User:MetaAlphaBeta/Castor californicus

Castor californicus is an extinct species of beaver that lived in western North America from the end of the Miocene to the early Pleistocene. Castor californicus was first discovered in Kettleman Hills in California, United States. The species was similar to but larger than the extant North American beaver, C. canadensis.

Unlike other members of the Castor genus, the Castor californicus has a total of three enamel folds, the folds of the internal enamel epithelium.

HOLOTYPE of Castor accessor Hay, 1927 N. Drake (1897) United States of America. USNM PAL USNM V11607

Source: NMNH Paleobiology Specimen Records (USNM)

Phylogenetic characteristics
The Castor californicus has been determined to be the earliest type of its genus to appear in North America.

Castor californicus is characterized by having short nasal passages. The backs of their skulls were quite wide in comparison to other members of the Castor genus. The coronoid process is more spread out. Their pterygoid muscles were on the larger side, and their neck muscles were broad. Due to these distinctions, Castor californicus had a slight physical advantage compared to Castor fiber. Each Castor californicus was slightly unique in its postcranial morphology. They had short femurs and elongated hind feet, which assisted them in moving with ease through water.

Compared to other species of beavers, Castor californicus had considerably wide metatarsals, which aided them in the swimming and digging process. Castor californicus has three folds of the internal enamel epithelium, unlike other members of the Castor genus.

Relative warp analysis of both cranial and dentary material revealed a close association between Castor canadensis and C. californicus. Dorsal views both show C. californicus fitting within the range of morphospace variation for DRW1 and 2 of C. canadensis (Figure 6), which are associated with shortened nasals, wide posterior cranium, and posterior positioning of the orbit (Figure 7). Lateral views also show close association between C. californicus and C. canadensis. All specimens fall in the range of morphospace variation for LRW1 and 3, where one specimen of C. californicus fell within the cluster of C. canadensis and two specimens fell just outside the morphospace range of C. canadensis (Figure 8). The lateral view grouping of C. californicus and C. canadensis are associated with elongated nasals and shortened posterior cranium between nuchal crest and occipital condyles (Figure 7). Relative warps of the ventral view did not show any clear separation between species, indicating an absence of consistent morphological differences between species in this view (Table 4). C. canadensis and C. californicus had more morphological distinctions within the dentary. For DenRW1 all three Castor species overlap, with C. canadensis showing a broad range of variation (Figure 10). Most specimens of C. californicus fell outside of the DenRW2 range of C. canadensis, with C. californicus having a wider separation between the condylar and articular processes and anteroventral positioning of incisor alveolus (Figures 9). Across relative warp analysis C. fiber consistently plotted separately from C. canadensis and C. californicus, showing distinct morphological differences between both extant species.

Canonical variate analysis (CVA) also showed a close association between C. canadensis and C. californicus. Cranial CVA in which C. californicus was classified a priori as unknown, one C. californicus overlapped with C. canadensis and the other was between the two extant species, closer to C. fiber (Figure 11), which were characterized to have more shortened nasals, posteromedial positioned orbit, and widening of the posterior cranium (Figure 12). In the classification stage, C. californicus had one specimen assigned to C. canadensis and one to C. fiber (Table 9). When species were all categorized a priori, C. canadensis and C. californicus did separate into distinct groups fairly well (Figure 14). Along canonical variate one, C. californicus was characterized to have elongated nasals, narrowed posterior cranium, and narrowed premaxilla more like C. fiber, while with canonical variate two C. californicus was associated with shortened nasals and broader posterior cranium like C. canadensis (Figure 13). The classification stage did result in one of two specimens of C. californicus assigned as C. canadensis in cross-validation (Table 10). Dentary CVA, where C. californicus was classified as unknown, grouped all C. californicus with C. canadensis (Figure 15) which are associated with anteroventrally positioned coronoid process, posterior positioning of condylar process, ventral position of angular process, and anterior position of pterygoid insertion (Figure 17). The classification stage primarily categorized C. californicus with C. canadensis (Table 11). When the dentary CVA had all species classified a priori, C. californicus and C. canadensis plotted near each other along both canonical variates (Figure 18), where both species displayed anterior positioning of the coronoid process (Figure 17). The classification stage had C. californicus classified as C. canadensis for one of three specimens (Table 12).

