What is the most reliable tail marker for *Crotalus molossus* identification?

A tail that is entirely black or very dark brown/gray up to the rattle

The namesake characteristic of the black-tailed rattlesnake provides the most consistent visual clue for identification, even though other features can sometimes be deceiving. The defining feature is the coloration of the tail segment extending immediately before the rattle assembly. This section is typically solid black, or in some instances, a very dark shade of brown or gray. While this marking is highly reliable in adults, it is noted that juvenile snakes may exhibit a lighter tip initially, which progressively darkens as the snake matures into adulthood. This consistent dark tail coloration contrasts with many other regional rattlesnake species, making it a primary field marker despite variations found in the snake's overall body pattern.