Beavers (Castoridae) are semiaquatic rodents that modify forest and aquatic habitats by exploiting trees as a source of food and building material. The capacity of beavers to transform habitats has attracted interest from a variety of researchers, including ecologists, geomorphologists and evolutionary biologists. This study uses morphological and behavioral evidence from the fossil record to investigate the evolutionary history of tree-exploitation and swimming in beavers. The findings suggest that both behaviors appeared within a single castorid lineage by the beginning of the Miocene, roughly 24 million years ago. Biogeographic results support the hypothesis that tree-exploitation evolved at high latitudes, possibly influenced by the development of hard winters.
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I am enormously grateful to K. K. Smith and V. L. Roth (Duke University) for discussion and reading earlier drafts. I also thank A. Ballantyne, P. Baker, M. Cartmill, W. W. Hylander, D. Schmitt (Duke University), W. W. Korth (Rochester Institute of Vertebrate Paleontology, New York), W. McLellan (University of North Carolina, Wilmington) and C. R. Harington (Canadian Museum of Nature) for feedback during the preparation of this manuscript. I am indebted to the numerous museums who provided both kind hospitality and access to specimens. In particular I am grateful (in alphabetical order by institution) to: J. P. Alexander, R. Tedford, J. Meng (American Museum of Natural History, New York), K. Shepherd and M. Feuerstack (Canadian Museum of Nature, Ottawa), M. R. Dawson, A. Tabrum (Carnegie, Pittsburgh, USA), J. Agusti (Institut de Paleontologia “Miquel Crusafont”, Sabadell, Spain), D. Heinrich, R. Schoch (Museum für Naturkunde, Humboldt University, Berlin), P. Tassy, X. Filoreau (Museum National D’Histoire Naturelle, Paris, France), R.W. Purdy, D. Levin (National Museum of National History, Washington, DC), B. Engesser and staff (Natural History Museum, Basel, Switzerland), T. Engel (Naturhistorisches Museum Mainz), K. Heissig (Paläontologisches Museum München, Germany), G. Storch and T. Dahlmann (Senkenberg-Museum, Frankfurt, Germany), M. Hugueney (Université Claude Bernard Lyon 1, France), L. Martin, D. A. Burnham, K. Gobetz, and D. Maio (University of Kansas, Lawrence), M. Voorhies and G. Corner (University of Nebraska, Lincoln). This research was supported by funding from an Aleanne Webb Dissertation Improvement Grant (Duke University), Duke Travel Grant, Sigma-Xi Travel Grant, Polar Continental Shelf Project, and NSF Dissertation Improvement Grant IBN-0073119.
Appendix A: Sources of anatomical data
Abbreviations: AMNH, American Museum of Natural History; BHI, Black Hills Institute; BSP, Bavarian State collection of Paleontology and Geology (Paläontologisches Museum, München, Germany); CM, Carnegie Museum; CMN, Canadian Museum of Nature; FAM, Frick collections of the AMNH; GPIT, Institut und Museum fuer Geologie und Palaeontologie der Eberhard-Karls Universitat Tuebingen, (Germany); KUVP, University of Kansas Natural History Museum; IPS, Institute of Paleontology “Miquel Crusafont” at Sabadell (Catalonia, Spain); UNSM, University of Nebraska State Museum; USNM, National Museum of Natural History, Smithsonian; SDSM, South Dakota School of Mines and Technology.
Agnotocastor coloradensis (Wilson, 1949)
Partial skeleton: KUVP 10986; skull and partial skeleton: KUVP 14674, mandible and mandible fragments: KUVP 8999, 11001, 10985, 8233; skull: SDSM 6441.
Agnotocastor praetereadens (Stirton, 1935)
Skull: SDSM 6420; mandible: SDSM 40168.
Anchitheriomys fluminis (Mathew, 1918)
Skull: USNM 299914; mandible fragments: AMNH 17213 (type), 64020, 22068-A, FAM 64024A, UNSM 85495; isolated P4: UNSM 85496; tibia fragment: UNSM 85573.
Anchitheriomys tungerensis (Stirton, 1934)
Mandible fragment: AMNH 26538 (type); skeletal fragments: AMNH 26539.
Anchitheriomys wiedenmanni (Roger, 1898)
Mandible fragments: GPIT 24352.
Casts: UNSM 123611 C; unnumbered cheek teeth, calcaneum, astragali specimens from Stätzling locality (Southern Germany). Originals housed in Palaontologische Sammlung des Naturhistorischen Vereins in Augsburg.
Texas A& M University specimens: Skull: 2665; Skeletal fragments and upper cheek tooth; isolated teeth 2667-2669. Specimens located at UNSM.
Articulated skull and partial skeleton: S. D. 232-4435 at AMNH.