The cluster analysis of cranial and dentary material showed a strong similarity between C. canadensis and C. californicus (Figure 19). C. californicus clustered within the groupings of C. canadensis cranial specimens, indicating shared morphological similarities between the two species (Figure 19). Dentary specimens showed greater separation between species, with C. californicus forming the outgroup from C. canadensis and C. fiber, suggesting more morphological differences distinguishing the species (Figure 20).

Overall, Castor canadensis shows high levels of variation in cranial morphology. The geometric morphometric analysis all resulted in widespread distribution of the species within morphospace. It has been noted in previous literature that C. canadensis is highly variable, as at one time it was separated into subspecies based on phenotypic characteristics and regional distribution across North America (Rhoads 1898; Jenkins and Buscher 1979; Long 2000). Specimens of C. canadensis used in this study were collected from across North America (Appendices A and B); therefore, the resulting variation seen within the species is a good representation of the variation seen across the continent in the recent past and present.

Castor californicus consistently plotted within the observed range of variation of C. canadensis across analyses (Figures 6, 8, and 10). This suggests cranial morphological features are more similar in C. canadensis and C. californicus than either is with C. fiber. Previous studies on the mitochondrial DNA of Castor canadensis and C. fiber show that the two species last shared a common ancestor as early as 7.5 million years ago (Horn et al. 2011). This timing corresponds with the oldest known record of C. californicus in North America from the Rattlesnake Formation in Oregon (Samuels and Zancanella 2011). Cranial morphological similarities between C. canadensis and C. californicus broadly include shortened nasals, widened posterior cranium, and posterior positioning of the orbit when compared to C. fiber. Dentaries of C. canadensis and C. californicus both display anterior placement of the anterior margin of the pterygoid insertion and widening between the posterior processes. This suggest both North American species have larger pterygoid muscles than C. fiber and broader nuchal region, which is the insertion of the neck muscles.

Semi-aquatic rodents exhibit a wide range of osteological specializations for their lifestyles (Howell 1930). Characteristics include shortening of the femur, robust limb elements, enlarged muscle attachment sites for the hind limb, and elongated hindfoot to aid in movement through the water (Samuels and Van Valkenburgh 2008). These characteristics hold true for C. canadensis and C. californicus. The femur anteroposterior diameter (FeAPD) in C. canadensis is low, exhibiting an extreme flattening of the femur while C. californicus exhibits a more robust anteroposterior diameter than C. canadensis (Figure 21). The femoral epicondylar breadth (FeEB) is wider in C. californicus than in C. canadensis (Figure 22). A wider FeEB would allow for greater muscle attachments to help with swimming (Samuels and Van Valkenburgh 2008) and would be expected in an animal of larger body mass. The anteroposterior and mediolateral diameters at the distal end of the tibia (TDEAPD and TDEMLD) are slightly wider in C. californicus than in C. canadensis, suggesting that C. californicus had more robust articular distal ends on the hindlimbs than C. canadensis (Figure 21 and 22). In the pes, the anteroposterior diameter of the third metatarsal (MT3APD) and mediolateral diameter of the fourth metatarsal (MT4MLD) are both more robust in C. californicus than C. canadensis (Figure 22). Increasing the size of the pes can aid in increasing the surface area of the hindfoot for increased propulsion through the water (Samuels and Van Valkenburgh 2008), which would also be expected at larger body mass. The articular width at the distal end of the humerus (HDAW) is wider in C. californicus than C. canadensis, suggesting that C. californicus had more robust articular distal ends on the forelimbs than C. canadensis (Figure 22), which may allow a wider range of motions and facilitate both swimming and digging.

Castor canadensis and C. californicus show high levels of variation in postcranial morphology. Coefficients of variation for both species were highly variable and significantly different (Figure 23 and Table 13), suggesting that differentiating species based on size is not a reliable metric. Previous studies of C. californicus described it as closely resembling the extant C. canadensis but larger in size (Stirton 1935; Shotwell 1970). As shown in other studies, size is not generally a reliable metric for identifying and distinguishing species (Emery-Wetherell and Davis 2018). Therefore, as C. canadensis has high variation within its morphology, size alone should not be used to distinguish C. californicus from C. canadensis. Although certain postcranial features showed differences in range between C. californicus and C. canadensis, most C. californicus elements measured in the study fit within the observed range of variation for C. canadensis.

