Below is a list and description of characters pertaining to endocasts and endocranial osteology. The character matrix is provided in Table 3, and the distribution of character states among mammals is discussed below. We report all the characters we examined regardless of informativeness with the reasoning that the characters that are uninformative for our study might be informative when examined in a broader taxonomic context.
We do not include characters pertaining to the petrosal, except for those structures that are typically represented on the cranial endocasts (e.g., parafloccular cast representing subarcuate fossa of petrosal). Osteological characters of the petrosal pertaining to Vincelestes and other crown mammals are discussed in detail in the literature (e.g., MacIntyre, 1972; Rowe, 1988, 1993; Wible, 1990; Rougier et al., 1992, 1996, 1998, 2004; Meng and Fox, 1995; Wible et al., 2001; Luo et al., 2002, 2003; Ekdale et al., 2004; Luo and Wible, 2005; Rougier and Wible, 2006). However, we do include osteological characters pertaining to the endocranial cavity that are not typically incorporated in phylogenetic analyses (e.g., presence or absence of ossified falx cerebri).
Percent of endocast composed of olfactory bulb casts: 6% or greater (large) (0), or less than 6% (small) (1). Based on the taxonomic sample we examined, 6% is the mean for this value. The casts of olfactory bulbs on endocasts provide a good approximation of the size and shape of the corresponding olfactory bulbs in extant mammals (Edinger, 1948; Bauchot and Stephan, 1967; Radinsky, 1968, 1973, 1975; Macrini et al., 2006), and olfactory bulb size is known to correlate with endoturbinal surface area and olfactory acuity in some extant mammals (e.g., Nieuwenhuys et al., 1998; Rowe et al., 2005). Therefore, this character has biological significance.
Olfactory bulb casts are large (in the sense mentioned above) in Triconodon, Vincelestes, and many of the therians examined in our sample. This finding suggests that the MRCA of therians and the MRCA of mammals had large olfactory bulbs. Reduction or complete loss of the olfactory bulbs occurs in the cetaceans and sirenians within Placentalia (e.g., Edinger, 1955; Jerison, 1973; Meisami and Bhatnagar, 1998; Marino, 2004; Colbert et al., 2005), and the aquatic platypus in Monotremata (Pirlot and Nelson, 1978; Macrini et al., 2006). These deviations from the primitive mammalian morphology were acquired convergently in those lineages and are presumably related to the reduced sense of smell in aquatic mammals (Negus, 1958; Pirlot and Nelson, 1978; Meisami and Bhatnagar, 1998).
Accessory olfactory bulb casts: absent (0), or visible on endocast (1). Accessory olfactory bulbs of extant mammals receive projections from the vomeronasal organ, an organ that functions in the detection of pheromones (Nieuwenhuys et al., 1998). The accessory olfactory bulbs are not often represented on endocasts of extant mammals (e.g., Bauchot and Stephan, 1967; Jerison, 1973; Macrini et al., 2006, 2007). In our sample, casts of these structures are only seen on Vincelestes, and also reportedly on the eutherian Kennalestes (Kielan-Jaworowska, 1984).
Olfactory bulb tracts: not visible on endocast (0), or visible on endocast (1). The olfactory tracts are the projections of the olfactory bulbs to the telencephalon (Butler and Hodos, 1996). Among the taxa we sampled, only Didelphis has olfactory bulb tracts represented on its endocast. The meninges and associated structures often obscure the visibility of olfactory bulb tracts on endocasts. The condition in Vincelestes is unknown because there is damage to the bones underlying the ethmoidal fossa (cavity for the olfactory bulb) in the corresponding skull.
