comment on this calibration


 node name
Fregatidae     Look for this name in NCBI   Wikipedia   Animal Diversity Web
  recommended citations Smith and Ksepka, 2015
  node minimum age
51.81 Ma
Both the holotype (USNM 22753) 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. 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.
  node maximum age
0 Ma
None specified.
 primary fossil used to date this node 
USNM 22753
Limnofregata azygosternon, Olson, 1977
Location relative to the calibrated node: Stem

[show fossil details]
     Locality: Green River Formation
     Stratum: Fossil Butte Member
     Geological age: Ypresian, Eocene, Paleogene, Cenozoic

More information in Fossilworks   PaleoBioDB

  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 delto pectoral 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.
  phylogenetic reference(s)
Olson, S.L. 1977. A Lower Eocene frigatebird from the Green River formation of Wyoming (Pelecaniformes: Fregatidae). Smithsonian Contributions to Paleobiology, 35:1–33.
Smith, N.D. 2010. Phylogenetic analysis of Pelecaniformes (Aves) based on osteological data: implications for waterbird phylogeny and fossil calibration studies. PLoS ONE, 5:e13354.
 tree image (click image for full size) 
tree image
Figure 3 from Smith and Ksepka (2015).