Overall, cranial, dentary, and postcranial morphology of Castor californicus has been shown to be highly similar to C. canadensis. The overall morphological similarities between C. canadensis and C. californicus likely indicate similarities in diet and locomotor ecology, which would highly suggest the two species are ecologically analogous. Notable cranial morphological differences in C. californicus include widened posterior cranium and posterior positioning of orbit. Dentary morphology in C. californicus was distinct with wider separation between condylar and articular processes and anteroventral position of incisor alveolus. Postcranial morphology of C. californicus had less dorsoventral flattening of the femur, increased hindlimb robustness, and increased metatarsal widths, representing some noticeable differences from extant species of Castor. These differences are likely a consequence of anagenetic changes in a species over several million years, with C. californicus being ancestral to C. canadensis.

Distribution and habitat
Modern habitat suitability for Castor canadensis, using only bioclimatic variables, fell into previously recorded distributions (Figure 24). Bioclimatic variables highly contributing to the model included precipitation seasonality, isothermality, and annual mean temperature (Table 15). Predicted distributions, using both bioclimatic variables and ecoregions and only ecoregions, modeled more restricted distributions (Figure 24). Model accuracy, determined from area under the curve (AUC) values produced by MaxEnt, were lower for the model using only bioclimatic variables compared to the higher AUC values produced for the other modern distribution models (Table 14). AUC scores were likely lower for predicting species habitat as C. canadensis inhabits a wide range of areas across North America (Figure 1). Previous works studying beaver habitats were conducted at more localized ranges, using variables including stream gradient, watershed size, and hardwood cover in riparian zones (Touihri et al. 2018).

The Pliocene model (3.3 Ma) for Castor showed similar ranges of suitability as today, with major restrictions of habitats in the northern and central regions of the continent (Figure 25). Bioclimatic variables highly contributing to the model include precipitation seasonality, annual mean temperature, mean temperature of driest quarter, and mean temperature of warmest quarter (Table 16). Variables affecting the model differ, likely because bioclimatic variables were absent for this dataset. The mid-Pliocene warming period, 3.3- 3.0 Ma, represents a period where global temperatures were warmer than today (Dowsett and Caballero-Gill 2010; Dolan et al. 2015). However, the Marine Isotope Stage M2 showed a period of cooling, which is represented by the bioclimatic variables used to produce the model (Figure 25) (Dolan et al. 2015). Loss of habitat suitability in the northern and central regions of North America and expansion into coastal and southern regions (Figure 25B) could be attributed to climatic cooling facilitated by the closing of the Isthmus of Panama and opening of the Bering Strait (Brierley and Fedorov 2016).

The Last Interglacial model (130 ka) showed high suitability habitats in the southern and western regions of North America, but low suitability in the far north and eastern regions (Figure 26). Bioclimatic variables highly contributing to the model included precipitation seasonality, isothermality, and mean annual temperature. Although regions in northern and eastern North America are predicted to be less suitable, it is uncertain what factors could be directly impacting this possible contraction in habitat suitability (Figure 26B). Global temperatures during the Last Interglacial period were approximately five degrees warmer than those seen today (Anderson et al. 2004). Recent studies suggest that precipitation seasonality during the Last Interglacial period was more variable, and that seasonal precipitation was lower in some regions of North America (Scussolini et al. 2019). The models produced by Scussolini et al. (2019) correspond with areas predicted to have lower suitability for Castor.

Castor distributions were highly restricted during the Last Glacial Maximum (21 ka), particularly in the northern latitudes across the continent (Figure 27). Bioclimatic variables highly contributing to the model included precipitation seasonality, isothermality, and mean annual temperature. Global temperatures during the Last Glacial Maximum were approximately four degrees cooler than today, with continental ice sheets covering much of the northern latitudes and decreased sea level (Otto-Bliesner et al. 2006). Northern latitudes significantly contract in habitat suitability due to the continental ice sheets extending further south into North America (Figure 27B). Habitats in southern latitudes, in Mexico and peninsular Florida, likely became more suitable as global temperatures cooled by, creating more suitable habitats in previously unsuitable areas of North America.

