(facepalm) Paleontologists have determined this based on morphological differences among other things. Extant coelecanths are approx. 50% larger than their distant ancestors. There’s also the fact the species lived hundreds of millions apart.
I’m waiting for your explanation for the vertical dino tracks and angular unconformities too. 
That’s not a (facepalm), it’s a major problem for your argument here. Biologists don’t define species based upon morphological differences at all. They define it based upon ability or inability to produce fertile offspring through interbreeding. Let’s decide if these are different species based upon morphology, shall we?
compared to
… yet, all dogs are the same species.
Yep, same species!
Sure, you can add crocodiles and turtles as well, although these are not identical to modern versions. Nobody holds that every family goes extinct at the same time or even ever. But here you have another problem in the opposite direction. Coelocanths or horseshoe crabs are never found with trilobites or any other Cambrian creature.
In a flood scenario, it is entirely legitimate to expect to find dinosaurs mixed with elephants, bears, wolves, or other animals of like size or distribution. These modern mammals are never found below the K-T boundary. There are zero dinosaur fossils above the K-T boundary.
How does this observation constitute evidence for a global flood?
No they don’t. Two extant populations are defines as different species if there in no gene flow between their respective gene pools in their natural habitat. Not that they can’t interbreed, that they don’t in the wild. Of course in the real world there are plenty of gray areas where similar species overlap i.e. grizzlies and polar bears.
Tell me how extant coelecants bred with ancestors which lives 100 MYA.
Oh, don’t forget the vertical dino tracks and angular unconformities. You bragged about Flood geology, time to back up the boasts.
Things don’t need to be identical to be the same species. Are you retracting your statement then?
Are you seriously suggesting that if I were to resurrect a fossil coelocanth and place it together with a modern version, they would be so far apart as to be unable to interbreed? Or that for some reason they would refuse to do so?
Are you really claiming every member of the order Coelacanthiformes is the same species??? 
This is getting pedantic. Turtles and crocodiles have never been designations for species.
Yep. He can’t deal with the scientific evidence so empty rhetoric is all he’s got.
Besides PDPrice is looking for any excuse to keep ignoring his Flood geology claims and his explanation for the vertical dino tracks and angular unconformities. Right PD?
Your inability to define something as a ‘separate species’ in the fossil record is going to be a major achilles heel for you!
No you are the one that made a claim (that they are not the same species). So back it up. Pointing to some minor morphological differences is not sufficient at all.
There you have YEC science at its finest folks. All turtles for the last 230 million years are the same species. All crocodile for the last 250 million years are the same species.
Easy.
Why coelacanths are not ‘living fossils’ A review of molecular and morphological data
Vertical dino tracks. Angular unconformities. Still waiting for your Flood explanation.
Never seen this paper before, thanks for the reference. After very briefly scanning through it, though, it seems to be arguing that there are morphological differences:
This result is additional support, if needed, that extant coelacanths have not remained morphologically static since the Devonian.
But that misses the whole point. Who was claiming there were no morphological differences? The problem is that you cannot assume that just because there are morphological differences that proves they are a different species. This paper seems to be scientific semantics and wordplay at its worst.
Do you think T. Rex was a different species from Triceratops? Why or why not?
Call 'em species, families, even kind if you want. There are still no duckbill dinosaurs buried with elephants. Of the hundreds of thousands of dinosaur fossils in catalog, none are from strata above the K-T boundary. Trilobites are found around the world, thick as lice, but none with any modern animal of any type.
Yeah, I think so. They had a completely different body plan. That’s a clear and obvious case of being a different species. Even the dogs I showed here have the same body plan, just very different dimensions.
LOL! Why did I know PDPrice would hand-wave and ignore the scientific data?
Just one more sad example of a YEC talker, not a thinker or a doer. Like most YECs he seems to have majored in Empty Rhetoric 101.
Oh, and he’ll never address the geology of those vertical dino tracks or angular unconformities. Like most YECs he’s only brave when he can find a pre-canned YEC article to C&P.
What you call ‘empty rhetoric’ I call critical thinking.
So they have morphological differences. Not sure how you’re defining “body plan” here.
How about sabre-toothed cats and hyenas? Do you think they’re different species?
