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Fig. 2. (A) Cretaceous–Pleistocene (C–Pl) time scale showing relative
cooling in benthic ocean temperature based on stable oxygen isotope ratios.
Warmest times were late Paleocene (Pa), while coolest were Pleistocene.
Numbers in circles mark the approximate timing of five major paleoceanographic
events: (1) Central American land bridge formation (3 mya); (2) beginning of
thermohaline circulation (22 mya); (3) closure of the Tethys seaway (25 mya);
(4) Antarctic glaciation and polar cooling (30 mya); (5) Northward tectonic
drift of Australia (36 mya). Data summarized from Macdougall
(1996); Fordyce and de Muizon
(2001); Graham and Dickson
(2004). Other Tertiary epochs
abbreviated on the left axis are: Eocene, E; Oligocene, O; Miocene, M;
Pliocene, P. (B) Phylogeny for the Sardini and Thunnini based on the earliest
known occurrences of fossil remains (orange circles) of two extant Sardini
genera (G, Gymnosarda; S, Sarda) and extant tuna genera
(A, Auxis; E/K, Euthynnus or Katsuwonus;
T, Thunnus). (Note that Thunnus fossils extend from the late
Paleocene to the Pliocene.) The extinct Paleothunnus (P) is
considered to be closely related to the tuna–bonito common ancestor.
Data from Bannikov (1985);
Carroll (1988); Monsch
(2000; personal
communication). Bonito genera abbreviations: S and G, as
above; C, Cybiosarda; O, Orcynopsis. Tuna abbreviations as
in Fig. 1; the two
Thunnus subgenera (Neothunnus and Thunnus), each
with four species, are indicated. (Note: terminal lines indicate number of
extant species in each tuna genus.)
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