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Sceloporus dixoni BRYSON & GRUMMER, 2021

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Higher TaxaPhrynosomatidae, Sceloporinae, Iguania, Sauria, Squamata (lizards)
Common Names 
SynonymSceloporus dixoni BRYSON & GRUMMER 2021
Sceloporus aeneus – DUELLMAN 1965 (in part)
Sceloporus aeneus – THOMAS & DIXON 1976 (in part)
Sceloporus aeneus – BENABIB et al. 1997 (in part)
Sceloporus aeneus aeneus – SMITH 1937 (in part)
Sceloporus aeneus aeneus – SMITH 1939 (in part)
Sceloporus aeneus aeneus – SCHMIDT & SHANNON 1947
Sceloporus aeneus aeneus – DUELLMAN 1961 (in part)
Sceloporus aeneus aeneus – MINK et al. 1996 (in part)
Sceloporus aeneus subniger – SMITH et al. 1993 (in part)
Sceloporus aeneus subniger – BRYSON et al. 2012 (in part)
Sceloporus subniger – GRUMMER et al. 2014 (in part) 
DistributionMexico (Jalisco)

Type locality: Nevado de Colima, 13.5 mi W Cd. Guzmán, municipality of San Gabriel, Jalisco (N 19.6427°, W 103.6236°, 2375 m; WGS84);  
TypesHolotype: UTA 61714, adult male, (field number RWB 0649), collected 24 June 2006 by R. W. Bryson Jr.
Paratypes: MZFC 22053, 22054; UTA 61713, 61715–61716). Michoacán: 11.7 mi W Zacapu on rd to Zamora (MZFC 22055, 22056; UTA 61699–61702). 22 km N Uruapan on Hwy 37 (UTA 61703, 61704), same data as holotype. 
DiagnosisDiagnosis: Sceloporus dixoni is a member of the S. scalaris group, sharing with other species in this group parallel lateral scale rows, femoral pore series that are either in contact or separated by no more than two scales, females with smooth preanal scales, and males with lateral abdominal color patches (Smith 1939; Smith et al. 1997; Watkins-Cowell et al. 2006). Sceloporus dixoni can be distinguished from other species in this group by the following combination of characters: single canthal on each side of the head, small adult size (maximum SVL = 54 mm, average 47.1 mm), 37–45 dorsal scales (average 41), 37–43 scales around midbody (average 40), 32–39 ventral scales (average 35), tibia length/head length proportion of 0.76–0.94 (average 0.86), 4–5 supralabial scales (mode of 5), 12–18 scales bordering the interpariatel scale (average 15), 31–36 femoral pores in males (average 34), adult females with lightly mottled venters, and adult males with extensive dark pigment on the venter, heavily mottled throats, and orange or rust-colored flanks (BRYSON & GRUMMER 2021).

Variation. Variation in meristic and mensural characters of male and female paratypes is summarized in Tables 3–4. All males have heavily mottled throats; in several, the mottling is so dense that the ventral surface of the head appears almost entirely black, as seen in the holotype. Ventral surfaces of males are similarly dark in preservative; in some, a pale-colored patch extends midventrally from about the intersection of the hindlimbs towards the front limbs. This lighter-colored section of the venter is especially evident in life, as seen in Fig. 5. Also noticeable in this image are the lateral blue patches on the venter and orange-red color of the flanks of males. In preservative, the ventral surface darkens considerably, presumably due to fixation in formalin. The dorsal surface of males ranges from weakly patterned to patternless. When patterned, the dorsal surface is marked by a pair of light-colored dorsolateral stripes, one-scale wide, that originates at the posterior margin of ear opening and extends onto the tail. A pale vertebral line, two scale-rows wide, is also present, beginning at the nape of the neck and extending posteriorly to tail. The region between the vertebral and dorsolateral stripe is marked with narrow, dark brown transverse bars on each side; in many individuals, these bars are dimly evident. Females possess lightly mottled throats, some with more mottling than others. The ventral surface of females is very lightly mottled. The dorsal surface of females ranges from strongly patterned to patternless. In strongly patterned individuals, dark transverse bars are sharply defined, often edged posteriorly by white (BRYSON & GRUMMER 2021).

Comparisons. Sceloporus dixoni is most similar to S. subniger and specimens from the Sierra de Mascota in western Jalisco, sharing with them a single canthal on each side of the head, relatively short legs (average tibia length/head length proportion less than 0.9), small adult size (maximum SVL less than 63 mm), 36–50 dorsal scales, extensive dark pigment on the venter of adult males, a black-barred or darkly mottled chin/throat in adult males, orange or rust-colored flanks in adult males, and oviparity. Sceloporus dixoni can be distinguished from S. subniger by the combination of its smaller adult size (maximum SVL = 54 mm in S. dixoni vs. 62 mm in S. subniger; average SVL = 47.1 mm vs. 48.6 mm), longer legs (average tibia length/head length proportion 0.86 vs. 0.83), fewer femoral pores in males (maximum of 36 vs. 40; average number 34 vs. 35), fewer scales around midbody (average of 40 vs. 41), more supralabial scales (mode of 5 vs. 4), and fewer scales bordering the interpariatel scale (average of 15 vs. 16). Female S. dixoni also have considerably less mottling on the ventral surface than female S. subniger. Sceloporus dixoni differs from specimens from the Sierra de Mascota in western Jalisco by the combination of their larger adult size (maximum SVL = 54 mm in S. dixoni vs. 47 mm in specimens from the Sierra de Mascota; average SVL = 47.1 mm vs. 45.4 mm), slightly longer legs (average tibia length/head length proportion 0.86 vs. 0.84), fewer ventral scales (a minimum of 32 vs. 37; average = 35 vs. 38), fewer dorsal scales (37–45, average = 41 vs. 41–47, average = 43), and fewer scales around midbody (37–43, average = 40 vs. 40–45, average = 43) (BRYSON & GRUMMER 2021). 
CommentFor additional references see Bryson & Grummer 2021 (not provided upon request)

Synonymy: Thomas & Dixon (1976) argued that S. a. aeneus and S. s. subniger were indistinguishable. Smith et al. (1993) challenged this conclusion, claiming it was based on misidentified specimens from Nevado de Toluca and therefore an inaccurate description of the status and distribution of S. a. subniger. Sceloporus subniger and S. aeneus were subsequently considered distinct species in checklists (Liner 1994; Bell et al. 2003)

Distribution: see Bryson & Grummer 2021: 342 (Fig. 2) for a (rough) map as S. subniger “West” (not dixoni). 
EtymologyThe specific epithet is a patronym honoring the late James R. Dixon for his decades of research on Mexican herpetofauna, including several insightful studies of the S. scalaris group. “Doc” Dixon took an early interest in the academic growth of the first author and made a profound and lasting impact. For this and for his encouragement and support, he will be truly missed. 
  • BRYSON, ROBERT W.; JR., JARED A. GRUMMER, ELIZABETH M. CONNORS, JOSEPH TIRPAK, JOHN E. MCCORMACK, JOHN KLICKA 2021. Cryptic diversity across the Trans-Mexican Volcanic Belt of Mexico in the montane bunchgrass lizard Sceloporus subniger (Squamata: Phrynosomatidae). Zootaxa 4963 (2): 335–353 - get paper here
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