Endocranium and ecology of Eurotherium theriodis, a European hyaenodont mammal from the Lutetian

Hyaenodonts are extinct placental mammals with a carnivorous diet. Their phylogenetic position among mammals and the relationships within Hyaenodonta are at present partially unresolved. The endocranium is a structure that has rarely been studied in this clade. Using 3D tomography, we studied the endocranium of the European hyaenodont Eurotherium theriodis, discovered in Egerkingen (Switzerland, Lutetian, middle Eocene). Eurotherium theriodis has an endocranium morphology that supports an increase in size of the cerebrum relative to the cerebellum over time within the Hyaenodontoidea. The endocranium also supports a complexification of the cerebrum (i.e., at least two furrows per cerebral hemisphere) within the Hyaenodontoidea and allows us to envisage an increase of the encephalization quotient (EQ), over time. Based on morphology, we consider that its endocranium does not depart from that of the hyaenodontoids known in the Lutetian, Bartonian, and Priabonian of Europe, being less complex than that of the hypercarnivorous Hyaenodon. However, the morphology of its olfactory bulbs and turbinates is similar to that observed in Hyaenodon. The large size of the turbinates of E. theriodis is regarded to be the result of a possible scavenger ecology and agrees with the meat/bone diet envisaged based on the analysis of the morphology of the skull and teeth.

The endocranium is the bony capsule enclosing the brain. The latter organ displays various morphological characteristics (including its size) that provide ecomorphological traits. It has been highlighted that an increased encephalization may be related to arboreality (Mace et al. 1981). Information relating to diet (Pirlot and Stephan 1970), or even to diurnal or nocturnal behavior (Mace et al. 1981) have also been studied.
Each structure of the endocranium can help to identify ecological behaviors, since each part of the endocranium is associated with a sensory or motor function (Barone and Bortolomi 2004). The turbinates are located in the frontal sinuses of the muzzle, in the nasal cavity (Barone and Bortolami 2004). Together with the olfactory bulbs, they are linked to olfactory capability (Barone and Bortolomi 2004). Because the turbinates are not part of the encephalon (grouping together cerebellum, cerebrum, and olfactory bulbs), they are often omitted in published drawings. However, some observations on living species indicate that these structures may reflect hunting strategies (Blanton and Biggs 1969;Van Valkenburgh et al. 2014;Negus 1954).
Finally, comparisons of the morphology of the endocranium between species among a mammal group can make it possible to reconstruct the evolution of the brain and encephalization (Dubied et al. 2019a).
The morphological characteristics of the endocranium and turbinates of this fox-sized mammal allow for a discussion regarding its ecological characteristics, such as its hunting strategy, as well as the evolution of the brain and encephalization among hyaenodonts.
Institutional abbreviations.-NMB, Natural History Museum of Basel, Switzerland.

Material and methods
Fossil specimen.-The fossil NMB.Em12 is a complete skull, found in the Swiss locality of Egerkingen, probably Egerkingen γ based on the date it was collected (Figs. 1, 2). It is the holotype of Eurotherium theriodis (Van Valen 1965). Egerkingen corresponds to karst fillings in an aberrant siderolitic facies. Three infillings have been excavated since 1890: these are designated α, β, and γ. Quarries α and β constitute the reference locality of reference level MP14 (generally noted as Egerkingen α + β), while Quarry γ is considered to be slightly older and referred with uncertainty to MP13? (Aguilar et al. 1997), possibly close to the MP13 reference level. Therefore, all the infillings are Lutetian (middle Eocene) in age. NMB.Em12 belongs to the old collections of Egerkingen permanently stored at the Natural History Museum of Basel (Naturhistorisches Museum Basel, NMB).
X-ray microtomography.-The X-ray microtomography was performed using a nanoCT® system nanotom® (phoenix X-ray, GE Sensing & Inspection Technologies GmbH, Wunstorf, Germany) hosted at the Biomaterials Science Center of the University of Basel (Switzerland). The scan parameters are 180kV and 30μA for a pixel resolution of 49μm. We digitally segmented the cranium and endocast of NMB.Em12 using AVIZO LITE 9.0. This method allows access to the inner structure of the specimen (herein the endocranium). AVIZO LITE 9.0 was also used for measuring the endocranial structures. The 3D models (skull and endocast) are available at MorphoMuseuM (Dubied et al. 2020).