Circular fissure: shallow or absent (0), or deep (1) on endocast. This character is modified from Luo and Wible (2005, character 418). The annular ridge of the frontal bone (Rowe et al., 2005; Macrini et al., 2007; “transverse ridge” of Nieuwenhuys et al., 1998:1642) is the bony element that sits in the circular fissure of the brain and separates the olfactory bulbs from the rest of the brain. Thus, the circular fissure marks the posterior extent of the olfactory bulbs. The circular fissure is prominent on endocasts of all the taxa we examined except Tachyglossus (Macrini et al., 2006), Phascolarctos, and Vombatus (Macrini et al., 2007); therefore, the MRCA of therians has a prominent fissure. However, because nonmammalian cynodonts have shallow circular fissures (Quiroga, 1980a, b; Macrini, 2006), the condition for this character is equivocal for the MRCA of mammals.
Surface of cerebral hemisphere casts: lissencephalic (i.e., smooth; 0), or gyrencephalic (i.e., convoluted; 1). Among some groups of extant mammals, gyrencephaly of brains correlates with body weight, brain weight, and isocortex volume (e.g., von Bonin, 1941; Elias and Schwartz, 1971; Jerison, 1973, 1982; Zilles et al., 1989, and references within).
Lissencephalic endocasts usually result from skulls that contained small, lissencephalic cerebral hemispheres. However, some mammals with large, gyrencephalic brains such as hominids, proboscideans, and cetaceans, have smooth endocasts (e.g., Osborn, 1942; Edinger, 1955; Tobias, 1971; Jerison, 1973; Holloway et al., 2004; Colbert et al., 2005) because meninges, cisterns, and other soft-tissue structures of the endocranial cavity, fill in the spaces in the sulci of the brain, producing a smooth, flattened surface that contacts the skull bones.
When dealing with fossils, one can only assume that mammals with small endocranial cavities and lissencephalic endocasts actually had lissencephalic cerebral hemispheres. Data from brains of extant mammals can also be useful for drawing inferences about the surfaces of the cerebrum from fossil endocasts. Empirical data suggest that extant mammals with brain weights under 5 g, for the most part, have relatively smooth cerebral hemispheres (Bauchot and Stephan, 1967; Elias and Schwartz, 1971; Zilles et al., 1989; Striedter, 2005). Assuming that brain tissue has an average density of 1.0 g/cm3 (following Jerison, 1973), Vincelestes had a brain weight of less than 2.4 g. The brain size data combined with the fact that the endocast is lissencephalic suggest that the cerebral hemispheres of Vincelestes were also lissencephalic.
The presence of lissencephalic endocasts is the plesiomorphic condition for Mammalia based on the endocasts of extinct mammals (Kielan-Jaworowska et al., 2004). Based on brains of extant mammals, gyrencephaly or the condition of having convoluted cerebral hemispheres has independently evolved several times in Placentalia, at least three times in Marsupialia, and at least once within Monotremata (Johnson, 1977; Nieuwenhuys et al., 1998; Striedter, 2005). This finding is corroborated using parsimony ancestral state reconstruction of the character on multiple topologies for major clades of extant mammals (e.g., Novacek, 1992; Horovitz and Sánchez-Villagra, 2003; Springer et al., 2005). Furthermore, the inferred ancestral pattern of fissurization varies between different extant mammalian groups (Nieuwenhuys et al., 1998). The presence of lissencephalic cerebral hemisphere casts was the likely condition for the MRCA of therians based on endocasts of the eutherians Kennalestes, Barunlestes, and Zalambdalestes (Kielan-Jaworowska, 1984, 1986); basal metatherians (e.g., Pucadelphys; Macrini et al., 2007); and Vincelestes.
Rhinal fissure on endocast: absent (0), or present (1). The rhinal fissure is defined as the external border between the isocortex (i.e., neocortex) and the piriform cortex of the telencephalon of mammals (Jerison, 1973, 1991; Rowe, 1996a, b; Nieuwenhuys et al., 1998). The mammalian isocortex is the six-layered portion of the dorsal pallium (or neopallium) that receives sensory projections from other portions of the brain (Butler and Hodos, 1996; Nieuwenhuys et al., 1998).
