New genus or new species, how do paleontologists sort things out?

[Reptilia]

Searching for some info on Wikipedia I stumbled upon this:



and I've been immediately reminded of this:



I wonder how all those oviraptosaurs can be classified as different genera while in Psittacosaurus case the differencies between individuals translate into different species in the same genus. What is the rule to establish a new genus? If there's any specific.

[ZoPteryx]

I think the oviraptorid chart is a little misleading on the matter because the feathers are obscuring any differences in the skull bones.  And of course there are differences in the rest of the skeleton, especially in the neck, hands, and tails of oviraptorids.  Psittacosaurs are pretty samey in post-cranial anatomy, mainly just differing in size.

To be honest though, it's pretty subjective and arbitrary.  All that matters is that the taxa, be it a genus or species, is a valid clade, not polyphyletic or paraphyletic in respect to other taxa.  So if some of those psittacosaurs have features in common to the exclusion of others, they could be split off into their own genus, provided they also form a clade to the exclusion of the others.

[Halichoeres]

Can confirm. I've been an author on one species description paper and even among us there was some debate about whether all of the things we were looking at warranted species status.

[Newt]

There have been some attempts over the years to create consistent criteria for genus-level distinctions, but none have really caught on. Edward Drinker Cope, a neo-Lamarckian, tried to establish changes in developmental timing (neoteny or hypermorphosis) as the distinction between genera. Some more recent workers have tried to use the estimated time of divergence or some measure of genetic difference. All are difficult to justify and to apply.

The trouble is, while "species" seems to be a more-or-less objective category with some real significance in nature (whether we recognize species correctly or not), genera and all higher taxa are artificial constructs we use to understand how these real units (species) are related to one another.

Some paleontologists have encouraged use of monotypic genera (genera with only one species) for dinosaurs, because the usual small sample size and incomplete nature of dinosaur remains makes establishing relationships among species especially difficult. This encourages researchers to treat every species as a distinct taxonomic unit, rather than assuming two species are closely related just because some other worker placed them in the same genus. Other paleontologists object that this makes genera redundant and fails to make use of the genus category for its intended purpose, to express hypotheses about species' relationships.

By the way, those of you who are mainly interested in dinosaurs and don't look too closely at other groups of organisms may be surprised to learn that the very small genera typical of dinosaur taxonomy are unusual. There are plenty of genera of extant animals and plants with over a hundred species each; some are several times this size.