.5	12.2	—