The rhinal fissure is frequently used as a marker for the base of the isocortex in mammals (Jerison, 1973, 1991), but the fissure is not always visible on the endocasts of some mammals, making it difficult to track the expansion of the isocortex based on this feature alone. Lack of a rhinal fissure on a cranial endocast is not necessarily an indication of absence of isocortex, because the rhinal fissure is absent on the endocasts of extant taxa that clearly possess the fissure on the exteriors of their brains (Jerison, 1991; e.g., Monodelphis domestica, Macrini et al., 2007).
The ancestral condition for the MRCA of mammals and the MRCA of therians is to lack a rhinal fissure on endocasts. This finding may simply be an artifact of size, such that the rhinal fissure does not leave an impression on the endocasts of mammals with small brains.
Lateral extent of cerebral hemisphere cast: most lateral point of cerebral cast is medial to or even with the parafloccular cast (0), or cerebral cast clearly extends laterally beyond parafloccular cast (1). Vincelestes exhibits the ancestral state (0) for this character and this finding is also the condition in the MRCA of therians.
Superior sagittal sinus cast: not visible on dorsal surface of endocast (0), or visible (1). The sss does not appear on endocasts if it is located deep within the meninges (e.g., Phascolarctos) or if the walls of the sss are completely surrounded by bone such as an ossified falx cerebri, as is the case with Vincelestes and the extant dolphin Tursiops truncatus (Colbert et al., 2005). In the latter case, this character is correlated with character 9. The lack of an sss cast on the dorsal surface of an endocast (state 0) is reconstructed for the MRCA of therians.
Ossified falx cerebri: absent (0), or present (1). The falx cerebri is a portion of the dura mater that occupies the median sulcus between the cerebral hemispheres. The ossified falx cerebri is examined in this study because of the potential that it is correlated with the depth of the median sulcus on an endocast. This character is also potentially correlated with the visibility of the superior sagittal sinus cast on endocasts (character 8).
The falx cerebri is known to ossify in Obdurodon dicksoni, a Miocene platypus, and Ornithorhynchus anatinus, the extant platypus (Macrini et al., 2006), some extant cetaceans (e.g., dolphins and porpoises, Klintworth, 1968; Nojima, 1988; Colbert et al., 2005), sirenians (Nojima, 1988), some pinnipeds (Nojima, 1988), and Vincelestes. Calcification of the falx cerebri is also documented in humans, most commonly among the elderly (Cheon et al., 2002). To our knowledge, an ossified falx cerebri is not documented for any other extinct or extant mammalian taxon than those mentioned above.
Based on the current phylogenetic (e.g., Luo and Wible, 2005; Rose and Archibald, 2005) and anatomical information, the ossified falx cerebri is independently derived in each of these taxa. However, many stem therians have poorly preserved cranial material, and it is possible that, as better cranial material is found for these taxa, the distribution of this character may change.
The presence of an ossified falx cerebri is currently considered the derived condition based on the known distribution of this state within extant placentals. However, using the Luo and Wible (2005) tree, the current ancestral state reconstruction for the MRCA of mammals is equivocal for this character. Future examination of serial sections or CT images of skulls of nonmammalian cynodonts and additional nontherian mammals might support a reversal of the polarity of this character.
Osseous tentorium: absent (0), represented by posteromedial ossification of tentorium cerebelli (1), lateral ossification of tentorium cerebelli (2), or complete ossification of tentorium cerebelli (3). Among extant mammals, an osseous tentorium is present in equids, carnivorans, pholidotes, cetaceans, and macropodid marsupials, but the degree of ossification of the tentorium cerebelli of the dura mater is variable among these taxa (Jollie, 1968; Klintworth, 1968; Bjerring, 1995; Solano and Brawer, 2004; Colbert et al., 2005). For example, in Felis catus (domestic cat), the entire tentorium cerebelli is ossified in adults (state 3), but in other mammals, only portions of the tentorium ossify (Klintworth, 1968). Only the posteromedial portion of the tentorium cerebelli ossifies (state 1) in some mammals such as the Canis familiaris (domestic dog), Mustela vison (mink), Tursiops truncatus (bottlenose dolphin), Delphinus delphis (common dolphin), Phocoenoides dalli (Dall's porpoise), Equus caballus (horse), and Thylogale browni (Brown's pademelon; Klintworth, 1968; Solano and Brawer, 2004). Only the lateral portions of the tentorium ossify (state 2) in Manis (pangolin; Jollie, 1968). In some extant mammals (e.g., carnivorans, macropodids, equids), the osseous tentorium is formed before adulthood, but in other mammals (e.g., delphinids and phocoenids), the ossification occurs during the course of adult aging (Nojima, 1988).
