FREGATIDAE, Degland & Gerbe, 1867

Smith, ND & Ksepka, DT, 2015, Five well-supported fossil calibrations within the " Waterbird " assemblage (Tetrapoda, Aves), Palaeontologia Electronica 18 (1), pp. 1-21 : 9-11

STEM FREGATIDAE

Frigatebirds

Node Calibrated. This node represents the split between Fregatidae and their extant sister taxon, the Suloidea (including Sulidae , Phalacrocoracidae , and Anhingidae ). Phylogenetic analyses of both molecular ( Ericson et al., 2006; Hackett et al., 2008) and morphological (Smith, 2010; Mayr, 2011b) data support a sister-taxon relationship between Fregatidae and Suloidea.

Fossil Taxon. Limnofregata Olson, 1977

Specimen. USNM 22753 (holotype of Limnofregata azygosternon ; Figure 1.1 ). Additional support for this calibration is provided by contemporaneous specimens of this species including UWGM 6919 (paratype) ; GMNH PV 167 (referred specimen from four meters above the " 18 inch layer") and of the species Limnofregata hasegawai including GMNH PV 170 (holotype; collected 2–3 meters laterally from GMNH PV 167 ) ; FMNH PA 719 (referred specimen from Thomson Ranch, F-2 Facies) . A new species of Limnofregata, UCMP 134932, represented by an articulated right coracoid and humerus recovered from UCMP locality V 70272 in the Wasatch Formation of Wyoming, also provides additional support for this calibration (Stidham, 2015) .

Phylogenetic Justification. Phylogenetic justification is based on analyses of osteological data from Smith (2010). The analysis of Smith (2010) recovered 18 unambiguous synapomorphies of a Limnofregata + Fregata clade, with six of these exhibiting no homoplasy on the most-parsimonious trees. Synapomorphies include cranial, axial, pectoral, and pelvic characters distributed throughout the skeleton. The synapomorphies supporting a Limnofregata + Fregata clade, presented in the format: "Character(character state)", with boldface indicating no homoplasy in the character on the most-parsimonious trees, include: 30(1) Quadrate, shape of otic head in dorsal aspect: compressed anteroposteriorly and distinctly elongate mediolaterally; 101(1) Number of cervical vertebrae: 15 or 16; 102(1) Osseous bridge from processus transversus to processus articularis caudalis on third cervical vertebra: present; 103(0) Cervical vertebrae 8-11 with processus carotici ankylosed along the midline, forming an osseous canal: no; 108(1) Sternum, shape and relative craniocaudal length to mediolateral width of dorsal surface of sternal body: square-shaped, sternal body wider than long; 114(2) Number of costal facets on sternum: six; 133(2) Sternum, relative convexity of ventral carinal margin in lateral aspect: extremely convex, approaching semicircular profile; 134(0) Sternum, apex carinae of sternum pointed and projecting far rostrally to coracoid sulci: no; 155(0) Scapula, relative cranial extension of acromion: short, does not extend cranial to articular facies for the coracoid; 201(2) Humerus, tuberculum m. pectoralis superficialis, pars deep depth: deep groove medial and distal to tuberculum, with distal portion of tuberculum hypertrophied as a round swelling; 206(1) Humerus, relative development and shape of deltopectoral crest: strongly protruding and triangular; 221(2) Humerus, shape of tuberculum supracondylare ventrale in medial (ventral aspect): distal half of tuberculum distinctly concave, giving the tuberculum a triangular, ‘pointed’ appearance in medial (ventral) aspect; 223(2) Humerus, relative location of muscle scar for insertion of M. pronator superficialis (= “m. pronator brevis”): only slightly posterior, and proximal to tuberculum supracondylare ventrale; 293(1) Manus, proximodistally elongate fenestra on the distal third of the blade of II-1: present; 316(1) Pelvis, interacetabular width relative to synsacral length: between 1/2 to 1/3; 394(1) Tibiotarsus, medial ridge of trochlea cartilaginis tibialis hypertrophied, robust, and mound-like: present; 404(1) Tarsometatarsus, proximodistal length of tarsometatarsus relative to the femur: short, tarsometatarsus less than 1/2 of the length of the femur; 408(2) Tarsometatarsus, development and orientation of eminentia intercondylaris (= “intercotylar prominence”): short, and rounded, weakly developed with no dorsal component. See Smith (2010) for full descriptions of characters and states, as well as original sources for characters.

Synapomorphies of the Limnofregata + Fregata clade that are present in UCMP 134932 include: characters 201(2), 221(2), and 223(2). Breakage of the deltopectoral crest of UCMP 134932 prevents assessment of character 206. The presence of several plesiomorphic (relative to crown frigatebirds) features in UCMP including: 144(0) Claviculae, fenestra (fenestra subacrocoracoidea claviculae) created by fusion of anterodorsal portion of processus omalis claviculae to coracoid: absent; and 219(0) Humerus, fossa olecrani pneumaticity: absent or extremely small pneumatic foramina scattered on surface; indicate its position outside of Fregatidae (Smith, 2010; Stidham, 2015).

Minimum Age. 51.81 Ma

Soft Maximum Age. None specified.