The future projection model (2081-2100) showed Castor canadensis distributed in similar ranges of suitability as today, with major restrictions of habitats in the north-central, south-central, and Great Lakes regions of the continent (Figure 28). The 2081-2100 model used a predicted middle range trajectory climate scenario (EC-Earth-Veg SSP3-7.0) to create bioclimatic variables used for the model. This means that distributions for C. canadensis could be predicted as drastically different for best- or worst-case climate scenarios. Northern latitudes, especially eastern and western portions of Canada and Alaska, were predicted to become more suitable (Figure 28). Southern regions, including northern peninsular Florida, north-central Mexico, and northern regions including the Great Lakes and northern Great Plains decreased in suitability (Figure 28). If beavers retreat from these areas of low suitability, that could cause catastrophic effects to water availability, water quality, erosion, and loss of biodiversity (Naiman et al. 1988; Rosell et al. 2005; Pollock et al. 2017; Touihri et al. 2018; Thompson et al. 2020).

Although model predictions do not extend back into the arrival of Castor into North America during the Miocene, Pliocene projections into the present can help us understand the expansion of Castor throughout the continent. Based on the resulting projection models, habitats in the late Miocene, although not mapped, might have been highly suitable across northwestern portions of North America due to warmer climatic conditions. Warming seen in the Miocene likely opened otherwise low suitable areas in Alaska and western North America, providing Castor with habitats suitable for their semiaquatic and dam building lifestyles. As climate cooled from the Miocene into the Pliocene, those corridors across the Bering land bridge likely became less suitable for Castor and pushed their distributions further south into North America.

The niche models for Castor distributions across North America showed distinct shifts in habitat suitability from the Pliocene to the present. Fossil occurrences of both Castor californicus and C. canadensis fell within suitable habitat ranges predicted in the distribution models (Figures 25A, 26A, and 27A). This suggests that the environmental requirements and distributions for C. californicus are like those of C. canadensis, as would be expected given the strong morphological similarity between the two taxa.

Castor distribution and habitat suitability expands and contracts throughout North America from the Pliocene to today. Pliocene distributions show fewer suitable habitats in the northern and central regions of the continent likely due to cooling, while southern regions become more favorable. Last Interglacial distributions show massive reductions in habitat suitability in the northern and eastern regions, likely from warming temperatures and changes in seasonal precipitation. Distribution in the Last Glacial Maximum show massive reduction in distribution in northern and central North America due to continental ice sheet extent. Fossil occurrences of Castor in North America fall within the high suitability regions projected by the models. This suggests that C. californicus and C. canadensis have similar habitat requirements and ecological needs, further asserting that the two species are analogs.

Comparison with C. canadensis
Overall, analyses employed here have documented strong morphological similarity between the late Miocene to early Pleistocene-aged beaver Castor californicus and the extant North American beaver C. canadensis. Both taxa show high degrees of variability in size, and substantial overlap in both skull (cranium and dentary) and postcranial size and shape. Subtle, but notable morphological differences in the skull of C. californicus include a wider occiput and posteriorly positioned orbit (Figure 15). Dentary morphology in C. californicus displays distinctly wider separation between condylar and angular processes and anteroventrally depressed incisor alveolus (Figure 15). Postcranial morphology of C. californicus is distinguished from extant species of beaver by less dorsoventrally flattened the femur, greater robustness of hindlimb elements, and greater metatarsal widths. These differences may be attributable to body size and allometry, but do not likely represent substantial differences in function of either the cranial or postcranial skeleton.

Overall, cranial, dentary, and postcranial morphology of Castor californicus has been shown to be highly similar to C. canadensis. The overall morphological similarities between C. canadensis and C. californicus likely indicate similarities in diet and locomotor ecology, which would highly suggest the two species are ecologically analogous. Notable cranial morphological differences in C. californicus include widened posterior cranium and posterior positioning of orbit. Dentary morphology in C. californicus was distinct with wider separation between condylar and articular processes and anteroventral position of incisor alveolus. Postcranial morphology of C. californicus had less dorsoventral flattening of the femur, increased hindlimb robustness, and increased metatarsal widths, representing some noticeable differences from extant species of Castor. These differences are likely a consequence of anagenetic changes in a species over several million years, with C. californicus being ancestral to C. canadensis.