Body mass.-The body mass has been estimated based on the equation provided by Morlo (1999): Log 10 (P) = [3.5104 × Log 10 ((ΣM)/3)] -2.6469. P is the estimated body mass in grams. ΣM is the sum of the length of the three lower molars in mm. To estimate the body mass of E. theriodis, we used the means of the lengths of the three lower molars provided by Van Valen (1965) (Table 1). Encephalization quotient.-The encephalization quotient (EQ) is brain size divided by expected body size for an average mammal of the same body size. As a ratio, it can be used to compare brain sizes among specimens with different body masses (Bertrand et al. 2017). All the EQs herein have first been estimated using the methodology provided by Jerison (1970Jerison ( , 1973; this equation has previously been used to calculate the EQ of Hyaenodon and Cynohyaenodon (Jerison 1973;Radinsky 1977Radinsky , 1978. The equation is: EQ = E / 0.12 P 0.67 ; where E equals volume of the encephalon in cm 3 ; P, body mass in grams. For the purposes of comparison, we also calculated EQ using the equation provided by Eisenberg (1981): EQ = E / 0.055 P 0.74 , where E is the volume of the encephalon (in cm 3 ) and P corresponds to body mass (in g). The encephalic volumes of C. cayluxi, H. horridus, H. crucians, and H. brachyrhynchus are based on the estimates made by Radinsky (1977). The cephalic volume of P. typica was estimated by Dubied et al. (2019a).
Diet.-Based on relative premolar size (RPS) and relative blade length (RBL) the extant Carnivora can be separated into three feeding types (Van Valkenburgh 1988): (i) meat, more than 70% meat in diet; (ii) meat/bone, more than 70% meat with the addition of large bones; (iii) meat/non-vertebrate, 50-70% meat with the balance of fruit and/or insects. The RBL tends to discriminate the meat and meat/bone eaters (highest values) from the meat/non-vertebrate eaters (lowest values), while the RPS discriminates the meat eaters (lowest values) from the meat/bone eaters (highest values). We estimated these two indices for some selected European hyaenodontoids from the Paleogene. The first index was adapted to Hyaenodonta by Friscia and Van Valkenburgh (2010). The method used herein follows that of Solé and Mennecart (2019). However, based on the discrimination generated by the values obtained for European hyaenodontoids, we are able to recognize three distinct diet types: the meat/non-vertebrate eaters have RBL below 0.7; the meat and meat/bone eaters thus have RBL above 0.7; the meat eaters have RPS below 2.65; and the meat/bone eaters have RPS above 2.65.
Description.-Endocranium: The olfactory bulbs (maximum length 14 mm, maximum width 20 mm) are tubeshaped (Fig. 3A 1 ). Olfactory tubercles are visible in ventral view (Fig. 3A 2 ). The encephalon is spherical, but its anterior part has no visible structure due to taphonomic modification; for instance, the anterior lobes are not observable due to poor preservation. The cerebrum (maximum length 22 mm, maximum width 34 mm) is similar in width to the cerebellum (maximum length 13 mm, maximum width 37 mm). The cerebrum is divided into two hemispheres (Fig. 3A 1 ). The sagittal sinus is present but weakly marked. A slight protuberance, which may be the result of damage during preservation, is present on the left side of the  Radinsky (1977) except that for Proviverra typica that is from Dubied et al. (2019a). The encephalization quotients were estimated using the equation provided by Jerison (1973) and Eisenberg (1981)  cerebrum, preventing observation of the different furrows on this side of the cerebrum. On the right side of the cerebrum, the lateral sulci are sigmoidal, with their anterior part concave (Fig. 3A 3 ). The suprasylvian sulci are concavely rounded and become rectilinear in their anterior part (Fig. 3A 3 ). Their exact course is difficult to define given the preservation of the specimen. The cerebellum is transversally wide. However, it is poorly preserved, which prevents a precise description of its morphology. We can only determine the presence of the vermis (Fig. 3A 3 ) and of the medulla oblongata (Fig. 3A 1 , A 2 ). The preservation