Description
Members of this genus have a number of features in common. All species have a relatively broad head with short fangs. A loreal scale is present, except in A. piscivorus. There are usually nine large symmetrical platelike scales on the crown of the head, but in all species these are often irregularly fragmented or have sutures, especially in A. bilineatus. All have a sharply defined canthus rostralis and a vertically elliptical pupil. There are 6-10 (usually 8) supralabial scales and 8-13 (usually 10-11) sublabials. The dorsal scales are mostly keeled and at midbody number 21-25 (usually 23), while A. piscivorus has 23-27 (usually 25). There are 127-157 ventral scales and 36-71 subcaudals. Of the latter, some may be divided. The anal scale is single. All have a color pattern of 10-20 dark crossbands on a lighter ground color, although sometimes the crossbands are staggered as half bands on either side of the body.
The phylogeny of the three species has long been controversial. Studies based on morphological (Gloyd & Conant, 1990) and venom characteristics (Jones, 1976) support the idea that A. bilineatus and A. contortrix are more closely related. However, an analysis of mitochondrial DNA was conducted by Knight et al. (1992), as well as more recent molecular studies (Parkinson et al., 1997, 1999) have concluded that A. bilineatus and A. piscivorus are sister taxa, with A. contortrix being a sister species to them both.
Read more about this topic: Agkistrodon
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