Morphological Similarities
Castor canadensis shows high levels of variation in skull morphology. The geometric morphometric analysis all resulted in widespread distribution of the species within morphospace. It has been noted in previous literature that C. canadensis is highly variable (Stefen, 2009), as at one time it was separated into subspecies based on phenotypic characteristics and regional distribution across North America (Rhoads, 1898; Jenkins and Buscher, 1979; Long, 2000). Specimens of C. canadensis used in this study here were collected from across North America (Appendix 1 and Appendix 2); therefore, the resulting variation seen within the species is a good representation of the variation seen across the continent in the recent past and present. Some of the variation seen in C. canadensis may also be partially attributed to ontogenetic changes in skull shape within older adults (greater than 5 years old) after reaching sexual maturity (Segura et al., 2023), as has been previously noted with the sagittal crest of extant beavers (Hinze, 1950). Castor fiber may also have similarly high morphological variability in the skull, suggested by the wide separation of MVZ 19229 from other specimens across analyses, but limited sampling here precludes rigorous evaluation of that possibility. Prior work has also documented high intraspecific variability in the dentition of both C. canadensis and C. fiber (Stefen, 2009), with much of the variation attributable to ontogenetic changes.

Castor californicus consistently plots within the observed range of variation of C. canadensis across both relative warp and canonical variate analyses of the cranium (Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9). Within the classification stage of both the cranial and dentary CVA, C. californicus primarily categorized with C. canadensis (Table 10, Table 11, Table 12, Table 13, Table 14, Table 15, Table 16). Castor californicus clustered within C. canadensis in the cranial hierarchical cluster analysis (Figure 10), indicating strong, shared morphological similarities between the two species.

This suggests cranial morphological features are more similar in Castor canadensis and C. californicus than either is with C. fiber. Across relative warp analyses, C. fiber consistently plots separate from C. canadensis and C. californicus, highlighting the distinct morphological differences between extant beaver species (Troszyński, 1975; Flynn and Jacobs, 2008; Danilov et al., 2011; Kauhala and Timonen, 2016). Previous studies on the mitochondrial DNA of C. canadensis and C. fiber show that the two species last shared a common ancestor as early as 7.5 m.y.a. (Horn et al., 2011). This timing corresponds with the oldest known record of C. californicus in North America from the Rattlesnake Formation in Oregon (Samuels and Zancanella, 2011).

s figure15Cranial morphological similarities between Castor canadensis and C. californicus broadly include shorter nasals, wider occiput, and posteriorly positioned orbits when compared to C. fiber (Figure 15). Dentaries of C. canadensis and C. californicus both display anterior placement of the anterior mar gin of the pterygoid insertion and greater spread of the posterior processes (coronoid, condylar, angular) (Figure 15). These findings suggest both North American species have higher mechanical advantage and potentially larger pterygoid muscles than C. fiber, and a broader nuchal region which represents the insertion area of the head-stabilizing neck muscles.

Studied specimens of Castor californicus fit largely within the wide range of morphological variation seen in C. canadensis postcrania (Table 17). The postcranial analysis of C. fiber is limited due to inadequate sampling, precluding detailed comparisons to either North American species. The postcranial elements, which were measured, did document differences from those of C. canadensis, though not enough data were collected to confidently evaluate morphological differences between these species.

Castor canadensis and C. californicus show high levels of variation in postcranial morphology, as evidenced by coefficients of variation, which were also highly variable for both species even with sample size corrections. Previous studies of C. californicus described it as closely resembling the extant C. canadensis but larger in size (Stirton, 1935; Shotwell, 1970). High intraspecific variability in both taxa and substantial overlap in measurements suggests that differentiating these species based on size is not reliable. Other studies have found size is not generally a reliable metric for identifying and distinguishing species (Koch, 1986; Stefen, 2010; Emery-Wetherell and Davis, 2018; Martin et al., 2018). Given the degree of morphological variation observed in extant C. canadensis (data presented here and for dentition by Stefen, 2009), size alone should not be used to distinguish C. californicus from C. canadensis. Although certain postcranial features showed differences in range between C. californicus and C. canadensis, most C. californicus elements measured in the study fit within the observed range of variation for C. canadensis.

Morphological Distinctions
Castor canadensis and C. californicus show some morphological distinctions within the dentary and postcrania. Most specimens of C. californicus fell outside of the range of C. canadensis, in the relative warp analysis of the dentary (Figure 5). The hierarchical cluster analysis of the dentary showed greater separation between species, with most specimens of C. californicus forming the outgroup from C. canadensis and C. fiber (Figure 11), reflecting morphological differences from the extant species.

Semi-aquatic rodents exhibit a wide range of osteological specializations for their lifestyles (Howell, 1930; Gingerich, 2003; Samuels and Van Valkenburgh, 2008; Calede, 2022). Postcranial characteristics include shortening of the femur, robust limb elements, enlarged muscle attachment sites for the hind limb, and elongated hindfoot to aid in movement through the water (Samuels and Van Valkenburgh, 2008). These characteristics hold true for Castor canadensis and C. californicus (Table 17, Appendix 3, Figure 12 and Figure 13).

