Why are there no intermediate necked plesiosaurs?
There appear to be short necked and long necked plesiosaurs, but why are there no intermediate necked plesiosaurs? Did they diverge somehow, and rather than one type being more fit for the environment than the other, and the less fit type dying out, they both lived?
In a sense, there are intermediate necked plesiosaurs. The early species of the genus Rhomaleosaurus, found in the lower Jurassic of Britain have relatively long necks and relatively large heads, although they are assigned to the pliosauridae - the large-headed, short-necked type. These animals came early in plesiosaur evolution, and although there were longer-necked and smaller-headed types around at the time, they did not reach the extremes found much later in the Cretaceous, where we find animals with extremely long necks (as in Elasmosaurus, with 72 neck vertebrae), and animals with very large heads (Kronosaurus was pretty well a giant head with a relatively small body attached to move it around).

One of the big questions in plesiosaur taxonomy (the study of their evolutionary relationships) is this: are the two body types separate clades (i.e. are all long-necked plesiosaurs more closely related to other long-necked plesiosaurs than they are to the large-headed types) or are they morphotypes (i.e. do evolutionary pressures produce the two types independently from basic plesiosaur stock). There is some evidence (though it is a matter of heated dispute) that the two body plans are morphotypes.

One of the main problems in studying plesiosaur evolution is that the fossil record is patchy. Most of the specimens we have come from a few places, so that although we know a lot about the plesiosaur fauna of the Oxford Clay, in the middle/upper Jurassic, and a fair bit about the middle Cretaceous fauna from North America, we know very little about what happened in between - a gap of 40 million years. A new location with good plesiosaur material from this gap could throw a lot of light onto their evolutionary relationships.