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Higher TaxaViperidae, Crotalinae, Colubroidea, Caenophidia, Alethinophidia, Serpentes, Squamata (snakes)
Common Names 
Metlapilcoatlus nummifer - TAYLOR, 1949
Metlapilcoatlus nummifer - CASTOE et al., 2009
Metlapilcoatlus nummifer - JADIN et al., 2010 
DistributionMéxico (Hidalgo, San Luis Potosí, Querétaro, Northern Veracruz)

Type locality: México, Querétaro, El Pilón (21.4998, -99.1738) 1134 m elevation  
TypesHolotype: MZFC 35381, adult female, collected by Jacinto Chávez, on 30 June 2015
Paratypes: MZFC 35382, adult male, collected by Jacinto Chávez, on 18 July 2015 
DiagnosisDiagnosis: Metlapilcoatlus borealis is a medium-sized, moderately robust viper (TL: males 657 mm, females 594 mm), has 22–25 rows of scales over the mid part of the body, nasorostral scales 4–6, ventrals 130–132, subcaudals 26–35, supraoculars 8–10, interoculabial 2–3, scales from 21–26 shields at the sides of the body before the cloaca. Additionally, Metlapilcoatlus borealis possess three unique nucleotides for cyt b at positions 21 (C) 369 (A), 378 (A); and three for ND4 at positions 192 (C), 405 (G), and 459 (T) (Tepos-Ramírez et al. 2021).

Comparisons: Externally, M. borealis can be distinguished from M. indomitus by having fewer ventral scales (130–132 vs. 133–142); in addition to the fact that the rows of scales in the mid part of the body do not exceed 25 (M. indomitus = 23–25, M. mexicanus = 22–27, M. nummifer = 23–28, M. occiduus = 23–27, M. olmec = 22–26); with regard to M. mexicanus and M. olmec, M. borealis can be distinguished because the supraocular scales are never divided, whereas in the other two species they can be; the postorbital stripe is narrower with respect to those of M. mexicanus and M. nummifer, covers a lower number of temporal scales (M. borealis = 3–5, M. indomitus = 4, M. mexicanus = 1–7, M. nummifer = 4–8, M. occiduus = 5–6, M. olmec = 3–6); has a lower average number of interoculabial scales (M. borealis = 2–3, M. indomitus = 4–5, M. mexicanus = 2–5, M. nummifer = 2–5, M. occiduus = 4–5, M. olmec = 4–6), and a higher number of scales in contact with the supraoculars (M. borealis = 4–6, M. indomitus = 4–5, M. mexicanus = 3–5, M. nummifer = 3–5, M. occiduus = 4, M. olmec = 3–5). Metlapilcoatlus borealis has a completely dark pigmentation in the last third of the body after the cloacal scale, as opposed to other species, where this region is black with light spots. (Tepos-Ramírez et al. 2021).

In addition to the differences in external morphology, the more distinctive traits are found in the hemipenial morphology, where M. borealis presents a strongly thickened sulcus spermaticus that turns ventro-diestrally to the apex of the lobes, while it is a thin structure that runs longitudinally along the center of the hemipenes sulcate side in the rest of the species of Metlapilcoatlus; the calyculate area in the interior region of the sulcate side of the hemipenis is laid out obliquely with respect to the spinous area, unlike in the rest of the species where the division of the spinous area and the calyculate area cuts transversally across the sulcate side of the hemipenis, forming a groove shaped like a ‘‘V’’. The exception to this is M. occiduus, where there is no division of the calyculate area, since this species does not appear to have spines in the hemipenes; M. borealis has the smallest hemipenis of the entire genus, even among similarly sized males (M. borealis HL = 13.21, HW = 2.21; M. indomitus HL = 27 mm, HW = 10.5 mm; M. mexicanus HL ‡ 20 mm, HW = 11. 25 mm; M. nummifer = HL ‡ 16 mm, HW = 9 mm; M. occiduus HL ‡ 20 mm, HW = 7.5 mm; M. olmec HL = 20 mm, HW = 10 mm). Metlapilcoatlus borealis can be distinguished from M. mexicanus–M. nummifer–M. olmec by its higher number of spines at the base of the lobes (M. borealis = 6, M. indomitus = 13, M. mexicanus = 4, M. nummifer = 3, M. occiduus = 0, M. olmec = 3) and a higher number of rows of calyces in the lobes (M. borealis = 6, M. indomitus = 13, M. mexicanus = 4, M. nummifer = 3, M. occiduus = 0, M. olmec = 3) (Tepos-Ramírez et al. 2021).

Color in life: Collected specimens of M. borealis presented a base coloration ranging from orange to dark brown. Dorsal and lateral blocks always darker than the base color, and sometimes the fringes of the blocks had darker colorations. Ventral coloration went from light yellow and white hues to yellow and orange in the gular region (Tepos-Ramírez et al. 2021). 
CommentSympatry: probably sympatric with Atropoides nummifer in middle Veracruz

Similar species: M. nummifer, M. mexicanus. 
EtymologyThe specific epithet, borealis, references the northernmost distribution of this taxon with respect to the other taxa of Metlapilcoatlus. 
  • Castoe, T. A., J. M. Daza, E. N. Smith, M. M. Sasa, U. Kuch, J. A. Campbell, & C. L. Parkinson. 2009. Comparative phylogeography of pitvipers suggests a consensus of ancient Middle American highland biogeography. Journal of Biogeography 36:88–103 - get paper here
  • Jadin, Robert C.; Ronald L. Gutberlet Jr and Eric N. Smith 2010. Phylogeny, evolutionary morphology, and hemipenis descriptions of the Middle American jumping pitvipers (Serpentes: Crotalinae: Atropoides). J Zool Syst Evol Res 48(4): 360–365; doi: 10.1111/j.1439-0469.2009.00559.x - get paper here
  • Tepos-Ramírez, Mauricio; Oscar Flores-Villela, Julián A. Velasco, Carlos Pedraza Lara, Oscar R. García Rubio & Robert C. Jadin 2021. Molecular Phylogenetics and Morphometrics Reveal a New Endemic Jumping Pitviper (Serpentes: Viperidae: Metlapilcoatlus) from the Sierra Madre Oriental of Mexico. Journal of Herpetology 55 (2): 181-191 - get paper here
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