Skull: AMNH 65023.
Castor fiber (Linnaeus, 1758)
Naturhistorisches Museum, Basel. Skulls: 5736, 5739, 5765, 5763; skeletons: 9312, 1112, 3024.
Castoroides ohioensis (Foster, 1838)
Skull: USNM 1634.
Skull: CMN 51751; partial skulls: CMN 51752, 51753; mandibles and mandible fragments: CMN 51754, 51755, 51758, 51759, 51760, 51762; isolated lower incisors: CMN 51756, 51757, 51761; femur: CMN 51763; humerus: CMN 51764; innominate: CMN 51765; tibia+fibula: CMN 51766, 51767, 51768, 51769.
Dipoides stirtoni (Wilson, 1934)
Unless otherwise indicated all are uncatalogued UNSM specimens from Locality FT-40, Frontier Co., Nebraska. Skulls: 3064-49, 3241-49; isolated incisors: 5014-72, 1105-47; mandibles: 1167-47, 1407-47, 3138-49, 666-48; humeri: USNM 123592, 5127-73; femora: 5095-73, 3074-49, 612-48, 1558-47, 1410-47, 5039-72; tibiae: 3211-49, 3018-49; calcanei: 1558-47, 5149-73A, 3065-49, 4926-38, 2005-70; astragali: 1558-47; metatarsals III: 703-48, 1558-47, 1558-47, 1266-47; metatarsals IV: 1558-47, 709-48, 1266-47; caudals: 616-48, 1410-47, 1266-47, 1410-47 “c”, 3007-49, 1248-46.
Dipoides tanneri (Korth, 1998)
Skull, mandibles and associated postcranial elements: FAM 64482.
Literature source: (Korth, 1998).
Dipoides c.f. tanneri (Korth, 1998)
Skull: FAM 64398, 64406; skull fragments: FAM 64403, 64371, 64405, 64402; mandibles and mandible fragments: FAM 64426, 64439, 64456, 64424; humeri: FAM 64479, 64481; femora: FAM 64480-F; tibiae: FAM 64481, 64393.
Eucastor tortus (Leidy, 1858)
Skulls: USNM NE 81-570 (in UNSM collections), USNM 352579; skull fragments: UNSM 46672, BW 106, FAM 64885, 64934, 64925; mandibles: UNSM 56645, 56636, 56647, 56644, 56633, USNM 352584, 352580; postcranial elements: FAM 64865 A+B.
Literature source: (Wahlert, 1972).
Euhapsis breugerorum (Martin, 1987)
Skull, mandibles, humerus: KUVP 28376.
Euhapsis ellicotae (Martin, 1987)
Skull: KUVP 48015.
Euroxenomys minutum (Von Meyer, 1838)
Skull fragments: BSP 1926 V 25, 1931 V I32, 1881 IX 590, IPMC SA 11741, 11769; mandible fragments: BSP 1993 IX 566, IPMC - SA 117722, 11763, 11734, 11735, 11730, 11732, 11739, 11731, IPMC - Castell de Barberà 1, 4, 8, 11, 13, 14; calcanei: IPS SA 11731; astragali: IPS SA 11730, BSP 1926 V 503, 1926 V 39.
Eutypomys thomsoni: (Mathew, 1905)
Associated cranial and postcranial elements: AMNH 12255, FAM 64002, 64003; partial skulls: FAM 12254 (type, includes mandibles), FAM 64996, 64002, 64006, 64008, 64007, 65296, 65297, 94996, AMNH 98531; mandible fragments: FAM 64005, 64009, 65300, AMNH, 94080, 65300, AMNH 1939; BSP 1981 I 27.
Fossorcastor greeni (Martin, 1987)
Skull and mandibles: KUVP 80845 (type).
Hystricops venustus (Leidy, 1858)
Skull: FAM 64044, 65032; mandible: FAM 64047.
Migmacastor procumbodens (Korth and Rybczynski, 2002)
Skull and mandibles: FAM 65723.
Palaeocastor fossor (Peterson, 1905)
Skull and associated postcrania: KUVP 28383, CM 11192, 1208, 1204, 1217 (type), FAM 65022; skulls and partial skulls: CM 1207, 1212, 1216, FAM, 64214, 64217, 65022, 64231, 64216; mandibles: FAM 64234; humerus: AMNH 81532; tibia and foot elements: KUVP 28390.
Palaeocastor magnus (Romer and McCormick, 1982)
Skulls: KUVP 28386, 28380, 29388.
Palaeocastor c. f. nebrascensis (Leidy, 1856)
Skull and associated postcranial elements: FAM 64153B; skulls and partial skulls: AMNH 55545, 21461, FAM 64127, 64158-A, 64224, 64223, 64222, 64167, 64122, 65722; mandibles and mandible fragments: FAM 64173, 64146, 64124, 64151, 64184.
