1 Introduction

1.1 The occipitum

The evolving biology of skull development in relation to ancestry does constrain the morphotype changes possible in the occipital bone (Gilbert, 2006) with genetic variants disposition to more or less mutation- thresholds through modularity (Conlon and Raff, 1999), and may induce various changes through correlated progression on the foramen magnum (Gilbert, 2006), the ratio of which size increases with skull index though not with shape (Ellis et al., 2009). Radiations and reintroduction of raccoons into new lands has been observed to impact skull size and shape negatively compared to fossil forms (Nowak, 1984; Nowicki et al., 2011). The squama occipitalis morphology in raccoon dogs has been reported to be trapezoid (Hidaka et al., 1997; Hatori et al., 2003) in Japanese raccoon dogs (Nyctereutes .procyonoides.Viverrinus) but more triangular in the species found around Finland (N.p. Ussuriensis) and Korea (Nyctereutes procyonoides Koreensis) (Kauhala et al., 1998), similar to those found in tropical environments (N.p cancrivorus). Buffering effects of developmental constraints on evolution of new phenotypes such as canalization may be overcome by stress factors in different environment (Halgrimson et al., 2004).

A dorsal median symmetrical division of the bone by a prominent crista occipitalis externa (Jurgelenas et al., 2007) forms the median demarcation of two lateral depressions. Epigenetic incident frequencies have been reported to be increased (Samuel et al., 2015)* independent of sex and side, the canalis condylaris varies in number and location between and within species. There is no consensus among several authors on the construction of this bone; little information exists on excavations and numerous foramina perforating the external aspect of this structure, along with bilateral grooves which indents the cerebral surface. The occipital sensory area of the brain resides in the saucer-like bone (Figuti, 2004). The Pars basilaris maintains a constant morphology among the subspecies (Hatori, 1997; Hidaka, 2003).

1.2 Anatomical and paleontological implications

Chromosomal similarities and differences among populations have been reported earlier (Nyctereutes procyonoides and Nyctereutes procyonoides Ussuriensis possess 54 chromosomes while N.p. Viverinnus possess 38) (Makinen et al., 1986; Ward et al., 1987), ten homologous autosomes have been recognized while the remaining chromosomes arms are homologous due to reduction in chromosomal number in Japanese raccoons via Robertsonian translocations; an occurrence known only in new species evolution (Mayr, 1976).

1.3 The foramen magnum and the contributions of allometry

Size, shape and architecture of the foramen magnum differ among species, gender and individuals (De La-hunta, 1983; Rholf, 2010). Foramen magnum index among the subspecies peaks within juveniles’ age group especially in the smaller subspecies (Zevellof, 2002) though foramen margin morphology remains poorly described, the occurrence of dorsal notches has not been properly documented in this subject as morphologic variants of supra-occipital bone ossification differences and not necessarily pathologic (Janeczek et al., 2008; Watson et al., 1989) and compatible with longevity more so, they are not associated with any other skull bone deformities except a degree of directional asymmetry observed in the construction giving rise to behavioral trait (handedness) in adults (Samuel et al., 2015)*. The most caudo-ventral portion morphology of the occipital condyles also offers variations which may be of further relevance in classification of the family.

1.4 Anatomical and clinical significance

Occipital index in adulthood in the subspecies of this family differs both from ecologic and subspecies point of view (Ewer, 1985; Baltrunaite, 2005; Gibson, 2000; Rusbridge and Knowler, 2006; Reid and Helgen, 2008). Females, in this regard has about 15% attenuated size development of the skull bones (IUCN, 2012). As in other canids, the possibility of cerebellar protrusion occasioned by volume reduction of the posterior fossa, syringomyelia and neurological disorders (Rusbridge and Knowler, 2006; Simoens et al., 1994) remains potent with observations of open dorsal notches which Janeczek et al (2008) as reported in dogs (canis familiaris) to be associated with captive breeding and domestication attempts. Variable topographic localization, number and shape of condylaris canalis ventralis observed in different subspecies of genetically related N.P. cancrivora as observed in tropical (Nigerian) subspecies do modify the location of occupant nerves and vessels thereby predisposing to neuro- vascular accidents (Bartussek, 2004; De Lahunta, 2006), possible causes of such occurrences may be due to delay in closure of anterior neuropore (Gilbert, 2006) and could be spatial or temporal (Watson et al., 1989; Gilbert, 2006). This is better studied using computed tomography (Alpak, 2003) compared to the conventional less reliable methods of analysis (Fike et al., 1984; Jurgelenas et al., 2007). This species has not been priory investigated in environmental and developmental perspectives especially in the tropics, linear cranial morphometric studies exists in Finland (Kauhala et al., 1998), Lithuania (Jurgelenas et al., 2007), Russia (Latrov, 1971), Japan (Hidaka et al., 1997), Korea (Ellerman and Morrison-Scott, 1951), Germany (Bartussek, 2005; Zevellof, 2002) and a few other European geographic locations (Ewer, 1985, Helle and Kauhala, 1991, Bartussek, 2004) on similar subject.