The femur anteroposterior diameter (FeAPD) in Castor canadensis is low, exhibiting an extreme flattening of the femur while C. californicus exhibits a more robust anteroposterior diameter than C. canadensis. The femoral epicondylar breadth (FeEB) is wider in C. californicus than in C. canadensis. A wider FeEB would allow for greater muscle attachments to help with swimming (Samuels and Van Valkenburgh, 2008) and would be expected in an animal of larger body mass. The anteroposterior and mediolateral diameters at the distal end of the tibia (TDEAPD and TDEMLD) are slightly wider in C. californicus than in C. canadensis, suggesting that C. californicus had more robust articular distal ends on the hindlimbs than C. canadensis. In the pes, the anteroposterior diameter of the third metatarsal (MT3APD) and mediolateral diameter of the fourth metatarsal (MT4MLD) are both more robust in C. californicus than C. canadensis. Increasing the size of the pes can aid in enlarging the surface area of the hindfoot for amplified propulsion through the water (Samuels and Van Valkenburgh, 2008), which would also be expected at larger body mass. The articular width at the distal end of the humerus (HDAW) is wider in C. californicus than C. canadensis, suggesting that C. californicus had more robust articular distal ends on the forelimbs than C. canadensis, which would accommodate larger size and may have allowed a wider range of motions used in both swimming and digging.

Taxonomy
Beavers (Family Castoridae) first appeared in North America during the late Eocene and from there dispersed into Eurasia (Korth 1994; Flynn and Jacobs 2008). The fossil record of beavers includes approximately 30 genera, with diverse lineages adapted for fossorial, terrestrial, and semiaquatic lifestyles (Martin and Bennett 1977; Martin 1989; Korth 1994; Rybczynski 2007; Samuels and Van Valkenburgh 2008; Samuels and Van Valkenburgh 2009). The semiaquatic lineage of beavers, consisting of both Castorinae and Castoroidinae, diversified in the Miocene (Rybczynski 2007; Rybczynski et al. 2010). The genus Castor likely appeared in the late Miocene, as represented by Castor neglectus from Germany (Hugueney 1999; Flynn and Jacobs 2008). Little is known about the dispersals of Castor between North America and Eurasia, though it is likely those migrations were facilitated using the Bering land bridge throughout the Cenozoic (Rybczynski 2007; Flynn and Jacobs 2008; Samuels and Zancanella 2011).

Castor has been present in North America since the late Miocene, around 7 million years ago (Samuels and Zancanella 2011). Castor californicus was first discovered in the Kettleman Hills in California (Kellogg 1911). Kellogg (1911) designated it as a separate species from C. canadensis based on upper 3rd molar (M3) dental features, including a greater anteroposterior diameter and three enamel folds (striations) along the outer tooth wall. Other specimens of C. californicus, including cranial, postcranial, and dental material, have been discovered and described in Miocene and Pliocene localities across the western United States, including Idaho, Oregon, and Nebraska (Zakrzewski 1969; Shotwell 1970; Kurten and Anderson 1980; Samuels and Zancanella 2011). Specimens described as Castor californicus show only subtle differences in tooth morphology from extant North American beavers (Kellogg 1911; Stirton 1935). A second species of fossil beaver in North America, Castor accessor, was initially described by Hay in 1927 and designated as a separate species based on differences in striae lengths compared to C. californicus and C. canadensis (Hay 1927; Kurten and Anderson 1980). Hay (1927) confined the species to the late Blancan through the late Irvingtonian. However, due to similarities between size and temporal distribution, C. accessor is generally combined with C. californicus (Stirton 1935; Flynn and Jacobs 2008).

Castor specimens from the Miocene and Pliocene of North America have been referred to as C. californicus, while those from the Pleistocene to recent are referred to the living species C. canadensis (Kurten and Anderson 1980; Flynn and Jacobs 2008). From the Miocene through the Pleistocene of North America, Castor seems to have gotten slightly smaller (Stirton 1935; Shotwell 1970), but otherwise changed little morphologically (Martin 1989; Samuels and Zancanella 2011). That raises the question of whether the two species are distinct or represent change in a single species over time.