Palaeocastor peninsulatus (Cope, 1881)
Skull: AMNH 6998 (type); mandible, calcaneum and astragalus: AMNH 7006.
Palaeocastor c.f. simplicidens (Mathew, 1907)
Skull with associated nearly complete skeleton: UNSM 2-17-7-34.
Palaeocastor wahlerti (Korth, 2001)
Skull: SDSM 40123.
Skull and associated complete skeleton: UNSM 1-17-6-33.
Paramys copei (Loomis, 1907)
Paramys delicatus (Leidy, 1871)
Priusaulax browni (Korth and Bailey, in press)
Skull: UNSM 119707; mandibles: UNSM 119711 (type), 119708, 119709, 119710.
Procastoroides idahoensis (Shotwell, 1970)
Literature source: (Shotwell, 1970).
Pseudopalaeocastor barbouri (Martin, 1987)
Skull and partial articulated skeleton: CM 1210 (type); skull: KUVP 48018; skull fragments: KUVP 49442; associated skeletal elements: KUVP 48025.
Pseudotomus robustus (Marsh, 1872)
Literature source: (Wood, 1962).
Sinocastor anderssoni (Schlosser, 1924)
Skull: FAM 64074, uncatalogued AMNH specimen from Ma-Chia-Liang-Kou, northwest Sanxi, China-42-L306; skull fragments: FAM 64069, 44070, 64070.
Steneofiber depereti (Mayet, 1908)
Museum National d’Histoire Naturelle, Paris. Locality abbreviation: Burdigalier Moyen Arteney, Ar. Skull: Ar 2529, 2830; humeri: Ar 3136, 3169; innominates: Ar 1584, 6001, 6004; femur: Ar 1584; tibiae: Ar 3182, 3183, 3188; calcanei: Ar 3224, 3226; astragalus: Ar 3233.
Steneofiber eseri (Von Meyer, 1864)
Museum National d’Histoire Naturelle. Locality abbreviation: Saint-Gerand le Puy, SG.
Skull and skull fragments: SG 3654, 3656, 3657, 3658, 3661, 3662, 3663, 3664, 3665, 3668, 3670, 3691, 3693, 3698, 3817, 13255; isolated bullas: SG 3834, 3835, 3836, 3867, 3868, 3869, 3870, 3871, 3872; isolated upper incisors: SG 4104, 4105; mandibles: SG 3699, 3700, 3712, 3741, 3747, 3805, 9332, 9336,13265; isolated lower incisors: SG 4092; 4094 scapula: 4115; humeri: SG 4133, 4135, 4143, 4149, 4151, 4188; ulnas: SG 12393, 13287; innominate: SG 4242; femora: SG 3782, 3963, 3982, 6637, 6638, 9427; tibias: SG 13401, 13403: astragalus: SG 9452; calcanei: SG 12396, 12397, 4268, 4269; third metatarsal: SG 9866; 4th metatarsals: SG 9862, 13303.
Naturhistorisches Museum Basel. Locality abbreviations: Laugnac (Lot-et-Garonne), CG; Montaigu-le-Blin, MA; Phryganid limestone from Montaigu, Ph;. Saint-Gérand-le-Puy, SG; Saulcet, Sau.
skull fragments: Sau 6, 794, SG 12248, 12249, 12250, 23251, 23254; MA 808, 810, 807, 2774; isolated upper incisors: Ph 2923, SG 7847, 18655; isolated bulla: SG 17358; mandibles: CG 190, SG 1780, 2775, 7843, 7844, 12251, 21747, Sau 797, 798, 799; humeri: MA, 843, 844, 847,1785, Ph 1360, SG 3, 7852, 10689, 12265, 18661, 20883, 21753; ulnae: MA 854, 2790, SG 12267, 18668, 18669, 23263, 23264; innominates: MA 4997; SG 17777, 4997, 18660, 20876, 20879, 20880; femurs: SG 5, 7854, 17317, 18676, 21764, 23258; tibia: SG 7855; astragali: MA 833, 884, SG 6824, 6826, 17262; calcanei: SG 6825, 7797, 18698, 77894; 3rd metatarsals: MA 750, 752, 753, 887, 893, 4994, 4995, Ph 2380, 2382, 2925, SG 10921, 10920, 10926, 12290, 12294, 12295, 15559, 18685, 18686, 18687, 18688, 18690, 18697, 23273; 4th metatarsals: MA: 751, 754, 886, 888, Ph 2383, 2385, 2386, 2926, SG 10922, 10923, 10924, 10925, 10929, 10930, 10931, 12288, 12291, 12292, 12293, 12297, 12298, 17330, 19439, 23271, 23272, 23276; 5th metatarsals: MA 756, SG 10927, 15945, 12300, 18696, 23276; cuboid: MA 903; caudals: MA 224, 816, 819, 822, 827, 865, SG 7857, 12284, 12285, 12887, 15560, 18587, 20887, 21772.