An osseous tentorium is also present, but damaged in the specimen of Vincelestes examined in this study (Fig. 5), the only documented occurrence of this osseous structure outside Theria. The osseous tentorium of Vincelestes represented a posteromedial ossification of the tentorium cerebelli (state 1).
The taxonomic distribution and differences in extent of ossification of the tentorium cerebelli suggest that the formation of an osseous tentorium was independently derived multiple times in Theriiformes. Lack of an osseous tentorium is the condition in the MRCA of therians and the MRCA of mammals.
Recognition of the variation of ossification of the osseous tentorium also has implications for higher level relationships within Placentalia. For example, molecular data routinely converge on a Carnivora and Pholidota sister relationship (Springer et al., 2005, and references within). Morphological support for this clade is weak, but one reported character supporting this relationship is the presence of an osseous tentorium in both carnivorans and pholidotes (e.g., Rose et al., 2005, and references within). As mentioned above, only the lateral portions of the tentorium cerebelli ossify in pangolins, whereas the medial portion or the entire tentorium ossifies in carnivorans. This morphological variation combined with the taxonomic distribution of an osseous tentorium suggests that presence of ossification of the tentorium cerebelli is plastic among mammals and is not a synapomorphy supporting the purported Carnivora and Pholidota sister relationship.
Exposure of midbrain (superior and inferior colliculi) on dorsal surface of endocast: absent (0), or present (1). The midbrain is not exposed on several endocasts because of coverage by sinuses (e.g., transverse sinus) and associated meninges, a posteriorly expanded cerebrum, or a combination of these structures (Edinger, 1964; Nieuwenhuys et al., 1998). Lack of exposure of the colliculi on endocasts does not necessarily correlate with lack of exposure of these structures on the dorsal view of the brain. For example, Didelphis virginiana (Dom et al., 1970), Monodelphis domestica (Macrini et al., 2007), and Tenrec ecaudatus (Bauchot and Stephan, 1967), all have their colliculi exposed in dorsal view of their brains but not on the corresponding endocasts.
The colliculi are exposed on the dorsal surface of endocasts of at least a few mammalian endocasts, including the fossil eutherians Asioryctes, Kennalestes, Zalambdalestes, and Barunlestes (Kielan-Jaworowska, 1984, 1986; Kielan-Jaworowska and Trofimov, 1986), and at least a few extant placentals (Bauchot and Stephan, 1967). But the taxa possessing state (1) are definitely in the minority among the taxa we sampled. The condition for this character in Vincelestes is uncertain because of damage to the skull in this region. The MRCA of therians most likely did not have dorsal exposure of the midbrain on its endocast based on the distribution of this character among the sampled crown mammals. The polarization of this character is also supported by the current knowledge of the endocasts of nonmammalian cynodonts (Hopson, 1979; Quiroga, 1980a, b; Macrini, 2006).
Cast of vermis of cerebellum: extends anterior to or even with the parafloccular casts (0), or vermis remains behind parafloccular casts (1). This character is modified from Luo and Wible (2005, character 415). State (0) is present in eutriconodontans (Simpson, 1927; Kielan-Jaworowska, 1986), the multituberculate Kryptobaatar (Kielan-Jaworowska and Lancaster, 2004), Vincelestes, Pucadelphys (Macrini et al., 2007), and Phascolarctos. In contrast, several crown therians including marsupials (Macrini et al., 2007), Leptictis (Novacek, 1982, 1986), and Zalambdalestes (Kielan-Jaworowska, 1984, 1986; Wible et al., 2004) have a small vermis cast that is positioned posterior to the parafloccular casts (state 1). The vermis was not observed on endocasts of nonmammalian cynodonts (Watson, 1913; Hopson, 1979; Quiroga, 1979, 1980a, b, 1984). Based on this distribution, the condition in the MRCA of therians is equivocal.