Age Justification. Both the holotype (USNM 22753; Figure 1.1 ) and referred specimen (UWGM 6919) of Limnofregata azygosternon were collected near Kemmerer in Lincoln County, Wyoming. Their exact locality data is not known, though more precise locality data is known for several referred specimens as well as the holotype and referred specimens of Limnofregata hasegawai (see "Specimens" section above). These specimens are from the F-2 Facies, in the middle unit of the Fossil Butte Member of the Green River Formation ( Grande and Buchheim, 1994; Grande, 2013). These deposits are late early Eocene, and multicrystal analyses (sanidine) from a K-feldspar tuff (FQ-1) at the top of the middle unit of the Fossil Butte Member, from Fossil-Fowkes Basin (locality: N41º47'32.2" W110º42'39.6") have yielded an age of 51.97 ± 0.16 Ma (Smith et al., 2010).

UCMP locality V70272, where specimen UCMP 134932 was recovered, is located in Sweetwater County, Wyoming. The specific locality is from Bitter Creek 22 Level 3, which is a lignite bed in the Main Body of the Wasatch Formation (Stidham, 2015). Mammal fossils from Bitter Creek have been interpreted as Graybullian, roughly corresponding to the Wasatchian North American Land Mammal Age (NAMLA 3–5 subzones of the Bighorn Basin) (Stidham, 2015). UCMP V70272 is also stratigraphically below Bitter Creek fossil lizard localities considered to represent the Wasatchian subzones late Wa4–Wa6 (Smith and Gauthier, 2013; Stidham, 2015), suggesting that these Limnofregata specimens are from an equivalent of the early Wa4 or older subzone. Current estimates for the age of the Graybullian (Wa3– Wa5) range from 53.9 to 55.1 Ma ( Chew and Oheim, 2013; Gradstein et al., 2012; Stidham, 2015). Although the evidence for UCMP 134932 being located stratigraphically below other Limnofregata specimens (and the possibility that it may be ~2 million years older than these fossils) is compelling, no direct radiometric dates are associated with this specimen and locality, necessitating a cross-basin correlation using mammal biostratigraphy (Stidham, 2015). For these reasons, the hard minimum age of UCMP 134932 would actually be younger than the minimum age that can be established for the Green River Formation Limnofregata specimens.

Phylogenetic position of Fregatidae and Limnofregata . Limnofregata azygosternon ( Figure 1.1 ) was originally described as a stem-frigatebird by Olson (1977) and placed in the monotypic genus Limnofregata , and monotypic subfamily Limnofregatinae , within the Fregatidae . An additional species, Limnofregata hasegawai , was subsequently described in 2005, differing only in overall size and several skeletal proportions from L. azygosternon ( Olson and Matsuoka, 2005) . The inferred close relationship between Limnofregata and Fregata has never been challenged, though Mayr (2005) noted similarities between Limnofregata and the plotopterid Phocavis maritimus ( Goedert, 1988) , represented by a single tarsometatarsus. However, the analyses of Smith (2010) represented the first rigorous tests of the phylogenetic relationships of Limnofregata , recovering it as the sister-taxon to Fregata with strong support.

The extant sister-taxon of Fregata has been slightly more contentious. Most traditional taxonomies and studies of morphological data suggested that Fregata represented the sister-taxon to a Pelecanus + Suloidea ( Sulidae , Phalacrocoracidae , Anhingidae ) clade ( Livezey and Zusi, 2007 and references therein). However, more recent morphological (Smith, 2010; Mayr, 2011b) and molecular datasets ( Fain and Houde, 2004; Ericson et al., 2006; Hackett et al., 2008) are consistent in recovering a sister-taxon relationship between Fregatidae and Suloidea. Additionally, molecular datasets have long contradicted a sister-taxon relationship between Pelecanus and Suloidea, typically recovering the former as closely related to the shoebill, Balaeniceps , and the hammerkop, Scopus (van Tuinen et al., 2001; Ericson et al., 2006; Hackett et al., 2008). More recently, morphological support has also been described for a clade uniting Pelecanus , Balaeniceps , and Scopus ( Mayr, 2011b) .

Fossil record of total group Fregatidae . The fossil record of total group Fregatidae is extremely depauperate, with multiple specimens from the three known species of Limnofregata representing the only Pre-Quaternary records ( Olson, 1985; Smith, 2010; Stidham, 2015). The Quaternary records are all from oceanic islands and can all be referred to extant species of Fregata ( Olson, 1985) . Thus, we have little evidence to support a maximum age for this clade.

Figures

FIGURE 1. Specimens representative of two taxa that provide fossil calibrations within the waterbird assemblage. 1, Limnofregata azygosternon (USNM 22753) from the early Eocene Green River Formation represents the sister taxon (with its congener Limnofregata hasegawai) to Fregata, and calibrates the node Fregatidae + Suloidea. 2, Rhynchaeites messelensis (SMF 218) from the middle Eocene of Messel. Together with other Messel Rhynchaeites messelensis specimens and Rhynchaeites sp. (MGUH 20288) from the early Eocene Fur Formation, these represent the sister taxon to Threskiornithidae, and MGUH 20288 calibrates the node uniting Threskiornithidae and its extant sister taxon. Scale bars equal 50 mm.