Université Claude-Bernard, Lyon I (France).
Partial skeletons: 4405 from Saulcet, 4416 from St. Gerand-le-Puy, 4409, 4410, 4412, 4414 plus unnumbered specimens from Chavroches, FSL 97870 to 97874 and unnumbered specimens from Montaigu-le-Blin; Skull and mandibles: 4411 from Chavroches; Skull: 4406 from Saulcet; vertebrae: 4417 from Chavroches.
Trogontherium cuvieri (Fischer, 1809)
Forschungsinst Senchenberg (Frankfurt, Germany). Mandible fragements and isolated lower teeth: Säugetière II 73/284, 293-299, 292.
Museum fur Naturkunde (Humboldt University, Berlin). Tegelen locality. Skull fragments: MB. Ma 4985, 4868, 5006, 4825, 5015, 4868, 6693, 6691; mandible fragments: MB. Ma 4849, 4938, 4858, 4833, 4832, 4854, 4851; humerus: MB. Ma 4720; femur: MB. Ma 4685; tibia+fibula: MB Ma 5087; calcaneum: MB. Ma 4714; metatarsal IV: MB. Ma 4677.
Naturhistorisches Museum Mainz. Skull fragments: 1961/888, 1955/533, 1962/430, 1955/531, 1955/997, 1961/885, 1954/584; mandible fragments: 1959/143, 1955/1086, 1960/205, 1957/668, 1969/65, 1956/682, 1955/851, 1961/175, 1959/493, 1967/1730, 1957/746, 1959/493, 1961/175, 1964/165, 1957/747; tibia: 1952/ no #, 1959/84, 1955/57, 1961/389, 1956/437, 1954/254, 1961/564, 1956/914; tibia: 1959/84, calcanei: 1959/178, 1957/703, 1962/1325; astragali: 1955/779, 1957/388, 1956/656, 1961/545; metatarsal IV: 1954/58; metatarsal V: 1958/425.
Skull Cast: UNSM 123610C, original from Forest Bed of East Runton, near Cromer (UK) Original in possession of the collector A. Savin of Cromer. See Schredude (1929, plate II, Fig. 4).
Literature sources: (Schreuder, 1929, 1951)
“New Taxon A”
Skull, mandibles and partial skeleton: KUVP 125061
Literature sources: Gobetz and Martin, pers. comm.
Appendix B: Character definitions
Nomenclature for cranial foramina follows Wahlert (1974). Upper teeth are denoted by upper-case letters; lower teeth are denoted by lower-case letters. Upper and lower premolars are designated “P” and “p” respectively, whereas upper and lower molars are designated “M” and “m” respectively. In addition, tooth designations are numbered from front to back. For example, “M1” is the first upper molar. Dental nomenclature from Stirton (1935).
M1 in occlusal view: (0) width greater than length; (1) width less than or equal to length.
Rostrum cross-sectional shape, height/width: (0) less than 1; (1) greater than, or equal to 1, but less than 1.15; (2) greater than or equal to 1.15. Rostral dimensions are measured at anterior limit of premaxillary-maxillary suture.
Anterior edge of nasal relative to that of premaxilla: (0) anterior; (1) level; (2) posterior. Skull is viewed in lateral view and is oriented so that the cheek-tooth rows are horizontal.
Width of nasals divided by width of rostrum: (0) less than 0.6; (1) greater than or equal to 0.6. Rostral width taken at the dorsal limit of the incisor, within the maxilla. The dorsal limit of the incisor can be readily identified because the lateral surface of the maxillary bone bulges where it overlies the incisor. The nasal width measurement is taken at the same position along the rostrum.
Lateral margin of nasals in dorsal view: (0) nearly straight or only slightly convex; (1) posteriorly, margin is straight or slightly concave, and anteriorly it is convex; (2) posteriorly, margin is straight and angled relative to midline, anterior margin is straight and parallel to midline; (3) strongly convex.
Position of posterior limit of premaxilla-maxilla suture in relation to posterior limit of incisive foramina: (0) posterior or level; (1) anterior. Character modified from Martin (1987).
Length of incisive foramina divided by diastema length: (0) greater or equal to 0.24; (1) between 0.15 and 0.24; (2) less than or equal to 0.15. Diastema length is the parasagittal distance from the anterior margin of the cheek tooth alveolus to the posterior margin of the incisal alveolus. Character modified from Korth (2002).