Cerebellar hemisphere casts: not visible on endocast (0), or well-developed on endocast (1). This character is modified from Luo and Wible (2005, character 417). The cerebellar hemispheres are gross anatomical divisions of the mammalian cerebellum and do not necessarily reflect functional differences (Nieuwenhuys et al., 1998). Instead the hemispheres indicate lateral expansion of the cerebellum in mammals.
Cerebellar hemisphere casts are present on the endocast of Vincelestes as well as on endocasts of monotremes (Macrini et al., 2006) and crown therians (Kielan-Jaworowska, 1984, 1986; Novacek, 1986; Macrini et al., 2007). In contrast, cerebellar hemispheres are notably absent from endocasts of multituberculates (Kielan-Jaworowska, 1986) and nonmammalian cynodonts (Watson, 1913; Hopson, 1979; Quiroga, 1979, 1980a, b, 1984). Therefore, the presence of cerebellar hemispheres in the MRCA of therians is a plesiomorphic condition.
Cast of the paraflocculus of the cerebellum: present on endocast (0), or absent from endocast (1). The degree to which the paraflocculus of the cerebellum filled the subarcuate fossa of Vincelestes is uncertain. A study of extant marsupials revealed that the paraflocculus fills at least a portion of the subarcuate space in all taxa examined, but to varying degrees in different taxa (Sánchez-Villagra, 2002). Based on extant phylogenetic bracketing (Witmer, 1995), at least a portion of this space was occupied by the paraflocculus in Vincelestes.
The presence of parafloccular lobes represented on endocasts is the plesiomorphic condition for the MRCA of therians. The paraflocculus is associated with coordination, balance, and vestibular sensory acquisition (Butler and Hodos, 1996; Nieuwenhuys et al., 1998).
Percent of endocast composed by parafloccular casts: less than 1% (0), or greater than or equal to 1% (1). The parafloccular casts constitute approximately 1% of the total endocranial cavity on average for the taxa we examined. The MRCA of therians exhibits the ancestral character state (0). However, some crown therians such as Dromiciops australis (monito del monte [little mountain monkey]) and Dasyurus hallucatus (northern quoll) show the derived state.
Parafloccular cast shape: cone-shaped (0), broad and rounded (1), large, posterolaterally oriented ovoid (2), or long and cylindrical without expansion on the distal end (3). Character states were determined from published descriptions of the subarcuate fossa of the petrosal, published descriptions of parafloccular casts of endocasts, and examination of digital endocasts (Kermack, 1963; MacIntyre, 1972; Novacek, 1982, 1986, 1989; Kielan-Jaworowska, 1984, 1986; Wible, 1990; Rougier et al., 1992, 1996; Wible and Rougier, 2000; Luo et al., 2002; Sánchez-Villagra, 2002; Kielan-Jaworowska and Lancaster, 2004).
Presence of cone-shaped parafloccular casts is the inferred ancestral state because it occurs in some nonmammalian mammaliaformes (Macrini, 2006). The MRCA of therians and the MRCA of mammals is reconstructed with broad and rounded parafloccular casts (state 1).
Depth of hypophyseal fossa relative to its length: fossa deeper than long (aspect ratio > 1.1) (0), fossa longer than deep (aspect ratio < 0.9) (1), or hypophyseal fossa depth and length approximately equal (aspect ratio = 0.9–1.1) (2). The hypophyseal fossa is a poor indicator of the size and shape of the pituitary gland in many mammals (Edinger, 1942), but we are confident that at least a portion of this space was occupied by the gland in Vincelestes. Based on distribution of this character in nonmammalian cynodonts, presence of a fossil that is deeper than wide is the plesiomorph condition (MaCrini, 2006). The condition for this character in the MRCA of therians is equivocal.