of the ventral part of the encephalon does not allow for description of all the nerves and sinuses (Fig. 3A 2 ). The anterior side of the ventral part is the best preserved (Fig. 3A 2 ). In ventral view, the pituitary appears between the canals for cranial nerves III, IV, and V 1 . Cranial nerve II stands out clearly on the anterior and relatively axial part of the encephalon. Just behind this, cranial nerves III, IV, V 1 , V 2 , and VI are located in a single expanded and curved canal (Fig. 3A 2 ). The preservation of the posterior part does not allow for further description of these nerve structures. The cerebellum is broken and heavily restored, preventing any description. The only element that can be distinguished is the vermis, but the latter cannot be described in detail. Nasal cavity: The study of the nasal cavity allows the recognition of five turbinates on each side of the rostrum; they are visible in dorsal view (Fig. 3A 1 ). The latter are non-rectilinear and are separated from each other. The mesethmoid appears thin and elongated (Fig. 3A 1 ). The turbinates widen and thicken towards the front of the skull, forming two fans (diverging 5-35° from the median axis). They thus form a considerable volume with significant relief; this volume is large compared to the encephalon.  (Radinsky 1977;Lange-Badré 1979;Fig. 4). Gervais (1870) attributed an endocast to the type species of Hyaenodon, H. leptorhynchus, but we have doubt regarding the identification of this specimen considering the recent description (Solé et al. 2018a, b) of the previously unknown skull of that species; it is thus not included in this analysis. Recent phylogenetic analyses (Borths et al. 2016;Borths and Seiffert 2017;Dubied et al. 2019a;Solé et al. 2020) indicate that Eurotherium is more closely related to the hyaenodontine Hyaenodon than to Proviverra and Cynohyaenodon. However, like Cynohyaenodon and Proviverra, it is not a hyaenodontine in the current definition of the clade (Fig. 4). The endocast of the basal hyaenodont Tritemnodon agilis described by Radinsky (1977) is here considered because it may shed light on the polarity of brain character evolution through time.
Eurotherium theriodis has an EQ that is slightly greater than those of T. agilis and C. cayluxi (Table 1) but it is very low in comparison to P. typica; it is worth remembering that the high EQ of Proviverra is unique among hyaenodonts and may be linked to an allometric effect, as Proviverra is very small (Dubied et al. 2019a). The EQ of E. theriodis is clearly less than to those calculated for H. crucians and H. horridus (Table 1). No obvious trend can be established Fig. 4. Schematic evolution through time of the size of the endocast of several hyaenodonts. Red, lateral sulcus; green, suprasylvia; blue, ectosylvia. Endocast morphology has been modified from Radinsky (1977) except that of Proviverra typica Rütimeyer, 1862 (Dubied et al. 2019a) and Eurotherium theriodis (Van Valen, 1965) (present paper). The endocasts are not to scale. The phylogenetic relationships are based on Solé et al. (2020). for the EQ, although an increase over time can be envisaged if P. typica is regarded as a very peculiar case.
In the cerebrum, T. agilis has only one furrow per hemisphere, E. theriodis has two furrows per hemisphere, like P. typica and C. cayluxi, whereas the hyaenodontines have three furrows per hemisphere (Fig. 4). We can also observe that the relative size of the cerebrum in comparison to the cerebellum increases from T. agilis (where the cerebrum is very small), via Proviverra, Cynohyaenodon, and Eurotherium (where the cerebrum and cerebellum are of similar width), to Hyaenodon (where the cerebrum is large).
Tritemnodon agilis, P. typica, and C. cayluxi have dropshaped olfactory bulbs, while in E. theriodis, the olfactory bulbs are tube-shaped. This olfactory bulb shape is similar to the one observed on the endocranium of some hyaenodontines, especially H. horridus and H. crucians (Radinsky 1977). These species have thick tube-shaped olfactory bulbs.
As indicated in the description, the cerebellum is poorly preserved and cannot be compared.
