Well, I have been taken to the woodshed by my many geologist friends for using the word “geologist” to describe those who use the Periodic Table to guide their expectations, which in turn are key to deciding whether or not a result is viewed as “extraordinary”, in the Sagan sense.

My bad. And to think of it, more correct for such folks would be the word “chemist”. After all, the Periodic Table is at the core of chemistry. Further, those whose work in geology departments centers on the Table might call themselves “geochemists”, to distinguish themselves from those whose work centers on hiking landscapes to plot strata.

Now, anyone who finds the string c-h-e-m-i-s-t somewhere in their CV does not a priori find it absurd to propose that an element below another element in the Table might substitute for an element above. Especially organic chemists. Indeed, the “halogen series” of compounds (fluorine replaced by chlorine replaced by bromine replaced by iodine) is a staple of physical organic chemistry; the changing reactivity of one set of compounds along that series is used to calibrate changing reactivity for many other sets.

This is even true for elements in the middle of the Table. People who base expectations using the Periodic Table (shall we call them X-chemists?) do not discard generally as absurd the notion that silicon might substitute for carbon in some contexts, or arsenic might substitute for phosphorus, with “trend-like” changes in behavior.

But Sagan’s aphorism (“extraordinary claims require extraordinary evidence”) was the focus of my comment, not the naming of fields. It is not easily applied, as “extraordinary” depends on context. If all that you know about chemistry is the Periodic Table, the claim of arsenate DNA might not strike you as requiring extraordinary evidence. Only if you know much more chemistry does this change.