GRATITUDE - The careful preparation of the holotype of the new species was not taken up by Leroy Glenn, who spent many hours on this task. Most of the skull and sternum were contained within smaller pieces of the counterplate (Figure 1). Prints of the alula and some primaries are clearly preserved on the right wing.
Most of the right scapula is exposed, while the left scapula, except for the last one, is covered. Also, the carina was removed from the main plate and joined to the rest of the sternum. Compared to the ulna of the holotype, however, it was found to be morphologically identical.
In the ventral aspect there is less ossification between the anterior parts of the palatine and the quadratojugal bar. There is a thin elongated scar anteriorly for the attachment of the lacrimal bone. The lower jaw is more similar to that of Fregata than to any other member of the order.
Like the axis, this vertebra differs from Sula in the much less development of the hypapophysis.
In Phaethon, the postzygapophyses of the fourth cervical line are extremely weakened, unlike in both sexes of frigatebirds. It is very similar to that of Fregata, but is proportionately shorter, with longer styloid processes and a deeper posterior surface of the center. The pelvis (Figure 10) of Limnofregata is short, wide and shallow, unlike all Pelecaniformes except Fregata and Phaethon.
In the other families of the order, the pelvis is long, laterally compressed (especially in the diving forms, such as cormorants) and deep (Figure 11). Limnofregata is similar to Fregata and differs from Phaethon in the greater expansion of the preacetabular ilia into broad, r o u n d e d shields. In b o t h Fregata and Limnofregata the sixth and seventh caudals differ from the rest of the series in h a v i n g well-developed hemapophyses which are larger in the seventh than in the sixth.
The transverse processes of the seventh caudal are better developed in Limnofregata than in Fregata. The haemapophyses of the sixth caudal in Limnofregata are slightly shorter and are well separated rather than fused as in Fregata. All the caudal vertebrae of Limnofregata differ from those of Fregata in not being inflated and, except for the 7th, in having shorter and broader transverse processes.
From this it can be deduced that the structure and shape of the tail of Limnofregata was probably different from that of Fregata. This corresponds to the deeper scapular facet of the coracoid in both sexes. The ventral face of the furcula is visible in the holotype of Limnofrigata (Figure 12), with all anterior articulation on the right side obliterated and the scapular tuberosity hidden on the left side.
Comparison of the furcula of Limnofregata with that of Fregata is made difficult because in the latter this bone is fused broadly posteriorly to the sternum and anteriorly to the coracoids, while in Limnofregata there is no such fusion. Despite the fact that it is not fused with the rest of the pectoral girdle, the furcula of Limnofrigata appears to be more similar to that of Fregata than to that of other members of the order. The sternum in the holotype of Limnofregata is well preserved and can be seen in the ventrolateral aspect of the left side (Figure 16).
The posterior margin of the sternum in Limnofregata is distinctly four-notched, with the lateral notches on each side being quite deep and the posterior lateral processes slender and elongated. Phaethon also has a four-notched sternum, but in this genus the medial notches are larger and the lateral ones are very reduced, sometimes to varying degrees on either side of the same individual. The sternum of Limnofregata is certainly closer to that of Fregata than to any other modern genus of the order.
T h u m e r u s of Limnofregata (Figure 18) resemble those of Phaethon and Fregata and differ from those of any of the Pelecani in having a large triangular deltoid crest. The shaft appears to have been relatively sturdier than in Fregata or Phaethon, but this may be partly due to deformation of the fossil specimens. This scar is less obvious in Phaethon and is located off the midline, more towards the deltoid crest, as is the condition in the other modern members of the Pelecaniformes in which this scar is visible at all.
The distal end of the humerus in Limnofregata differs considerably from that in Fregata, and is more similar to Phaethon or even Sula. Fregata differs in having a highly inflated, elongate ridge extending proximally from the entepicondylar prominence, in the much greater development of the ectepicondylar area, and in the more bulbous and inflated external condyle, which lacks the small hook at the proximal end seen in Limno frigate. Limnofregata differs from Phaethon a n d Sula and is closer to Fregata in its much greater attachment of the anterior ligament and the greater development of the ectepicondylar region.
The papillae for the secondaries are not nearly so developed as in Fregata (Figure 21), although it appears that the inner surface of the shaft was probably flatter, as in Fregata, r a t h e r t h a n being somewhat flattened. The most distinctive feature of the ulna of Limnofregata is the very large, roughly triangular projection for the anterior articular ligament (Figure 22), which has a conical extension along the lower margin of the impression of the distal end of the M.Th e. the ulna in Limnofregata is quite similar to that of Fregata a n d differs markedly from Phaethon, in which the carpal tuberosity is large and hooked and bears a deep groove between it and the inner condyle.
The radius of Limnofregata is more sharply triangular in cross-section than that of Fregata and thus more similar to Phaethon. The left ulnare of the holotype is mainly visible in its internal aspect and has a tendinous groove that is much deeper than in Fregata or Sula, but similar to that in Phaethon. The proximal end is not as extended and projects more clearly from the shaft in Limnofregata than in Fregata.
The femur of the holotype of Limnofregata azygosternon is somewhat difficult to interpret, as both parts are partially hidden. In having the head flat and lying below the level of the trochanter, Limnofregata differs from Phaethon and all Pelecani and agrees only with Fregata (Figure 25). The femur of Limnofregata differs from Fregata in being proportionately longer, in having the attachment for the ligamentum teres deeper and more distinct, and in the somewhat greater development of the fibular condyle.
T h e outer cnemial crest is also well developed and unsiform, and is less reduced than in any of the m o d e r n Pelecaniformes except Phalacrocorax and Anhinga, which are otherwise quite different. T he distal end of the tibiotarsus of Limnofregata is similar to that of Fregata and differs from the other genera in having a distinct internal ligamental process. In frontal view, the internal and external condyles in Limnofregata are roughly triangular in shape, the apex of the triangles pointing towards the midline.
In posterior view, the crest of the internal condyle is better developed than in Sula or Phaethon and more closely resembles the condition in Fregata. The distal foramen is well developed and circular in Limnofregata and opens onto the posterior surface of the shaft, and is similar in all respects to Sula, although relatively smaller. In posterior view, the most notable feature of the trochleae of Limnofregata is that the articulating surface of the middle trochlea extends proximally in an elongated triangular shape as in Sula and is not short and truncate as in Fregata.
Limnofregata provides some new information on the affinities of the Fregatidae, as it presents us with a frigate without many of the obscuring specializations of the Frigate. The last is undoubtedly a primitive character, and most or all of the others might be as well. Note that the body size of the two species is almost identical, as evidenced by the length of the sternum and pelvis.
A l t h o u g h i n d l i m b of Fregata is more reduced than in Limnofregata, that of the latter shows that by the early Eocene the h i n d l i m b of frigate birds has already become more shortened than in other Pelecaniformes. It includes elongation of the wing, fusion in the pectoral girdle, and extensive pneumatization of the skeleton. The fact that they also differ from Limnofregata is not particularly problematic when assessing the fossil's relationship.
The distal end of the tarsometatarsus or that of the tibiotarsus could be referred to Sulidae, while the proximal end of the tarsometatarsus would be impossible to assign to a modern family. From its structure it can be guessed that Limnofregata probably occupied a place somewhat similar to that of modern gulls of the genus Larus.