Width of hypophyseal fossa relative to its length: wider than long (aspect ratio > 1.1) (0), longer than wide (aspect ratio < 0.9) (1), or hypophyseal fossa width and length approximately equal (aspect ratio = 0.9–1.1) (2). The presence of a hypophyseal fossa that is wider than long is the ancestral state for this character, based on the condition in nonmammalian cynodonts (Macrini, 2006). Therefore, presence of this condition (state 0) in the MRCA of therians is plesiomorphic.
Position of aperture of canals transmitting the carotid arteries into the hypophysis: located in posterolateral portion of hypophysis (0), or anterolateral portion of hypophysis (1). The MRCA of therians exhibits state (0) for this character.
Cavum epiptericum: confluent with cavum supracochleare (0), or cavum epiptericum and cavum supracochleare separated by at least a partial bony wall (1). This character was taken from Wible (1990).
The cavum epiptericum is the space between the primary and secondary braincase walls of mammals. In nonmammalian cynodonts, the medial wall of the cavum epiptericum is formed by the pila antotica, an anterior extension of the prootic, which acts as the primary wall of the endocranial cavity (Presley, 1980; Maier, 1987; Novacek, 1993; Rougier and Wible, 2006). The primary bony wall of crown mammals is greatly reduced and the cavum epiptericum is incorporated in the endocranial cavity (Kühn and Zeller, 1987; Novacek, 1993). Therefore, the lateral wall of the cavum epiptericum forms the secondary wall of the braincase in crown mammals and the composition of this wall is an important source of phylogenetic data among crown mammals (see discussions in Presley, 1980; Kühn and Zeller, 1987; Maier, 1987; Hopson and Rougier, 1993; Novacek, 1993; Kielan-Jaworowska et al., 2004).
The contents of the cavum epiptericum are variable among extant mammals. The cavum epiptericum of Ornithorhynchus anatinus, the platypus, houses the semilunar (Gasserian) ganglion of the trigeminal nerve (V), a portion of the otic ganglion, the geniculate ganglion (for cranial nerve VII), and portions of cranial nerves III–VII (Zeller, 1989b). However, in Tachyglossus aculeatus, the short-nosed echidna, the sphenopalatine ganglion for the greater petrosal nerve is also incorporated in the cavum epiptericum but the geniculate ganglion remains extracranial (Kühn and Zeller, 1987). In extant therians, the sphenopalatine and otic ganglia are extracranial but the geniculate ganglion is typically housed in the separate cavum supracochleare of the petrosal bone (Kühn and Zeller, 1987; Rougier et al., 1996).
The posterior wall of the cavum epiptericum of Vincelestes is pierced by an opening (fenestra semilunaris), which connects with the cavum supracochleare (Rougier et al., 1992). According to Rougier et al. (1992), the fenestra semilunaris was probably covered by a dense fibrous connective tissue that did not permit transmission of major structures between the two cava. Therefore, based on the cranial osteology and on extant phylogenetic bracketing, the cavum epiptericum of Vincelestes likely housed the semilunar ganglion and portions of several cranial nerves, but not the geniculate ganglion.
The presence of at least a partial bony separation of the cavum epiptericum and cavum supracochleare is a synapomorphy for the least inclusive clade containing Vincelestes and Theria. However, this is not an unequivocal synapomorphy because the cavum supracochleare is also separate from the cavum epiptericum in Tachyglossus (Kühn and Zeller, 1987) and Eutriconodonta (Kermack, 1963). The presence of a fenestra semilunaris in Vincelestes, at least one other stem therian (Wible et al., 1995), some extant marsupials (Wible, 1990), and “zhelestid” eutherians (Ekdale et al., 2004) suggests this feature should also be reconstructed for the MRCA of therians.