Posterior half of incisive foramen. Vertical position of lateral border of foramen in relation to midline ridge: (0) ventral; (1) level; (2) dorsal. Character modified from Martin (1987).
Muscle scar indicating presence of antorbital portion of deep masseter: (0) absent; (1) present, depression not pronounced; (2) present, depression confined to maxilla, anterior limit of muscle scar might be marked by a low, rounded ridge; (3) Present, depression largely located in maxilla, but also formed in the premaxilla. Anterior limit of depression marked by a sharp ridge.
Anterior margin of infraorbital foramen in lateral view: (0) concave; (1) straight and perpendicular to toothrow; (2) angled so foramen opens somewhat dorsally (see Martin, 1987); (3) angled so foramen opens somewhat ventrally (see Xu, 1994, Fig. 3). Modified from Xu (1995) and Martin (1987).
Origin of superficial masseter: (0) located on zygomatic arch; (1) located ventral to infraorbital foramen, with tendon scar facing predominantly ventrally; (2) located on tubercle that is ventral to infraorbital foramen, with tendon scar roughly parasagittal; (3) dorsal edge of tendon scar is lateral to infraorbital foramen and scar facing ventrolaterally; (4) located on tubercle ventral to infraorbital foramen with tendon scar facing predominantly ventrally.
Anterolateral border of orbit: (0) formed by jugal with small jugal-lacrimal contact and large maxilla-lacrimal contact; (1) formed by jugal with large jugal-lacrimal contact and small, or absent, maxilla-lacrimal contact; (2) formed mostly by jugal with no jugal-lacrimal contact; (3) formed mostly by maxilla; (4) formed equally by maxilla and jugal; (5) jugal-maxilla suture appear to be fused dorsally. Character is modified from Martin (1987) and Korth (2002). The suture pattern in the antero-lateral border of the orbit of some castorid individuals can be very difficult to interpret. In at least one Steneofiber eseri specimen (France, Lyon: 4411), and a “Palaeocastor” specimen from the University of Nebraska collections (UNSM 81361), the presence of supernumerary sutures in the antorbital region make it difficult to identify the boundaries of the contributing elements. Possibly the supernumerary sutures seen in castorids are divisions of the jugal. A similar condition is seen in some modern humans (Schwartz, 1995).
Shape of foramen formed in choanal roof between presphenoid and palatine: (0) oval; (1); thin slit; (2) absent.
Anterolateral eminence on bulla: (0) absent; (1) present. In lateral view, the anterior eminence occurs at the anterolateral edge of the bulla. The eminence is sub-oval in shape and partially encircled by a groove. It occurs on the lateral surface anterior to the acoustic meatus and is variably sized, ranging from roughly 1/4 to 1/9th of the bulla’s lateral surface. See Wagner (1983, Fig. 2B).
Jugal thickness divided by jugal height: (0) greater than or equal to 0.31; (1) less than 0.31. Jugal height is a maximum, measured perpendicular to tooth row, in the plane of the lateral surface of the zygoma. Jugal thickness (mediolateral) is measured at the ventral margin of the zygomatic arch, immediately posterior to the jugal-maxilla contact.
Maxilla in palatal view, medial to cheek teeth: (0) flat, or slight midline ridge; (1) surface recessed, weak or rounded midline ridge; (2) surface recessed, prominant midline ridge; (3) deep, slit-like, lateral groove and rounded midline ridge.
Maxilla in palatal view, anterior to cheek teeth: (0) roughly flat with lateral, parasagittally oriented ridge; (1) with midline ridge and lateral ridge (lateral ridge usually weak); (2) with midline groove, possibly a slight midline ridge formed within the groove. This character is modified from Korth (2002). The appearance of character state 1 is highly variable within taxa. For example within Castor the lateral ridge can be sharp and well-defined, or relatively weakly developed. Character state 2 is associated with elongated upper incisors. The base of an elongated upper incisor appears just anterior to the upper cheek teeth and is demarcated on the skull surface by the presence of low “capsular processes”. See Fig. 8A, E in Martin (1987). Elongated upper incisors appear to characterize tooth-digging taxa.
Maxilla-alisphenoid contact: (0) absent; (1) anterior limit located posterior to M3; (2) anterior limit located dorsal to M3; (3) anterior limit located dorsal, or dorsal and anterior to, M2-M3 junction. Character modified from Wahlert (1972).
Posterior maxillary foramen/notchborder composition: (0) between maxilla and palatine; (1) formed mostly within palatine; (2) formed entirely within palatine.
Ventral surface of palatine: (0) flat, or slightly recessed, possibly with a rounded midline ridge; (1) deeply recessed with a sharp midline ridge; (2) lateral, parasagittal, slit-like groove. Character modified from Korth (2002).
