Data from: Cranial endocast of Anagale gobiensis (Anagalidae) and its implications for early brain evolution in Euarchontoglires
Data files
Apr 17, 2023 version files 100.58 KB
Abstract
Anagalids are an extinct group of primitive mammals from the Asian Palaeogene thought to be possible basal members of Glires. Anagalid material is rare, with only a handful of crania known. Here we describe the first virtual endocast of an anagalid, based on the holotype of Anagale gobiensis (AMNH 26079; late Eocene, China), which allows for comparison with published endocasts from fossil members of modern euarchontogliran lineages (i.e. primates, rodents, lagomorphs). The endocast displays traits often observed in fossorial mammals, such as relatively small petrosal lobules and a low neocortical ratio, which would be consistent with previous inferences about use of subterranean food sources based on heavy dental wear. In fact, Anagale gobiensis has the lowest neocortical ratio yet recorded for a euarchontogliran. This species was olfaction-driven, based on the relatively large olfactory bulbs and laterally expansive palaeocortex. The endocast supports previous inferences that relatively large olfactory bulbs, partial midbrain exposure and low encephalization quotient are ancestral for Euarchontoglires, although the likely fossorial adaptations of Anagale gobiensis may also partly explain these traits. While Anagale gobiensis is a primitive mammal in many aspects, some of its derived endocranial traits point towards a new, different trajectory of brain evolution within Euarchontoglires.
Methods
The endocast of Anagale gobiensis was generated from CT scan data of AMNH 26079.
The endocast of Anagale gobiensis (Fig. 3) is compared to the sample of previously published endocasts pertaining to early members of Euarchontoglires. In particular, the comparative sample includes early Glires, that is, “ischyromyids” (Paramys, Pseudotomus, Reithroparamys, Rapamys, and Ischyromys; Bertrand & Silcox, 2016; Bertrand et al., 2016a, 2019), fossil sciuroids (Bertrand et al., 2017, 2018), Rhombomylus (Meng et al., 2003) and a stem lagomorph (López-Torres et al., 2020); Primates, including “plesiadapiforms” (Silcox et al., 2009, 2010a; Orliac et al., 2014) and early Tertiary fossil euprimates (Kirk et al., 2014; Harrington et al., 2016, 2020; Ramdarshan and Orliac, 2016); and the most primitive apatemyid known from endocranial data (Labidolemur kayi; Silcox et al., 2011). A sample of extant rodents (Bertrand & Silcox, 2016; Bertrand et al., 2017, 2018, 2019) and extant lagomorphs (López-Torres et al., 2020) were used to provide context to the fossil data in the quantitative analyses.
Surface area, length and volumetric measurements were taken on the endocasts to calculate several ratios using Avizo 9.0.1 (Table 1). The volume of the endocast of Anagale gobiensis was obtained by isolating the more complete half of the endocast using the ‘volume edit’ module and doubling its volume. The volume of the olfactory bulbs was obtained by isolating the two olfactory bulbs, also using volume edit. The petrosal lobules were re-segmented in the transverse plane (XZ dimension) using a distinct labelfield module and the volume measured from the resulting reconstruction. To obtain the neocortical surface area, the area above the rhinal fissure was selected, whereas the circular fissure and the confluence of sinuses were excluded. We followed Jerison (2012) and Long et al. (2015), who measured only one side of the neocortex (the more complete hemisphere), excluding the superior sagittal sinus, and then doubled the area of the hemisphere.
For comparative purposes, we calculated the encephalization quotient (EQ) using equations from Jerison (1973) and Eisenberg (1981). For the calculation of EQs, it is important to have an accurate estimation of the body mass of the animal in question. We consider the best estimate for a range of body masses for AMNH 26079 to be 1122.77—1464.68 g.
We generated a series of bivariate plots as well as boxplots based on the data in Table 1, combined with values collected from the literature (endocranial volume and surface area, olfactory bulb volume, petrosal lobule volume, neocortical surface areas; Gingerich & Gunnell, 2005; Silcox et al., 2009, 2010a, 2011; Kirk et al., 2014; Orliac et al., 2014; Harrington et al., 2016, 2020; Ramdarshan & Orliac, 2016; Bertrand et al., 2019, 2021; López-Torres et al., 2020; see data in Table 1 and in López-Torres et al., 2022, tables S1-S4) using the software PAST (Hammer et al., 2001).
Usage notes
All files are submitted in .xlsx format, and are readable by Excel.