Eurotherium theriodis presents striking anatomical features at the level of turbinates: they are highly developed and fan-shaped, whereas the turbinates are straight and poorly developed in P. typica and C. cayluxi (Lange-Badré 1979;Dubied et al. 2019a, b). The turbinates of H. brachyrhynchus are also well developed and fan-shaped (Lange-Badré 1979).
To conclude, E. theriodis has an endocranium morphology that supports an increase in size of the cerebrum relative to the cerebellum over time within the Hyaenodontoidea as observed by Dubied et al. (2019a). The endocranium also supports a complexification of the cerebrum (i.e., at least two furrows per cerebral hemisphere) within the Hyaenodontoidea and allows us to envisage an increase of the EQ over time (Proviverra being considered as a special case). The overall morphology of the cerebrum of E. theri odis is like those of Proviverra and Cynohyaenodon and clearly less complex than that of Hyaenodon, despite that Eurotherium is closely related to Hyaenodon (Fig. 4). Its morphology is thus similar to that of the other hyaenodontoids recorded in the Lutetian (Proviverra), Bartonian, and Priabonian (Cynohyaenodon) of Europe. It is worth reminding, however, that Eurotherium shares with Hyaenodon the presence of tube-shaped olfactory bulbs and well-developed and fan-shaped turbinates. The presence of such developed turbinates is unusual among hyaenodonts; however, one must keep in mind that these structures are rarely analyzed among hyaenodonts and this condition cannot be regarded, at this stage, as a diagnostic character either for Hyaenodontidae or Hyaenodontinae.
Ecological aspects.-The reconstructed body mass of E. theriodis is 7 kg, thus the size of a fox. The diet of extinct predators is difficult to reconstruct, but some morphological features provide useful information. In E. theriodis, the sagittal crest (formed by the parietals) is highly developed on the posterodorsal part of the skull (Fig. 5). The strong development of the sagittal crest is similarly observed in some extant hyaenids (Crocuta crocuta, Parahyaena brunnea, and Hyaena hyaena) (Joeckel 1998). One can imagine that the skull of Eurotherium was also similar to these hyaenids in having a vaulted forehead (Fig. 5); this striking morphology, which results from the presence of caudally elongated frontal sinuses that completely overlie the brain cavity, plays a critical role in dissipating stress away from the facial region (Werdelin 1989;Joeckel 1998;Tanner et al. 2008). However, this impression surely results from post-mortem deformation of the skull around the nasal-parietal contact (Fig. 5). When deformation is considered (i.e., when the dentition is aligned with the occipital condyles as usually observed in Table 2. Estimations of the relative premolar size (RPS) and the relative blade length (RBL) values of several hyaenodonts from Europe. The method used for estimated these indexes follows that of Solé and Mennecart (2019)   European hyaenodontoids), the skull of Eurotherium seems morphologically similar to that of other hyaenodontoids known from the Eocene of Europe. Despite the probable lack of a vaulted forehead, the strong development of the parietals and sagittal crest suggest a durophagous diet, it is worth remembering that extant hyaenids have a broad range of cranial and dental adaptations for durophagy, not just a vaulted forehead (Werdelin and Solounias 1991). Indeed, a long and tall sagittal crest implies strong development of the temporalis muscle, one of the powerful adductor muscles of the mandible (Popowics and Herring 2006).
Concerning the mandible and the lower teeth of Eurotherium theriodis, the transverse width of the premolars (i.e., robustness), the crowded premolars (i.e., absence of diastema), the horizontal abrasion of the premolars, as well as the depth of the mandible (noted by Van Valen 1965), indicate that this hyaenodontid would have been capable of powerful crushing. The secant morphology of the molars, which is characterized by the reduction of the talonid and molars, implies that the crushing function was mainly carried out with the premolars.