Horizontal location of palatine’s posterior margin: (0) posterior to M3; (1) level with posterior margin of M3; (2) level with M3; (3) level with M2/M3 junction.
Location of posterior palatine foramen: (0) palatine bone; (1) palatine-maxilla suture. Character from Wahlert (1972).
Location of sphenopalatine foramen: (0) between orbital process of maxilla and orbital process of palatine; (1) within maxilla. Character from Wahlert (1972).
Location of ethmoid foramen: (0) frontal; (1) frontal-orbitosphenoid suture. Character from Wahlert (1972).
Location of dorsal palatine foramen: (0) palatine; (1) maxilla-palatine suture; (2) maxilla-orbitosphenoid suture; (3) maxilla; (4) maxilla-alisphenoid suture; (5) absent. Character from Wahlert (1972).
Interorbital foramen(ina) posterior to optic foramen: (0) absent; (1) present. The presence of one or more foramina posterior to the optic foramen is considered to be a shared-derived trait of Eutypomys and Castoridae (Wahlert, 1972).
Sphenofrontal foramen: (0) present; (1) absent. Character from Wahlert (1972).
Shape of anterior margin sphenoidal fissure in lateral view: (0) anterior margin is concave; (1) anterior margin is nearly straight, and is roughly vertical or slopes so that the fissure opens dorsally.
Masticatory and buccinator foramina: (0) separate; (1) conjoint. Character modified from Wahlert (1972).
Foramen ovale accesorius: (0) present, forming a closed foramen; (1) present, but foramen not fully enclosed; (2) absent. Character modified from Wahlert (1972).
Middle lacerate foramen: (0) absent, or obscured by bulla; (1) present and separate from foramen ovale; (2) present and confluent with foramen ovale. Character modified from Wahlert (1972).
Posterior alar fissure: (0) absent, large alisphenoid-bulla contact present; (1) present, and forming a closed foramen; (2) present, and fissure open ventrally. In character state “1” a foramen occurs between the alisphenoid wing and bulla. There is a dorsal and ventral alisphenoid contact. In character state ‘2’ a small alisphenoid-bullar contact might occur dorsally but the alishpenoid-bulla contact is absent ventrally.
Sculpting (rugosities) on dorsal surface of parietals: (0) absent; (1) present. In Castor sculpting is most pronounced in adult specimens, consequently, this character was evaluated only in adult specimens.
Interparietal: (0) present; (1) absent or fused.
Sagittal crest(s): (0) single, or closely opposed; (1) double for length of braincase; (2) double in parietals, but single in interparietal region (= lyrate shaped). Because the formation of the sagittal crest varies ontogenetically (see Voss, 1988, Fig. 15) this character was evaluated in adult specimens only.
Ventral projection of squamosal separating external auditory meatus from mastoid: (0) absent; (1) present. Character from Martin (1987).
Mandibular glenoid (squamosal) extent: (0) medial but not posterior to jugal; (1) medial and posterior to jugal with lateral margin of posterior region angled posteriorly toward midline; (2) medial and posterior to jugal with lateral margin of posterior region angled parallel to midline. Character modified from Martin (1987).
Temporal foramen(ina): (0) single; (1) multiple. Character from Wahlert (1972)
Auditory tube: (0) absent; (1) present.
Auditory bulla in ventral view with medial process: (0) absent or forming only a very small bump; (1) present, forming a rugose flange that projects anteriorly; (2) present, forming a rugose flange that projects antero-ventrally.
Evidence for the presence of a stapedial artery: (0) present; (1) absent. Character from Wahlert (1972).
Basioccipital ventral surface: (0) with a midline ridge; (1) deep, sub-rectangular recess occupying entire space between bulla, flat dorsally; (2) deeply recessed anteriorly, midline bump located posteriorly.
Angle of occiput: (0) approximately perpendicular or sloping anteriorly up to 10°; (1) sloping anteriorly up to 10°–20°; (2) sloping anteriorly more than 20°. Character modified from (Martin, 1987). Skull viewed in lateral view with cheek teeth horizontal. Condyles were not considered in determination of occiput slope.
“Squamoso-mastoid” foramen: (0) absent; (1) present. The “squamoso-mastoid” foramen is a small foramen, visible in occipital view, and is located at the lateral margin of the mastoid.
Upper incisor shape. Anterior surface of incisor; blade edge in anterior view: (0) convex; convex; (1) convex; straight; (2) flat; straight. Modified from Stirton (1935).
Upper incisor cross-sectional shape. Incisor width divided by length: (0) less than or equal to 0.8; (1) between 0.8 and 1; (2) greater than or equal to 1.
