Mixcoatlus, Robert C. Jadin, Eric N. Smith & Jonathan A. Campbell, 2011

Robert C. Jadin, Eric N. Smith & Jonathan A. Campbell, 2011, Unravelling a tangle of Mexican serpents: a systematic, Zoological Journal of the Linnean Society 163, pp. 943-958 : 951-953

MIXCOATLUS GEN. NOV.

Type species: Agkistrodon browni Shreve, 1938 , by present designation.

Etymology: The generic name is derived from the Náhuatl word Mixcoatl, meaning ‘cloud serpent,’ a god of the Aztecs and several Mesoamerican civilizations. The name alludes to the restriction of this clade to high elevations. The gender of this name is masculine.

Content: The genus Mixcoatlus contains Mixcoatlus barbouri , Mixcoatlus browni , and Mixcoatlus melanurus . Similar to Ophryacus , Mixcoatlus is a pitviper genus endemic to the highlands of southern Mexico. Mixcoatlus barbouri and M. browni are restricted to highland humid pine-oak and cloud-forest habitats of the Sierra Madre del Sur in Guerrero, Mexico ( Fig. 6 ), whereas M. melanurus occurs in highland arid tropical scrub, high deciduous forest, and seasonally dry pine-oak forest in southern Puebla and northern Oaxaca (Campbell & Lamar, 2004: map 83). This limited distribution of southern Mexico makes this genus the most restricted of New World pitvipers.

Common name: Mexican montane pitvipers

Definition and diagnosis: Rostral broader than high, front surface flat to moderately concave ( M. melanurus ); preoculars two ( M. barbouri and M. browni ) or three ( M. melanurus ), upper preocular largest and squarish. In M. melanurus , middle preocular separate from supralacunal, lower forming posterior border of pit and excluded from orbit; single, large, flat, platelike supraocular above eye ( M. barbouri and M. browni ) or two to three supraoculars along dorsal margin of eye including supraocular horn (single scale above eye forming flattened horn, dorsoventrally compressed in cross section, occupying most of dorsal margin of orbit, tip broadly rounded; adjacent scales along dorsal ocular margin slightly modified, projecting slightly or not); seven to 14 supralabials (usually eight in M. barbouri and M. browni and 11 in M. melanurus ); lip margin strongly scalloped in M. melanurus ; eight to 13 infralabials; canthals and internasals relatively large, flat to rounded; crown of head covered with relatively large, flat scales with keeling beginning in parietal area ( M. barbouri , M. browni ) or covered by small keeled scales ( M. melanurus ); intersupraoculars one ( M. browni ), three to four ( M. barbouri ), or nine to 13 ( M. melanurus ); second supralabial discrete from prelacunal (these scales may be separated by two rows of small subfoveals in M. melanurus ); supralabial and subocular series in contact ( M. barbouri , M. browni ) or separated by two to four rows of small, roundish scales ( M. melanurus ); one to two postoculars; 17–21 middorsal scale rows; mid-dorsal scales at midbody moderately slender and pointed in M. barbouri and M. browni and broad and obtusely rounded in M. melanurus ; keel generally extending to tip of scale or nearly so, apical pits not apparent; free portion of apex of dorsal scales moderate in extent; 129–148 ventrals in M. barbouri and M. browni , 137–169 in M. melanurus ; subcaudals undivided, 26–35 in M. barbouri and M. browni and 42–64 in M. melanurus ; tail spine straight or distally curved upwards, moderately long. In M. barbouri and M. browni dorsum usually with ill-defined zigzag stripe bordered narrowly with black, sometimes broken into discrete blotches; 25–28 dark brown lateral body blotches; dorsal ground colour reddish brown. In M. melanurus dorsum with zig-zag pattern; ground colour reddish brown, olive brown, or grey; dorsal scales usually finely mottled or speckled with black, although this pattern may be apparent only under microscopic examination.

In M. barbouri and M. browni lateral edge of nasal bone expanded into roughly triangular shape; frontal bones mostly flat, dorsal surface with slightly elevated margins, longer than wide; postfrontal moderate in size, reaching frontal; transverse distance of postfrontal about equal to its distance along parietal bone; posterolateral edges of dorsal surface of parietals forming moderately distinct raised ridge continuing posteriorly on parietal to about level posterior to quadrate; junction between parietal and pro-otic rounded; squamosal extending posteriorly to level about equal to posterior edge of exoccipital; ectopterygoid much shorter than expanded, flattened base of pterygoid (posterior to the articulation with ectopterygoid), with flat shaft gradually tapering posteriorly; dorsal edge of palatine rounded. Three palatine teeth; ten to 12 pterygoid teeth; eight to 12 dentary teeth; pterygoid teeth not extending posterior to level of articulation of pterygoid with ectopterygoid; maxillary fang relatively short, being about equal in length to height of maxilla; fang at rest extending to level of about middle of supralabial 5.

In M. melanurus frontal bones with concave dorsal surface, strongly elevated margins, moderately longer than wide; postfrontals relatively small, not contacting frontal, comprising considerably less of dorsal perimeter of orbit than parietals; posterolateral edges of dorsal surface of parietals forming distinct flat shelf not continuing onto the parietal as a raised ridge; junction between parietal and pro-otic irregular, not particularly angular; anterior portion of ectopterygoid possessing shallow depression on medial side accommodating attachment of ectopterygoid retractor muscle; ectopterygoid noticeably longer than expanded, flattened base of pterygoid (posterior to articulation with ectopterygoid) with flat shaft tapering posteriorly; apex of choanal process positioned at about midlength on palatine, process greatly reduced in height, apex broadly rounded; dorsal surface of parietal roughly triangular; three palatine teeth, seven to ten pterygoid teeth, seven to nine dentary teeth; pterygoid teeth extending to level of articulation of pterygoid with ectopterygoid; maxillary fang relatively short, only slightly longer than height of maxilla, at rest extending to level of suture between supralabials 6–7 or supralabial 7; splenial and angular bones separate; haemapophyses separate distally.

The highland isolation of Mixcoatlus results in its allopatry to most species of pitvipers. However, these three species are sympatric with O. undulatus throughout parts of their range but are distinguished by morphological features listed above. Additionally, M. barbouri and M. browni may be broadly sympatric with Crotalus intermedius and Crotalus ravus but are distinguished from these species by not having a rattle at the end of their tail.

Figures

Figure 6. Distribution map of Mixcoatlus barbouri and Mixcoatlus browni.