Proper taxonomic names, with their formal suffixes, can be confusing at the best of times. But when they include identical prefixes their meaning is further complicated. For example, to what taxonomic group does the colloquial term ‘plesiosaur’ refer: Plesiosauria, Plesiosauroidea, Plesiosauridae, or Plesiosaurus? With all these unfortunately similar sounding names it’s no surprise that laypersons, and specialists alike, may have different concepts of what a ‘plesiosaur’ actually is.
To understand the colloquial terminology we must first clarify the scientific terminology – those suffixes have a discreet meaning after all. I’ll start off by explaining them each in turn, in order from lowest ranked to highest ranked. A taxon (or taxonomic group) of low rank is very specific, whereas a taxon of high rank is broader. Apologies in advance, I realise that this might get a bit technical and dry.
Plesiosaurus. This refers to the genus Plesiosaurus. In this case the suffix ‘-us’ is of no significance, but genera are always italicised.
Plesiosauridae. This refers to a family that includes Plesiosaurus. Animal families strictly end with the suffix ‘-idae’. The term Plesiosauridae isn’t used much these days because most species previously regarded as Plesiosauridae have since been allocated to the Elasmosauridae. This has left Plesiosaurus as the only taxon in the family, making the family more or less redundant. [EDIT - A recent analysis by Ketchum and Benson (2010) has proposed a new and more inclusive definition for Plesiosauridae, which I strongly endorse.]
Plesiosauroidea. This refers to the superfamily that includes Plesiosauridae and Elasmosauridae. Animal superfamilies strictly end with the suffix ‘-oidea’. The second (and only) other superfamily of note here is the Pliosauroidea.
Plesiosauria. This refers the higher taxon that includes the superfamilies Plesiosauroidea and Pliosauroidea. It is usually regarded as an Order today, but it’s rank has varies historically. Orders often end with the suffix ‘-ia, but the strict use of suffixes is not applied beyond the taxonomic level of Superfamily.
Consequently, these taxa are nested within each other: Plesiosaurus belongs to the Plesiosauridae, which belongs to the Plesiosauroidea, which belongs to the Plesiosauria. I’ll edit this post with an image at a later date.
There is a standard colloquial terminology for some of the scientific bames: Plesiosaurus is the same, and Plesiosauridae is plesiosaurid. (Notice that the colloqial names are lower case whereas the scientific names start with a capital). But as we enter ‘plesiosaur’ territory things get complicated, because as far as the published literature goes, there is no standard colloquial terminology for Plesiosauroidea and Plesiosauria: both are frequently referred to as plesiosaurs.
Plesiosaur meaning 1. The name ‘plesiosaur’ is often used to refer to the Plesiosauria, the largest most inclusive taxon – pliosauroids (or pliosaurs) included. In this case, the symmetrical terms plesiosauroid and pliosauroid can be used colloquially to distinguish between the superfamilies Plesiosauroidea and Pliosauroidea. The word pliosaur works well for the Pliosauroidea too here, although it is not symmetrical in this case (note that there is no such group as Pliosauria). I almost always use the word plesiosaur in this way, this is evident in the title of my website (the plesiosaur directory) and this blog (Plesiosaur bites). I don’t discriminate against pliosaurs, remember pliosaurs are plesiosaurs too! Or are they?
Plesiosaur meaning 2. An equally acceptable use of the word ‘plesiosaur’ refers to the Plesiosauroidea. This colloquial meaning is commonly adopted (in books and websites) because it sorts these critters into two manageable groups: plesiosaurs and pliosaurs (=Pliosauroidea). This makes sense because it is symmetrical. (Note again that there is no such group as Pliosauria, so meaning 1 can’t be symmetrical in the same way.) In this case, the colloquial term plesiosaurian (with an extra ‘n’) can be applied to the Plesiosauria, which would seem to tie everything up neatly. Maybe I should change my website to ‘The Plesiosaurian Directory’?
To summarise, we have two competing colloquial meanings for the word plesiosaur and I’m not entirely convinced one is better than the other. I’m probably making a mountain out of a molehill (this post got far longer than I intended!) but I think this is a topic that could do with discussion. Which do you use? Or which to you prefer? Comments are very welcome!