Upper incisor enamel: (0) smooth; (1) grooved; (2) faintly grooved or rugose. Character from Korth (2002). A small number of Castor canadensis individuals exhibit weakly grooved incisal enamel (pers. obs.).
Upper third premolar: (0) present; (1) absent. From Korth (2002). Only adult dentitions are considered. A deciduous P3 (DP3) has been reported to be present in Anchitheriomys (Voorhies, 1990) and Neatocastor (Korth, 1996). In Neatocastor described in Korth (1996) the DP3 appears to have been lost during subsequent preparation. A deciduous p3 has also been reported in some rare specimens of Castor. A study of a population of C. canadensis found Dp3 to appear at a rate of about 2%. In all cases the extra tooth appeared in very young individuals, suggesting that they were lost by adulthood (Pilleri, 1983).
Distance between tooth rows anteriorly divided by the distance between tooth rows posteriorly: (0) greater than or equal to 0.92; (1) between 0.92 and 0.7; (2) less than or equal to 0.7. Distance between toothrows anteriorly measured at midpoint of the anterior rim of the left and right P4 alveoli. Distance between toothrows posteriorly measured at midpoint of the posterior rim of M3 alveoli. Modified from Korth (2002).
Comparison of M2 and M3, mesiodistal (= anteroposterior) dimensions: (0) similar, or M2 slightly greater than M3; (1) M2 less than M3. Measurement taken along occlusal surface of the tooth. Only adult specimens are considered. Character modified from Korth (2002). Also see Hugueney (1999).
Molar shape: (0) brachydont, with distinct, convex-sided crowns, multiple roots visible near alveolar rim; (1) No flexus/id, multiple fossettes on occlusal surface; (2) hypoflexus/id present, multiple fossettes on occlusal surface, mesoflexus/id may be present but is not persistent (i.e., stria/id associated with mesoflexus/id is very short); (3) “Castor occlusal pattern” (see below), hyposodont with three roots; (4) “Castor occlusal pattern” (see below), very hypsodont and with reduced number of roots; (5) “Dipoides occlusal pattern” (see below), hypsodont, but closed rooted; (6) “Dipoides occlusal pattern” (see below), hypseledont (i.e., open-rooted). This character refers to the first and second molars of both the upper and lower molar teeth. Teeth which were heavily worn or newly erupted were not considered. In the “Castor occlusal pattern” the hypoflexus/id and mesoflexus/id are present and persistent, and there is also a large parafossette/id or paraflexus/id, and metafossete/id or metaflexus/id. In the “Dipoides occlusal pattern” the only major enamel features are a persistent hypoflexus/id and mesoflexus/id. In Dipoides these form a distinctive “S-pattern” on the surface of the tooth. Modified from Xu (1995) and Korth (2002).
Lower incisor shape. Anterior surface of incisor; blade edge in anterior view: (0) convex; convex; (1) semi-flat; convex, (2) semi-flat; straight; (3) flat; straight. Character modified from (Stirton, 1935). “Semi-flat” lower incisors are flat near the medial edge of tooth but curved toward lateral edge.
Lower incisor cross-sectional shape. Incisor width divided by length: (0) less than or equal to 0.7; (1) between 0.7 and 1; (2) greater than or equal to 1.
Lower incisor interstitial wear facet: (0) present; (1) absent. Interstitial wear near the apex of the incisor indicates symphyseal mobility.
Vertical position relative to tooth row of mandibular diastema; of posterior rim of lower incisor alveolus: (0) level; level or above; (1) below; level or above (2) below; below. Modified from Korth (2002, p. 274). Mandible viewed in lateral view.
Digastric process in lateral view: (0) absent; (1) small, barely visible in lateral view; (2) or well developed.
Horizontal position of digastric process relative to anterior margin of p4: (0) Anterior; (1) Level; (2) Posterior. Mandible viewed in lateral view with tooth row oriented horizontally.
Fossa located near anterior limit of pterygoid fossa: (0) absent, or slight; (1) present and well-defined. Character, as it appears in Trogontherium, is described in Schreuder (1929, plate V, Figs. 9–10).
Ascending ramus of mandible in lateral view obscures: (0) part of m3; (1) m3 and up to half of m2; (2) m3 and more than half of m2, but not m1; (3) m3, m2 and part or all of m1. Note the mandible is viewed in true lateral view.
Line drawn from posterior tip of coronoid to posterior tip of angle, in posterior view: (0) located just medial to, but contacting or, nearly intersecting condyle; (1) located at least1/2 a condyle-width lateral to medial edge of condyle. Character modified from Xu (1995) and Korth (2002).
Mandibular condyle width divided by length: (0) less than, or equal to 0.61; (1) between 0.61 and 0.75; (2) greater than, or equal to 0.75.
