The phrase "et al" in a bibliography or other citation simply means "and others." This phrase comes about as an abbreviation of a phrase in Latin. The original Latin phrase is "et alia." "Et" means "and" and "alia" means "others."
When you see this phrase in a bibliography or a citation, it can mean one of two things. If the work is being cited for the first time, the "et al" means that work has at least six authors. If the work has three to five authors, all of the authors are listed in the first citation. In later citations, only the first author's name is used with "et al" to show that there are more authors.
In your case, then, Shemwell was the first author named, but he or she had coauthors. This is why the "et al" is used.