Timon

Tschopp, Emanuel, Villa, Andrea, Camaiti, Marco, Ferro, Letizia, Tuveri, Caterinella, Rook, Lorenzo, Arca, Marisa & Delfino, Massimo, 2018, The first fossils of Timon (Squamata: Lacertinae) from Sardinia (Italy) and potential causes for its local extinction in the Pleistocene, Zoological Journal of the Linnean Society 184 (3), pp. 825-856 : 851-852

PALEOBIOGEOGRAPHY OF TIMON

The herein reported fossils represent the first occurrence of Timon in Sardinia, which is outside the current geographical range of the genus, and lies in between the eastern and western areas inhabited by the three distinct species-groups of Timon ( Ahmadzadeh et al., 2016) . However, this is perhaps not so surprising given that, according to distribution models based on current environmental factors, all four western species of Timon would be adapted to inhabit Sardinia ( Ahmadzadeh et al., 2016). The occurrence of Timon in the Pleistocene of Sardinia thus further corroborates a wider distribution of the genus in the recent past, as proposed by Ahmadzadeh et al., (2016).

POTENTIAL CAUSES FOR LOCAL EXTINCTION

Timon is not the only reptile known from Monte Tuttavista that became extinct in Sardinia. The fossil record also documents the presence of agamid lizards, vipers and amphisbaenians ( Abbazzi et al., 2004; Delfino et al., 2008; Delfino, Bailon & Pitruzzella, 2011), which are absent on the island nowadays. Other localities from the Miocene and Pliocene testify for the former presence of crocodiles, trionychid turtles, non- Anguis anguid lizards, aniliid and erycine snakes ( Kotsakis, Delfino & Piras, 2004; Delfino et al., 2011; Georgalis et al., 2017).

One of the reasons for local extinction could be the arrival or evolution of new predators. A causal relationship with the first arrival of humans was initially proposed as potential cause for an apparent major faunal turnover during the Pleistocene ( Sondaar et al., 1986). Two faunal complexes are recognized based on distinct compositions of the mammalian fauna (the Nesogoral and Microtus complexes), with two subcomplexes each ( Abbazzi et al., 2004; Palombo, 2006, 2009; Palombo & Rozzi, 2014; Vigne, 2014). The findings at Monte Tuttavista confirm the presence of such a turnover ( Abbazzi et al., 2004) but more recent data indicate that this turnover has not been as severe as previously thought ( Palombo et al., 2017). Also, it currently seems rather improbable that humans really established sustained settlements on the island before the Holocene, which would have been necessary to have such an important impact on the local fauna ( Palombo, 2006, 2009; Masini et al., 2008; Leppard, 2014; Palombo et al., 2017). In any case, at Monte Tuttavista, Vipera was only found from sites attributed to the Nesogoral complex, whereas Timon and the agamids occur in both faunal complexes, and only seem to disappear at the beginning of the so-called ‘Dragonara’ subcomplex within the Microtus complex ( Abbazzi et al., 2004; Palombo, 2006, 2009; Delfino et al., 2008; Palombo & Rozzi, 2014). The extinction of these large lizards does, therefore, not correlate with the main faunal turnover, but rather with the change from the so-called ‘Orosei 2 subcomplex’ to the ‘Dragonara’ subcomplex, confirming a rather gradual faunal change as proposed by Palombo et al. (2017).

Whereas an influence of humans on the local extinction of Timon and agamids appears unlikely, several carnivore mammals first occur on Sardinia during the ‘Dragonara’ subcomplex. These include three members of Lutrinae, and the small-sized canid Cynotherium sardous ( Abbazzi et al., 2005; Palombo, 2006, 2009; Palombo & Rozzi, 2014). The lutrines are specialized to prey on aquatic animals ( Willemsen, 1992), and can thus probably be excluded as predators of the large lizards. However, the appearance of the endemic canid Cynotherium sardous is of particular interest with respect to the extinction of Timon and possibly also the vipers and agamids. Cynotherium sardous likely evolved from larger canids that first arrived on the island during the Early Pleistocene (just before or during the main faunal turnover: Abbazzi et al., 2005; Lyras & van der Geer, 2006; Lyras, van der Geer & Rook, 2010; Palombo & Rozzi, 2014). Throughout the Pleistocene, these earlier forms became progressively smaller and adapted to prey on small-sized animals like the small lagomorph Prolagus ( Lyras & van der Geer, 2006; Lyras et al., 2010), and thus possibly also large-sized lizards such as Timon , whereas smaller lizards remained safe from predation by this newly evolved predator (see below). Larger canids, as well as hyaenids, are reported from the early Pleistocene of Sardinia ( Rook et al., 2004; Abbazzi et al., 2005; Lyras & van der Geer, 2006), but these were most likely less adapted to feed on smaller prey like Prolagus , large lizards and snakes, and became extinct during the faunal turnover ( Rook et al., 2004; Palombo, 2006, 2009; Palombo & Rozzi, 2014).

The body size decrease in Cynotherium , and thus the evolution of C. sardous , can also be traced among the sites at Monte Tuttavista ( Abbazzi et al., 2005). Whereas elements attributable to the genus are found nearly throughout all stratigraphic layers, only bones from the two stratigraphically highest sites, ‘VI-Banco 6’ and ‘VII-2’, can be tentatively ( C. cf. sardous at VI-Banco 6; Abbazzi et al., 2005) and definitively referred to the small-sized C. sardous (VII-2; Abbazzi et al., 2005). This size decrease apparently correlates, in particular, with the disappearance of Timon and agamids: VI-Banco 6 produced six remains attributable to Timon , and VII-2 none (see Fig. 1 and Supplementary Material), and the last occurrence of agamids is from ‘XI-canide’, which is just below the two sites hosting C. sardous . Indeterminate snakes and the small-sized, possible Podarcis continued to thrive throughout the ‘Dragonara’ subcomplex, and were among the most frequent finds in VI-Banco 6 ( Abbazzi et al., 2004). Around the Pleistocene–Holocene boundary, Vulpes vulpes also arrived on Sardinia ( Palombo, 2006), which has been shown to prey on lizards and snakes ( Díaz-Ruiz et al., 2013).

The Pleistocene was a time of marked climatic oscillations between warmer and colder periods. The end of the Pleistocene corresponds to a cold period (the Younger Dryas; Miraldo et al., 2013), which seems to have had an important impact on the fauna in Corsica ( Vigne, 2014) and on the distribution of Timon lepidus on the Iberian Peninsula ( Miraldo et al., 2013). However, current Timon populations inhabit variable climatic zones around the Mediterranean, and some species seem to have speciated quite fast to adapt to new environmental conditions and ecological niches ( Ahmadzadeh et al., 2016). Therefore, it seems most plausible that the cause for extinction of large lizards on Sardinia was a combination of the evolution or introduction of new predators preying on small-sized animals (in particular Cynotherium sardous ) and climatic changes.

Figures

Figure 1. Biochronology of selected Pleistocene Sardinian vertebrates. Temporal ranges of Carnivora and Timon sp. are indicated, and localities from Monte Tuttavista, from which the stratigraphic position is known, are plotted against geological time. Localities producing Timon sp. are highlighted in bold and the number of specimens attributed to Timon sp. per group of localities is indicated in brackets next to the line indicating the temporal distribution of Timon sp. Note that the local extinction of Timon sp. coincides with the evolution of Cynotherium sardous. Figure modified from Palombo & Rozzi (2014), with data added from Abbazzi et al. (2004, 2005).