Tetraodon

Benton, MJ, Donoghue, PCJ, Vinther, J, Asher, RJ, Friedman, M & Near, TJ, 2015, Constraints on the timescale of animal evolutionary history, Palaeontologia Electronica (Florence, Italy) 15 (1), pp. 1-107 : 46-47

TETRAODON – TAKIFUGU (40)

Node Calibrated. Divergence between Tetraodon and Takifugu .

Fossil Taxon and Specimen. Archaeotetraodon winterbottomi from the Pshekhsky Horizon of the lower Maikop Formation , north Caucasus, Russia (holotype PIN 3363 /111 and 111a, Palaeontological Institute of the Russian Academy of Sciences, Moscow, Russia) .

Phylogenetic Justification. Archaeotetradon winterbottomi is described in detail by Tyler and Bannikov (1994) and Carnevale and Tyler (2010). Formal cladistic analyses (Santini and Tyler, 2003: figure 4) and verbal arguments (Carnevale and Tyler, 2010: p. 297-298) based on morphology suggest the exact position of Archaeotetraodon winterbottomi to other Tetradontidae is unclear. However, combined analysis of morphological and molecular data nests Archaeotetradon winterbottomi high within the tetraodontid crown as the sister lineage of a clade comprising Tetraodon , Canthigaster , Monotrete , Auriglobus , Chonerhinos , Chelonodon , and Arthrodon ( Arcila et al., 2015).

Minimum Age. 32.02 Ma

Soft Maximum Age. 56.0 Ma

Age Justification. The age of the Psekhsky Horizon is constrained by foram, nannoplankton, and dinocyst biostratigraphy ( Leonov et al., 1998; Benton and Donoghue, 2006; Benton et al., 2009). The top of this horizon coincides with the first appearance of Sphenolithus predistentus and the base of Paleogene Nannoplankton Zone NP23. The base of NP23 is dated as approximately 32.02 Ma (Anthonissen and Ogg, 2012), which we specify as a minimum age for the last common ancestor of Tetraodon and Takifugu .

We propose a soft maximum for Tetraodon - Takifugu divergence based on the Ypresian London Clay of southeast England and ‘Calcari nummulitici’ of Bolca, Italy ( Ellison et al., 1994; Papazzoni and Trevisani, 2006). These deposits yield a diversity of gymnodont tetraodontiforms, including triodontids, diodontids, and stem tetraodontids (Tyler and Patterson, 1991; Tyler and Santini, 2002; Santini and Tyler, 2003; Arcila et al., 2015), implying that crown tetraodontids would have been sampled were they present. The base of the Ypresian is dated to 56.0 Ma, defining an estimated soft maximum for the divergence between Takifugu and Tetraodon .

Discussion. The selection of an appropriate calibration for the Tetraodon – Takifugu split hinges on resolution of the relationships among living pufferfishes. Although early molecular analyses (Alfaro et al., 2007), including those based on mitochondrial genomes (Yamanoue et al., 2011), yield alternative tree shapes, most molecular studies indicate that the last common ancestor of Takifugu and Tetraodon is identical to the tetraodontid crown node ( Holcroft, 2005; Betancur-R. et al., 2013; Santini et al., 2013; Arcila et al., 2015). Apart from Archaeotetradon winterbottomi and its congeners, the only definitive body-fossil remains of crown tetraodontids are those of Sphoeroides hyperostosus from unit 3 of the Yorktown Formation, Lee Creek Mine, North Carolina, USA (Tyler et al., 1992; Carnevale and Tyler, 2010). Pufferfish jaw fragments from the Miocene and Pliocene of France and Italy have been assigned to Tetraodon , but Tyler and Santini (2002) regard these materials as not sufficiently informative to specify such a precise placement within Tetraodontidae . Snyder et al. (1983) assigned the Yorktown Formation to the uppermost portion of Planktonic Foraminiferal Zone N18 to the middle of Planktonic Foraminiferal Zone N20. The base of the overlying N21 which has been dated as 3.35 Ma ( Hilgen et al., 2012; Anthonissen and Ogg, 2014), making Sphoeroides hyperostosus substantially younger than A. winterbottomi or other species of Archaeotetraodon (Carnevale and Tyler, 2010) . There is no doubt that Sphoeroides hyperostosus is a member of the crown tetraodontid genus Sphoeroides based on presence of trituration teeth in the upper jaw and well-developed remnants of the anterior edge of the dorsal roof of the myodome, combined with the absence of a dorsal flange of the parasphenoid in the orbit, diagnostic features of Sphoeroides not found in close relatives within Tetraodontidae (Tyler et al., 1992, p. 473) . More specifically, S. hyperostosus appears to be a close relative of the extant S. pachygaster on the basis of three characters of the skull not present in any other species of Sphoeroides : lateral ethmoid with broad upper surface with sloping anterior edge reduced or absent; posterodorsal region of the orbit roofed by expansions of the frontal and spehnotic; ridges on the frontals extending from the orbital region to a posterior extension of the bone overlying the epiotic (Tyler et al., 1992, p. 478). S. hyperostosus has been used by some studies to calibrate the Tetraodon – Takifugu divergence (Alfaro et al., 2007) due to ambiguous relationships between modern tetradontid lineages in past analyses (Santini and Tyler, 2003). However, unless subsequent analyses show that Archaeotetraodon is a stem tetraontid, calibrations based on S. hyperostosus substantially underestimate the minimum age for crown Tetraodontidae .