2 Materials and Methods

2.1 Literature review

For the purpose of the review, articles were assessed from basic searches from data base of PubMed using terms like; occipital bone, development, evolution and Nyctereutes procyonoides subspecies. The manuscripts and books consulted were from 1951 to 2015. Thus the aim of this enquiry was to review literature information on the occipital bone construction in six known and unreported subspecies of procyonoides skulls from other lands.

Fossil skulls literatures on N.p obtained from prefectures (Bjork, 1973, Hatori et al., 2003; Hohmann et al., 2001) and museums (Hidaka et al., 1997), those obtained from freshly hunted animals taken from ecologic environments (Kauhala et al., 1998; Samuel et al., 2015*) and those from Pliocene medieval periods (Hohmann et al., 2001) were variously consulted for this review.

Figure 1 Image of occipital area in Nyctereutes .procyonoides .cancrivora showing morphologic variations in condylar construction and dorsal notch in foramen magnum

3 Results

3.1 Functional morphology of the occipital bone

The occipital bone at the caudal and caudo-ventral part of the neuro-cranium is essentially, externally a joint surface area with the axial skeleton (atlanto-occipital joint) and partake in modularity of other skull parts as well for lodgments of ligamentum nuchae (pars occipitalis) on the crista nuchae externii dorsally, cleido-occipitalis sternocephalicus laterally, rectus capitis dorsalis, rectus capitis lateralis, obliquus capitis cranialis ventro-laterally (Popesco, 1977), occipito-mandibularis on the paramastoid process closely related with the supplying glossopharyngeal and hypoglossal motor nerves and the external carotid artery. The digastricus as well as occipito-hyoideus are for manipulative movements of the lower jaw (Sisson, 1975; ICVGAN, 1994) and generating characteristic bite force used in prey summarization and post-mortem processing (Ellis et al., 2009). Functional use of the head in this mesaticephalic skull for prehensile and masticatory activities varies with size and shape of the bone, as well as muscle mass attached (Dyce et al., 2002). The development of this structure is influenced by ecology and diet components in various environments as suggested by (Baltrunaite, 2005, 2006; Kauhala, 1998, Fernandes et al., 2008; Zevellof, 2002). In comparing skull dimensions in N.p. viverrinus and N.p. Usuriensis; N.p.cancrivora differs in diet typical of the cancrivorus subspecies (De La Rosa and Nocke, 2000) consisting of 40% invertebrates, 33% plant materials, 27% vertebrates (Zevellof, 2002). This carnivore-omnivore thus possesses a peculiar occipital morphology relative to similar sized carnivores (McClintock, 1981; Reid and Helgen, 2008). The glossopharyngeal, vagus, accessory, facial and mandibular nerves all contribute to the extremely fine use of the head (Ewer, 1985) in the species and has been observed to exhibit polymorphic variations in course through epigenetic and inconstantly (McClintock, 1981; De Lahunta, 1983; Gibson and Wagner, 2000) located canals and sometimes multiple foramina in the area among studied populations (Samuel et al., 2015).

3.2 Ontogenetic patterns

The cranial neural crest cells are known precursors of the mesenchyme of 3^rd, 4^th and 5^th pharyngeal arches with early instructions on cell types to form (Noden and De Lahunta, 1985) and develop into parts of the skull whose fates are determined by Hox genes (Hoxa-3).

Allometric changes within pups, juveniles and adults age groups with reference to occipital area metric changes are presently on-going to compare different subspecies data. Correlated progression in some embryonic skull parts may have induced the phenotypic shape plasticity observed with consequences on muscle and nerve placements (Noden and De Lahunta, 1985; Gilbert, 2006) under various developmental constraints in the subspecies environments.

4 Conclusion

N.p. procyonoides, being one of the most variably sized species (McClintock, 1981; Reid and Helgen, 2008) with wide geographic radiations which have demonstrated different morphotypes in occipital area may be pre-existent mutants. Probably, the N. p. viverrinus is a more recent form than ussuriensis or procyonoides. No literary evidence to the best of our awareness is available on tropical variants of cancrivoral subspecies. We undertook this review of the specified area of the skull with a view to provide an updated appreciation of the cranial morphology of known subspecies and an awareness of developmental modeling influences which may produce morphotypes by selection predisposed by environmental stress factors and how fundamental developmental processes structure variability. The study shall also be useful in population studies, skull modeling, species anthropometry as well as solving taxonomic ambiguities in the species for conservation purposes.

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