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After reviewing Dwight Platt's 1969 publication on the Natural History of Hognoses I thought I would post some of the interesting information on their evolutionary history.

The author states that the fossil record of hognoses rests for the most part on scattered vertebrae around 5+ million years ago. Apparently hognoses are a primitive group of colubrids, but obviously they are very specialized. Heterodon retained a number of primitive viper like characters, while Xenodon evolved musculature resembling advanced colubrid snakes.

In his 1952 publication, Edgren claims that the Western species is the most "bizarre" and most highly evolved of the hognoses. The eastern species more closely resembles a generalized colubrid with a smaller rostral scale with less projection, a longer head, a somewhat longer tail, and it's larger size is probably a primitive trait as well.

Now here is when it gets really interesting: the Eastern and the Western species apparently were already separated evolutionarily before they could be classified as the genus Heterodon. My take on this is that they separated from their ancestral stock, and what we see today with the three species of Heterodon is a perfect example of parallel evolution. In order to help you understand this, let's not forget that our system of classification is inherently flawed since we are subjectively drawing lines where one species ins and where another begins. In other words, we all know that the Western, Eastern, and Southern hognoses are similar because not only are they classified together in the same genus, but they at least superficially resemble each other. BUT! It is entirely humans that have put these animals together that may not necessarily belong together in the same genus.

I have learned that literature like this may be extremely dated and that some of this information may not at all be true. However I have not come across other literature that describes an updated version of the evolutionary history of hognoses, although according to Eckerman's review, there are some recent publications. Once I've posted this I plan on sending a link to several people who may be able to provide some input and perhaps we can have a good discussion about this topic and get some of the details ironed out.
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...the oldest task in human history: to live on a piece of land without spoiling it."
Aldo Leopold (1938)

"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)

Your observation on the western and eastern hognoses being placed in a genus by convention is well placed.

Indeed, it appears by all lines of evidence (genetic, immunological, morphological) that the eastern and western hognose snakes have been separated for a fairly lengthy period. However, when you include the hognose snake species in with the other Xenodontines (the closest known relatives), the hognose species always come out together.

In actuality most of the confusion has come from where H. simus belongs. It is pretty clear to me now that H. simus is very closely related to H. nasicus based on all lines of evidence. Pinou, in her dissertation, made the suggestion that H. simus was closely related to H. platyrhinos but there are many reasons why the immunological data she was examining would not apply to relatively recent divergences.

Anyway, taxonomic classifications are indeed somewhat arbitrary on some level. We use very strict guidelines to name and classify organisms but the most important variable to be as acurate as possible is the one thing we don't have... the timing of events. As a result, Heterodon, as a genus is a much older genus than many other snake genera. Does this mean that they aren't part of a genus? As a matter of convention it is perfectly acceptable in my opinion. The fossils that indicate that H. nasicus and H. platyrhinos do not have a common ancestor still show all of the hognose traits and so still share traits through a common ancestor though not an immediate one.

Hognose snakes are very unusual not only for their morphology or behavior. They seem to be the last of a line of snakes going back many millions of years. This is the very reason why the treatment of the classification of the Xenodontines as a group has received many treatments and all are at odds with one another in some way or another. The next closest relative to Heterodon is either Diadophis, Farancia or Contia and support for the different associations is found in different places. (Disclaimer: I do not agree with the Contia connection named by Pinou. Someday I'll actually finish sequencing my contia samples and show it.)

So it is an interesting discussion from a taxonomic standpoint because of its relevance to how we classify organisms. Until we have a better way of estimating the timing of events we will always have discrepencies in taxonomic levels. For hognose, I don't think the genus classification will change anytime soon. There are only three species and so there is little to go on in trying to find those defining evolutionary events in the group.

Curtis Eckerman
My homepage

This information is almost exactly what I found out today, but from a different source. Because I only talked to someone I cannot refer you to a webpage or direct quote, but the quote in the first post of this thread is close enough to get what i had meant
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1.1 Columbian red tail
1.1 western hognose
1.0 tricolor hognose
0.0.3 kenyan sand boas
1.1 cornsnake
0.0.1 burmese python
0.1 chinese water dragon
0.0.1 tokay gecko