Considering feeding habits inferred from the RPS and RBL ratios, the values estimated for E. theriodis (RBL = 0.89; RPS = 2.92; Table 2) support a meat/bone diet hypothesis (Fig. 6). This agrees with the morphology of the sagittal crest, mandible, and premolars. One can note from Fig. 6 that E. theriodis is situated in proximity to genera with which it is united in the Eurotherium clade (as defined in Solé et al. 2020): Cartierodon egerkingensis and Prodissopsalis eocaenicus. However, it is interesting to note that Eurotherium matthesi from Geiseltal is situated far from these taxa. This highlights the necessity to revise the genus Eurotherium. E. theriodis is also close to the three Matthodon species in Fig. 6. These species, which are phylogenetically closer to Hyaenodon than the different genera in the Eurotherium clade , have been regarded as potentially having a bone/meat diet (Solé et al. 2014).
The RPS and RBL values also highlight one interesting case for the present discussion, Hyaenodon, because its values are close to those of E. theriodis (Fig. 6). Hyaenodon is a geographically widespread (across the whole of Laurasia) and long-lived genus. It is characterized by robust premolars and the very derived blade-like morphology of the molars, especially in the m3 (Lange-Badré 1979; Bastl 2012). Interestingly, based on microwear analysis and enamel microstructure, Bastl et al. (2012) demonstrated the inclusion of tough food items like bone in Hyaenodon's diet.
Therefore, E. theriodis can be considered as having had a meat/bone diet, probably including carcasses. Due to its fox-size, hyaenas are not the best extant analogs for the ecological reconstruction of E. theriodis. The black-backed jackal (Lupulella mesomelas) is a better analog because its body mass is close to that of E. theriodis. This canid has an opportunistic feeding behavior, being capable of hunting (Krofel 2007), but also being a proficient scavenger (Rowe-Rowe 1983;Hiscocks and Perrin 1987). E. theriodis might thus have been at least partly scavenging on leftovers from other animals' kills, for instance, from the large hyaenodontid Cartierodon from Egerkingen (Solé and Mennecart 2019, or on carcasses of ungulates such as palaeotheriids (Perissodactyla). However, further analyses, such as a study of the evolution of enamel thickness among hyaenodonts, are necessary to test this hypothesis.
This passive predation strategy could be directly related to the morphology of E. theriodis' turbinates. A scavenger needs a well-developed sense of smell to detect carrion at a long distance (Stoddart 2012). This is, for instance, the case Fig. 6. Bivariate plot representing relative premolar size (RPS) versus relative blade length (RBL) for some selected hyaenodonts from the Eocene of Europe. Abbreviations: C., Cartierodon; E., Eurotherium, H., Hyaenodon, M., Matthodon, P., Prodissopsalis. of the black-backed jackal, which has an acute sense of smell (Fox 1971). Therefore, an olfactory membrane with a large air contact surface is common among scavengers, and some are even nocturnal and have very developed turbinates; this is, for instance, the case in hyaenids (Van Valkenburgh et al. 2014). The turbinates seen in E. theriodis thus testify to a strong expansion of the olfactory membrane of the sinuses. The morphology of its olfactory bulbs could therefore correspond to an adaptation of this hyaenodontid species to a scavenging way of life. It is worth remembering that E. theriodis shares with Hyaenodon the presence of well-developed, fan-shaped turbinates and that the diet of Hyaenodon included tough food items like bone ).

Conclusions
Comparison of the endocranial anatomy of several hyaenodonts shows that the endocranium of Eurotherium theriodis is not that different from that of some hyaenodontoids recorded in the Lutetian and Bartonian of Europe, such as observed in the genera Proviverra and Cynohyaenodon. The endocranium of E. theriodis is less complex than that of the hypercarnivorous Hyaenodon species in the absence of the ectosylvia. The morphology of the olfactory bulbs and turbinates is, however, similar to those of Hyaenodon species. This could either represent a convergence due to similar ecology or a synapomorphy.
The study of the nasal cavity of E. theriodis provides further information on its ecology. The presence of well-developed turbinates recalls the conditions observed in extant scavenging predators, in agreement with the ecomorphological characters observed on the skull and teeth of E. theriodis, and probably indicates a meat/bone diet. Therefore, E. theriodis could have been a scavenger living similarly to the extant black-backed jackal.
Investigations on a large array of hyaenodont endocrania are in progress and will provide further information on the evolution and ecology of this diverse and successful Paleogene group, as well as on the relationships among the sub-groups thanks to the establishment of phylogenetic characters related to the morphology of the endocranium.