On Internet discussions, I often use text and cladograms to explain why birds are dinosaurs. But people often end up completely ignoring them and bringing up completely unrelated topics. Either that, or they don't even understand what I'm saying and aren't willing to confess they don't. So I put together this handy Venn diagram that is, I hope, easier to comprehend.
As we see, birds are nested inside dinosaurs, not a branch lying outside. And that's all there is to it. It's that simple.
What really baffles me is that a lot of these people aren't BAND supporters (which is, by the way, an equally ridiculous position to take) but accept that birds are of dinosaurian origin. And they often come up with completely ridiculous "arguments" to support their idea, none of which hold any water. I suppose that only shows they learn the fact but don't understand the implications.
For example, they often get confused between two clades being "related" to each other and one being "inclusive" of the other. Deinonychosaurs and avians are related to each other. They share a common ancestor but don't include each other. If you examined a deinonychosaur and an avian carefully enough, you'd probably be able to find features that distinguish the deinonychosaur from the avian. (But you'd have to look very carefully! After all, several basal deinonychosaurs were once thought to be basal avialians!) But maniraptors and deinonychosaurs have a different relationship. Maniraptors include deinonychosaurs. You can distinguish deinonychosaurs from other maniraptor clades (such as oviraptorosaurs) but it'd be impossible to distinguish between "deinonychosaurs" and "maniraptors". All maniraptors besides deinonychosaurs don't share any characteristics that deinonychosaurs themselves don't also share. This applies to birds being dinosaurs in the same way. You could distinguish birds from sauropods or ceratopsians or even oviraptorosaurs, but you won't be able to find any features that all non-bird dinosaurs share but birds do not. You can tell a dog and a cat apart, but that doesn't mean they're not both mammals. Telling "dinosaurs" and "birds" apart is as ridiculous as telling "amphibians" and "frogs" apart. It's physically and logically impossible. But can you tell "frogs" and "salamanders" apart? Certainly.
Related to the above, there's never a point where an animal stops being the member of a clade its ancestors were in. It is by definition impossible. It is impossible by evolutionary law. It doesn't even make sense logically. Microraptor zhaoianus
looked a lot more like Jeholornis prima
that it did to Triceratops horridus
, but people who argue birds aren't dinosaurs would rather put Microraptor zhaoianus
and Triceratops horridus
in the same "group" and leave Jeholornis prima
out. I'm completely in the dark as to how anyone could come to such a conclusion.
It doesn't matter that birds don't look like the earliest dinosaurs. Nor did sauropods or ceratopsians, but no one doubts that they are dinosaurs. I'd argue that birds look more like early dinosaurs than did sauropods and ceratopsians! Besides, the earliest vertebrates looked nothing like trout or frogs or cats, but no one doubts those animals are vertebrates.
Most other dinosaurs being extinct means nothing. There are many extinct mammal lineages, but no one doubts that there are still living things that are mammals today. There are a lot of extinct bird lineages, but no one doubts there are still living things that are birds today.
Also, birds being dinosaurs doesn't mean they stop being birds. A dog can be a canid, a carnivoran, a mammal, an amniote, a tetrapod, a vertebrate, an animal, a eukaryote, etc. "Dinosaur" simply encompasses a far larger group than just "avian".
The strangest argument is the argument from common usage. So the argument goes, no one says, "I see a dinosaur in the garden" when they're looking at a robin, so the robin is not a dinosaur. For starters, since when did common usage affect scientific terminology? A lot of people say, "'Pterodactyls' are flying dinosaurs", but does that make pterosaurs dinosaurs? No! If I saw a robin in my garden I'd most likely say, "There's a robin in my garden" instead of "There's a turdid passerine neoavian neognath neornithine ornithurine ornithothoracine pygostylian avialian eumaniraptor paravian aviremigian maniraptor maniraptoriform tyrannoraptor coelurosaur avetheropod tetanuran averostran neotheropod theropod eusaurischian saurischian dinosaurian dinosauriform dinosauromorph ornithodiran avemetatarsalian archosauriform archosauromorph archosaurian saurian diapsid sauropsid amniote cotylosaurian reptilomorph tetrapod tetrapodomorph stegocephalian sarcopterygian osteichthyan eugnathostome gnathostome vertebrate craniate euchordate chordate deuterostome bilaterian animal opisthokont unikont eukaryote organism in my garden", but that doesn't mean robins don't belong to any of those clades. Common usage is used only informally and is nothing more than a convenience. It has absolutely no bearing on the relationships between living things.
There isn't the slightest difference between saying that a sparrow is a dinosaur and that a dog is a mammal. Saying the birds aren't dinosaurs makes about as much sense as saying bats aren't mammals or ants aren't insects, that is, none at all. If you can accept that dogs and bats are mammals and ants are insects, there's no reason not to accept birds are dinosaurs.
Here's a good video
that talks about the point I'm making here, this time applied to humans being monkeys. (Warning: some way into the video it discusses characteristics all monkeys share, and one of the photographs is of a monkey with a visible penis. It's only there for a few seconds, but if you're sensitive to that kind of thing...)