The Trimorphodon biscutatus (Squamata: Colubridae) Species Complex Revisited: A Multivariate Statistical Analysis of Geographic Variation

The Western Lyresnake (Trimorphodon biscutatus) inhabits arid regions from the desert southwestern United States southward along the Pacific lowland versant to northwestern Costa Rica and exhibits substantial geographic variation in size, squamation, and color pattern across its range. We examined patterns of geographic variation within T. biscutatus using multivariate statistical analyses of 33 morphological characters scored from 429 specimens. Principal components and discriminant analysis revealed six morphologically distinct groups that are generally concordant with lineages recovered in a phylogeographic analysis of mitochondrial DNA and with taxa traditionally recognized as species or subspecies. We conclude that Trimorphodon biscutatus (sensu lato) comprises six evolutionary species (including the recently elevated T. vilkinsonii) and recommend elevating T. biscutatus (sensu stricto), T. lambda, T. lyrophanes, T. paucimaculatus, and T. quadruplex to the species level. A key to the species of Trimorphodon is provided.

Resumen

Trimorphodon biscutatus habita regiones áridas desde los desiertos del suroeste de Estados Unidos hacia el sur, a lo largo de las tierras bajas del pacífico de México hasta el noroeste de Costa Rica. Esta especie presenta considerable variación geográfica en tamaño, escamación, y patrones de coloración a lo largo de su rango, y por lo tanto ha tenido una inestable historia taxonómica. Investigamos poblaciones de T. biscutatus usando análisis estadísticos multivariados de datos morfológicos para delimitar especies dentro de este taxón politípico y de amplia distribución. Análisis de componentes principales y análisis discriminante de 33 caracteres de 429 especímenes resultaron en seis grupos morfológicamente distintos, consistentes con taxa reconocidos tradicionalmente como subspecies. Concluimos que Trimorphodon biscutatus (sensu lato) comprende seis linajes evolutivos (incluyendo a T. vilkinsonii, recientemente designada como especie) y recomendamos elevar a T. biscutatus (sensu stricto), T. lambda, T. lyrophanes, T. paucimaculatus, y T. quadruplex al rango de especie. Una clave al nivel de especie de Trimorphodon es incluida.

Fig. 1

Geographic variation within members of the Trimorphodon biscutatus complex (A–G) and Trimorphodon tau (H). (A) T. b. lyrophanes: San Diego Co., California (TJD 088); (B) T. b. lambda: Cochise Co., Arizona (LSUMZ 83684); (C) T. vilkinsonii: Presidio Co., Texas (TNHC 66511); (D) T. paucimaculatus: Jalisco, México (UTA R 52919); (E) T. b. biscutatus: Jalisco, México (UTA R 52932); (F) T. b. quadruplex: Guanacaste Province, Costa Rica (specimen not collected); (G) presumed hybrid between T. b. lambda and T. vilkinsonii illustrating reduced head pattern characteristic of T. vilkinsonii: Cochise Co., Arizona (LSUMZ 83685); (H) T. tau: Sinaloa, México (MZFC 19476). All photos by T. J. Devitt.

Fig. 2

Distribution of the Western Lyresnake (Trimorphodon biscutatus) complex after Gehlbach (1971) and Scott and McDiarmid (1984a).

Fig. 3

Principal components scatterplots for males (A) and females (B).

Fig. 4

Canonical analysis of discriminance scatterplots for males (A, C) and females (B, D).

Contributor Notes

Associate Editor: D. Kizirian.

¹ Museum of Natural Science and Department of Biological Sciences, 119 Foster Hall, Louisiana State University, Baton Rouge, Louisiana 70803, E-mail: (TJD) tdevitt@berkeley.edu. Send reprint requests to TJD.

² Section of Integrative Biology and Texas Natural Science Center, The University of Texas at Austin, Austin, Texas 78712-1064.

³ Present address: Museum of Vertebrate Zoology and Department of Integrative Biology, 3101 Valley Life Sciences Building, University of California, Berkeley, California 94720-3160.

Received: 16 Feb 2007

Accepted: 03 Oct 2007