In part 1 of this series we looked at what a dinosaur is. We dove head first into the complicated world of dinosaurs and grouping them into the Linnaean system. We even talked about specific and interestinggroups of dinosaurs, leading up to a look at how modern birds are actually grouped in as well, existing as living dinosaurs. But now I want to discuss something that often annoys me.
I was in the post office the other day and I noticed a new selection of stamps for the month. Dinosaurs of Canada, it read. Being an amateur paleoartist I looked over the artwork when I suddenly grimaced. Now I will admit, this sort of thing annoys me, but not anger-induced rant sort of annoyed. Out of the six available stamps of "dinosaurs", only five were actually dinosaurs.
This comes up a lot. Often there are many creatures that get lumped in with dinosaurs as a matter of convenience. I suppose most people don't want to say, "dinosaurs and other prehistoric reptiles." But I am not so sure that is simply the case. I think it is a matter of everyone easily mistaking all ancient reptiles as dinosaurs. So let's set this issue straight (then you can bust out the facts at those cocktail parties we still aren't being invited to).
In the first part we talked about that specially adapted hip socket that set dinosaurs apart. It was simply that that made them a distinct group of vertebrates. But without studying the skeletons of every ancient creature, how do we know at a glance which ones are and aren't dinosaurs. I'll admit, there can be some deceptive creatures that might trick you, but most are pretty obvious. So let's look at the biggest offenders.
This is one I see in almost every grouping of dinosaurs. Many even get referred to as "flying dinosaurs", but as we have now learned from the previous entry, the only flying dinosaurs were birds and select bird-like dinosaurs such as Archeopteryx. Pterosaurs, as they are properly called, lacked the hip-socket feature to qualify it as a dinosaur.
So what were they? In one word, amazing. Pterosaurs ('winged-lizard') were the first vertebrates to gain the power of flight. Unlike birds, who evolved a different manner of flight mechanics, Pterosaurs had long thin arms that stretched their skin out into wings to catch the air. They ranged from the tiny Nemicolopterus (less than 10 inches in length), to the whopping Quetzalcoatlus (with a 52 foot wingspan), the largest creature ever to fly.
Remember when I said that dinosaurs were not lizards? Well they also were not crocodilids, turtles, or snakes. Just as we still have many living lizards and land-reptiles, we also have numerous extant marine reptiles. And while variations of turtles and crocodiles have been around since the dinosaurs, they are not at all related. We have to go up our Linnaean chart to the Chordata phylum to find any real connection. And yet, often, extinct aquatic reptiles get lumped in with dinosaurs. Ichthyosaurs ('fish-lizard') were the ancient reptile equivalent of dolphins. And like whales and dolphins, evolved from earlier land-based vertebrates. Plesiosauria ('near-lizard') evolved similarly. It is one of the few I am more likely to forgive for being included with dinosaurs, as they are somewhat sauropod-like. But even their name shows they are closer to a lizard than a dinosaur or a fish (although, don't get me started on what a fish is).
Synapsids are a strange group. Many of this group look sort of dinosaur-like, and I know I have seen them on stamps and posters. Hell, when I was a child, Dimetrodon was my favorite "dinosaur". Oh poor naive me. Synapsids, such as Dimetrodon and others, are sometimes called 'protomammals' as they include the mammal-like reptiles of the Permian period (before the dinosaurs). Technically Synapsids include everything in the 'mammal branch' of the tree of life, but we are specifically focusing on the mammal-like reptiles. These creatures existed long before the dinosaurs evolved, which means they didn't have the distinct hip-socket adaptation, and were therefore not dinosaurs. Sorry Dimetrodon and childhood me.
This group of reptiles would eventually take on more mammal like qualities by the time of the dinosaurs. Mammals did exist by the time dinosaurs were around, but were more often snacks for hungry therapods. It wasn't until after the extinction event at the end of the Cretaceous that mammals would have the opportunity to evolve beyond small rodent like creatures and fill the broader ecological niches of the extinct dinosaur species.
Next time: Part 8 - Prehistoric Time