Anterior portion of cavum epiptericum leading to the sphenorbital fissure: anterior portions of right and left cava are at least partially separated at sphenorbital fissure (0), or cava are completely confluent at sphenorbital fissure (1). The MRCA of therians is reconstructed as having the anterior portions of the right and left cava at least partially separated contrary to the derived condition seen in Vincelestes. In at least some crown therians (e.g., Monodelphis domestica), the presphenoid forms a partial barrier between the portion of the right and left cava leading to the sphenorbital fissures.
An encephalization quotient (EQ) is the ratio of actual to expected brain sizes for a particular taxon (Jerison, 1973). Expected brain size is calculated using equations relating brain mass (or endocranial volume) to body mass; these equations are determined by comparison of these variables among several closely related taxa (Jerison, 1973). Encephalization quotients are widely used in the literature but also frequently criticized (e.g., Deacon, 1990; Striedter, 2005). Even so, EQs are useful for determining allometric relationships between endocranial volume and body mass for extinct animals. In addition, the degree of encephalization in mammals appears to correlate with several anatomical and functional variables (Jerison, 1973; Eisenberg and Wilson, 1978, 1981).
In this study, we do not address methodological issues associated with encephalization quotients, but instead compare the EQ calculated for Vincelestes (Table 4) with those EQs determined from several fossil and extant crown mammals. We make comparisons between EQs calculated using the equation of Eisenberg (1981), because this equation was determined using a large amount of empirical data from a wide range of extant mammalian taxa. We also include the olfactory bulb casts in the determination of endocranial volume when comparing EQs of different taxa. Exclusion of the olfactory bulb casts when determining the endocranial volume is unjustified, because the olfactory bulbs receive olfactory sensory input and process that information to a certain degree and as such are part of the brain (Nieuwenhuys et al., 1998; Rowe et al., 2005). Furthermore, it is not possible to accurately determine the volume of the olfactory bulb casts for many endocranial volumes provided in the literature. However, for the sake of completeness, we calculated EQ for Vincelestes using both endocranial volumes that include and exclude the olfactory bulb casts volume, and using two different EQ equations (Table 4).
Table 4. Encephalization quotient values for Vincelestesa
|EQ (with olfactory bulbs)d||0.27||0.37|
|EQ (without olfactory bulbs)e||0.24||0.33|
The EQ for Vincelestes calculated using the total endocranial volume, a body mass of 619 g, and the Eisenberg (1981) equation for the one specimen examined in this study is 0.37. The EQ for Vincelestes overlaps the lower range of EQs calculated for basal eutherians (0.36–0.80; Novacek, 1982, 1986; Kielan-Jaworowska, 1984, 1986), and surpasses EQs calculated for the metatherian Pucadelphys (0.32) and Didelphis virginiana (0.34; Macrini et al., 2007). Therefore, the relative brain size of Vincelestes is close to the bottom range of EQs for crown therians.
However, the EQs of multituberculates (0.54–0.64; Kielan-Jaworowska, 1983; Krause and Kielan-Jaworowska, 1993) and eutriconodontans (0.49; Jerison, 1973) are higher than the EQ of Vincelestes. In addition, crown monotremes have considerably higher EQs (0.75–1.00; Macrini et al., 2006) than Vincelestes, as do at least some crown marsupials (Eisenberg and Wilson, 1981; Macrini et al., 2007). This finding suggests that relatively large brains evolved multiple times within Mammalia, particularly within Theriimorpha, the clade containing multituberculates, eutriconodontans, Vincelestes, and crown therians (Fig. 1).
However, the taxonomic sampling for these data is sparse, and EQs for different taxa might be modified in the future as methodologies for determining endocranial volumes and body masses are standardized. For example, some of the discrepancy between the EQs determined for Vincelestes and other extinct mammals (e.g., multituberculates, eutriconodontans, basal eutherians) might result from methodological differences in determining endocranial volume (e.g., estimating from natural endocasts versus measuring on digital endocasts) and estimating body mass. In addition, the effects of phylogenetic nonindependence have not been examined for these data (Felsenstein, 1985; Harvey and Pagel, 1991). These and other problems associated with EQs need to be addressed in future studies to properly examine the evolution of brain size across the major lineages of mammals.