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Note: this thread should be viewed as a thought experiment, not necessarily fact. I am still too new to these concepts to put my foot down yet, but enjoy mentally turning them over A paleontologist at his core is an evolutionary biologist. The goal of an evolutionary biologist is to study the dynamics of evolution, and not just "species" for their own end. After all, the concept of a species only exists as a man made construct that humans use to pin down certain moments in the path of change in lineages. Where one decides to put way points (species) in that gradient doesn't matter, so long as there are at least two way points there so that comparisons can be made. If that was confusing, forget what I said for now and consider this: Animals change from one to another through cumulative steps and changes. Under textbook conditions, the graph of change from species to species is a smooth slope, not stair steps. To illustrate this point, consider the color gradient below. This gradient represents a change from one animal (blue) to another animal (red). Humans assign names to color the same way we assign species to animals. Artificially. At the top left, the image is blue. At the bottom right, the image is red. Now, at what exact point on this image does blue change to red? We can never know- there is no answer. Such a change is too gradual to point to one spot and say "now the blue just turned to red". But, to make sense of the change, we can point to the middle phase and call that phase "purple". "Purple" is simply a way marker we put down to digest the change of blue to red. I could have put such a way point down anywhere in that gradient, said "that's the moment of change!" and been equally right. As a society, we've just decided the arbitrary "purple" point is where we'll dig in, so that we may have a consensus. The same idea applies to the gradient of animal change. There's no point where one animal suddenly becomes the next, we just assign points in that gradient and call each point a "species" for our own sanity, to make sense of change. BUT! Here's the thing: In paleontology, we do not have the curse of a perfectly filled out gradient, with every animal that's ever lived represented as a fossil that can be easily traced from one to the next. We do not really "choose" our way points. We're forced to use the few animals that fossilized as way points, to fill in the gradient because they're the only data we have. I guess that makes the paleontologists job easier. For example, we know that the mosasaur Clidastes, by virtue of it simply existing, is a way point in it's respective spectrum. We can tell that the mosasaur Mosasaurus is a further developed way point in that spectrum. There are no fossils in between them, so we physically can't assign a metaphorical "purple" in between them, but then Jormungandr is discovered, and now we can. Thus, we were forced to use Jormungandr as our way point to define the Clidastes---->Mosasaurus change. ----------------------------------------------------------- This is the inherent flaw of the "missing link" argument. No one "missing step" exists between two species. Since change is gradual, one can divvy up a perfect gradient into 1000 steps and each will still be slightly different from each other. It's a game you will play for infinity. When working with fossils, you're lucky to find one step in between two others, and so when you do, that's the representative of change you're forced to acknowledge. Now, such a gradient only considered the change from the blue species to the red species. In real evolutionary environments, this gradient branches off in many directions, changing to all sorts of wonderful colors, from greens to yellows to blacks and whites. I chose the two color gradient for ease of concept. Now that we understand that way points are artificially planted in a perfect gradient, we can understand the core of phylogenetic technique. Putting just one point down in this below gradient is useless. We have nothing to compare it to, no way to quantify change. If I put two points down though, NOW I have a way to quantify change. I can say with authority that point B is more red than point A, or vice versa. _______________________ This is what "out-groups" exist for in phylogenetic analysis. An out-group is an animal that you can point to say "this animal represents the most primitive version of the group I'm interested in". The out-group is your anchor of comparison - without it, you would be trying to understand the rest of the group in a vacuum. To say "Clidastes-like mosasaurs evolved forwards, turning into Jormungandr-like mosasaurs and then to Mosasaurus-like mosasaurs" is an empty statement unless you decide on a primitive group first to enable such comparisons to be made. Among mosasaurs, the primitive condition is what's seen in land lizards like Varanus (monitor lizards). NOW I can argue that "Clidastes is most like the primitive Varanus, making Clidastes most primitive mosasaur in our group of three, and from there, Jormungandr is next, and then Mosasaurus is least like Varanus of the three, making it the most derived. So, if you want to understand change in any animal (in my case mosasaurs), you must understand the usual out-group (in my case Varanus). If you know the states of the skeleton in Varanus, you know the anchor to make comparisons off of. -The more your mosasaur of interest looks like Varanus, the less change it experienced and thus the more primitive it is. -The less your mosasaur looks like Varanus, the more change it had to undergo to accomplish that, so the more derived it is. Easy peasy. (note: this gets more complicated sometimes - derived animals can also evolve back into primitive traits. But don't worry about that for now) So, if you're interested in mosasaurs, look at Varanus. I fortunately found the absolute bible of Varanus anatomy today - I was so delighted I felt instantly obligated to make this post. Illustrated skeletal elements in great detail and descriptions of its morphology and physiology. This is invaluable to those interested in any lizards. Enjoy! https://www.notesonzoology.com/vertebrates/varanus/skeleton-of-varanus-with-diagram-vertebrates-chordata-zoology/8396