I think everyone on the Internet is aware that words mean things. In science, words often are communicating fairly complex ideas and so proper technical word choice can make the difference between saying something that makes sense and saying something that makes no sense. I’m going to talk about an example of the latter.

First, some context

Until we have reason to reject the hypothesis, it’s assumed that every living thing on Earth is related and that every organism shares a common ancestor somewhere in time with every other organism. The primary way of illustrating evolutionary connections between different organisms is through evolutionary tree diagrams, which show “branches” of life splitting off into smaller and smaller branches. The general term for these diagrams are phylogenetic trees, they’re tree-like diagrams which express phylogenetic hypotheses, the hypotheses of how life is related.

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On any phylogenetic tree every branch and group of branches is a definable unit of life. It’s a thing that can have a word attached to it, and then when scientifically writing a person uses the proper noun version of that word to mean “the biological unit that contains all of these individuals” and the common noun version to mean “an individual of that biological unit.” Primates is the biological unit that a human and a lemur are both in; a human or a lemur is a primate.

This can start to get tricky when talking about a mixture of modern and prehistoric life. Because everything has common ancestors if you rewind the tape long enough 2 separate units (cats and dogs) become 1 unit (the ancestor of cats and dogs). But that ancestor of cats and dogs was neither a cat nor a dog nor even a catdog. That ancestor of cats and dogs is a carnivoran, with a more specific identity depending on whose phylogenetic hypotheses you’re following.

This is the part where I finally have images

So there’s a recent paper which used some words in an inconsistent manner. It’s a neat paper that looked at the development of skin coverings in some reptiles in order to help answer some questions on what exactly is going on with hair, scales, and feathers. Its authors looked at skin covering development in three species of reptiles and made comparisons between what they saw in those reptiles and what we know about mammalian and avian skin. Putting up a phylogenetic tree of those five biological units looks like this.

Pogona vitticeps (the central bearded dragon) and Pantheroophis guttatus (the corn snake) are closely related to one another; they’re both squamate reptiles and they’re both lepidosaurs. Crocodylus niloticus (the Nile crocodile) is closely related to all birds (here represented by the domestic chicken, Gallo gallo); they’re both archosaurian reptiles. Lepidosaurs and archosaurs are, more distantly, related to one another, and mammals like humans (Homo sapiens) are distantly related to all four of those species.

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And only one thing of what I just said is under any kind of real debate in biology, and that’s only because some biologists use the word reptile inconsistently. And this paper, about reptiles, gives an example of this reptilian inconsistency.

Reptile definition #1

skin appendages of sauropsids (birds and reptiles)

This wording throws some words onto the phylogenetic tree, arranging it like so:

In this usage, reptiles are the non-bird part of a unit called Sauropsida. They’re a grade of evolution from which birds evolved, and because they’re a grade, I use quotes around their unit, “Reptilia”, to indicate that it’s a paraphyletic unit.

Reptile definition #2

hairs, feathers, and scales of extant species are homologous structures inherited, with modification, from their shared reptilian ancestor’s skin appendages

that mammalian hair evolved as mechanosensory appendages in interscale regions of the skin (that is, hinges of scales) of their reptilian ancestor

This wording throws some different words onto the phylogenetic tree, arranging it like so:

In this usage, reptiles are an even less-poorly-defined grade of evolution because they’re what birds and mammals evolved from. For bonus confusion, this is internally inconsistent with the earlier statement that sauropsids and mammals are distinct. If reptiles are sauropsids, and mammals are descended from reptiles, then it would follow that mammals are sauropsids. So the paper kind of bites itself with its own word choices.

Reptile definition #3

I think it’s entirely possible to talk about these units of animals in consistent and not-self-contradictory ways, but it requires doing something which some biologists seem to have some hesitation about.

I’ve barked up this tree before so I apologize to anyone who has already read me bark about this but in science journalism (and even scientific publication) the same poor word usage keeps occurring because people have confusions over what a reptile is. Reptilia is a unit that includes birds and it shares a common ancestor with mammals like ourselves.

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And any other definition dilutes the meaning of reptile into some much less useful jumble of words.

Top image modified from Wikimedia file posted by DiBgd. Phylogenetic tree silhouettes from PhyloPic. Crocodile by Steven Traver, agamid lizard by Michael Scroggie, colubroid snake by Michael Scroggie, Gallus gallus by Steven Traver, Homo sapiens by T. Michael Keesey.