Base of lower incisor, demarcated on the lateral mandibular surface by a distinctive bump (= “capsular process”): (0) present; (1) very small. In some taxa the capsular process is enlarged to form a protuberance that is readily visible in anterior or posterior view and appears to be a characteristic of tooth-digging rodents (Rinaldi and Cole, 2000).
Location of base of capsular process relative to condyle: (0) below and anterior; (1) below; (2) below and posterior. Mandible viewed in lateral view with tooth row oriented horizontally.
Shape of mandibular angle in lateral view: (0) rounded; (1) pointed. If anterodorsal point of angle forms an angle of over 90°, it is considered rounded.
Location of mandibular angle dorsal-margin relative to toothrow: (0) below; (1) approximately level; (2) above.
Width of acromiom process of scapula relative to spinous process: (0) less than or equal to; (2) greater.
Entepicondylar foramen of humerus: (0) absent; (1) present.
Shape of metacarpal III. Proximal end in dorsal view of articulated manus: (0) roughly symmetrical; (1) lateral process is much larger.
Shape of metacarpal V articulation for metacarpal IV and hamate: (0) angular; (1) globular. That latter permits a greater range of movement.
Cross-sectional shape of iliac blade. Development of crista lateralis; Size of dorsal (gluteal) fossa relative to ventral (iliacus) fossa: (0) weak; similar (cross-sectional shape roughly triangular); (1) strong, similar (cross-sectional shape T-shaped); (2) strong; greater. The appearance of this character in Castor and Trogontherium is described in Schreuder (1929, p. 221, Fig. 9).
Femur length/tibia length ratio: (0) greater than or equal to 1; (1) between 0.9 and 1; (2) less than or equal to 0.9.
Cross-sectional shape of femur below third trochanter. Anteroposterior distance/mediolateral distance: (0) greater than or equal to 0.8; (1) between 0.52 and 0.8; (2) less than 0.52.
Proximal shaft of tibia. Depth of lateral fossa relative to posterior fossa: (0) lateral deeper; (1) posterior deeper, (2) similar. The lateral fossa and posterior fossa serve as attachment for muscles that dorsiflex foot, and plantarflex the foot, respectively.
Distal ends of fibula and tibia: (0) unfused; (1) fused.
Shape of calcaneal lateral surface. Laterally projecting process; facet for fibula: (0) present; absent; (1) absent; absent; (2) absent, present. See Schreuder (1929, p. 215–222) for discussion of this character in Trogontherium.
Shape of calcaneoastragalus facet. Parasagittal length/mediolateral width: (0) greater than 1.5; (1) between or equal to 0.7 and 1.5; (2) less than 0.7.
Sustentacular facet shape: (0) round or sub-round; (1) parasagittally elongated and is not continuous with cuboid facet; (2) parasagittally elongated and is continuous with cuboid facet.
Angle between long axis of calcaneal tuber cross-sectional shape and sustentacular facet: (0) roughly 90°; (1) less than 70°.
Astragalus. Location of posterior limit of navicular facet in relation to astragalar trochlea: (0) anterior; (1) level; (2) posterior. Astragalus examined in lateral view with the astragalar neck oriented horizontally.
Astragalus. Relationship of sustentacular facet and navicular articulation: (0) distinct and separate; (1) confluent; (2) distinct, but in contact.
Length of metatarsal III relative to metatarsal IV: (0) metatarsal III longer; (1) metatarsal IV longer.
Midshaft width of metatarsal III relative to metatarsal IV: (0) metatarsal III greater; (1) metatarsal IV greater; (2) Similar (within 10%).
Shape of articular surface on metatarsal IV for metatarsal V: (0) angular, restricting mobility of metatarsal V; (1) hinge-like, permitting mediolateral movement of metatarsal V.
Metatarsal V and cuboid: (0) metatarsal V articulates with lateral surface of cuboid; (1) no contact.
Distal caudal vertebrae. Shape of centra: (0) round or mediolaterally compressed; (1) dorsoventrally compressed.
Shape of transverse process of distal caudal vertebrae: (0) small/absent; (1) expanded. Character state “1” includes a range of morphologies, for example, the transverse process might possess a foramen, or be bifurcated.
Appendix C: Data matrix used in cladistic analysis
For a given taxon, the appearance of multiple character states in bold and underlined indicates the presence of character state uncertainty. Multiple character states indicated in bold, and not underlined, are polymorphic. Abbreviations: simplicid., simplicidens; nebrasc., nebrascensis.
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Rybczynski, N. Castorid Phylogenetics: Implications for the Evolution of Swimming and Tree-Exploitation in Beavers. J Mammal Evol 14, 1–35 (2007). https://doi.org/10.1007/s10914-006-9017-3
- Behavioral evolution