A close look at the developement of the prehistoric part of Ecco the Dolphin for the Genesis.
Everyone who honestly played the first Ecco game from start to beginning will remember the intriguing prehistoric part, announced by the Asterite's words "Use the machine to travel to the distant past before your kind existed. You will go back 55 million years!". And once you arrived there you knew, you are in a time setting where dinosaurs still rule the earth, extinct sea creatures swim around, pteranodons inhabiting the skies.
When I found a prehistoric themed book with illustrations by Zdenek Burian, there was a remarkable picture of a pteranodon inside. While at first I just thought it reminded me of its companion in the game I double-checked on my Mega Drive and indeed, the two looked exactly the same!
I've taken a closer look at the book, and found even more resemblances from the book and the game! Additionaly I noticed that the creatures have been taken from many different time periods. The variation is spread so wide that some of the creatures were actually already extinct at the time Ecco was. Let's take a closer look at this!
Of course I wanted to find out who was responsible for these graphics in Ecco games. The name is Zsolt Balogh, he is credited as a "great art talent" in Ecco 1. He must've been quite impressed with Burian's art, in fact it is very impressive! I've taken some time and integrated Balogh's pixel artwork from Ecco into the original pictures by Burian. You can easily compare both versions by that. Let's see if you can spot every piece by Zsolt...! ;-)
We'll list up all the lifeforms here and go into a bit more detail about them. We'll also try to put them in the correct time period. Take a geological time table, so you can keep track. There are gonna be some quite interesting results!
Let's first keep in mind that the Asterite said he'd send Ecco back 55 million years. Well, OK, if there's one thing most of us know about Earth's history, it's that dinosaurs became extinct 65 million years ago, at the end of the Cretaceous time period. So either the Asterite, being old and all, was a bit mistaken, or the Atlantean time machine was, or the game developers were. :-) In any case, it's safe to assume that we are supposed to be at the end of the Cretaceous. Times change often and sometimes heavily in geology, due to different dating methods and correlation. It's safe to assume that the developers got an "outdated" date for the Cretaceous upper boundary.

Jellyfish are one of the oldest groups of multi-cellular creatures on Earth. They've been recorded back till the Precambrian, the first ones being preserved in the so-called Ediacara fauna with around 600 million year old samples. They most likely floated around in the Cretacious like today.

Trilobites are a bit more complicated. They appeared with the beginning of the Cambrian (~540 m.y. ago) and have lived and evolved through the whole Palaeozoic era. They died out at the end of the Permian (~250 m.y. ago). So this is a first problem, Trilobites were already long extinct in the Cretaceos. The fact that this form Paradoxides lived only in the Mid-Cambrian doesn't help that fact!
The second problem is the way they are presented in the game. Everyone remembers those little pests swimming after you with high speed nagging at you draining your life meter. Let me tell you, they could never actually do that! Trilobites are rather crab-like creatures, crawling comparatively slowly on the ground, looking for carrion to eat with their small mouths. Actually I would've liked them better that way. But very weird indeed!

Snails, like all mollusc types, have appeared in the Cambrian. Most of their groups still live today and they haven't changed a lot in Earth's history. These should've been in the Cretaceous just like that.

Cephalopods have also appeared during the Cambrian times. These guys are supposed to be Nautiloids, and rather old forms of them. They rolled up their bodies during later stages in Earths history, and were eventually displaced the the well-known spirally Ammonoids (except for the Nautilus, of course). The last straight-shelled Nautiloids have disappeared at the end of the Triassic (~200 m.y. ago). I actually wonder why they didn't take in the pretty cool ammonites after all!

Let's take a look at some reef dwellers. These shells belong to brachiopods. They are not related to molluscs, and they just look similar to them at the first glance. I will not go deeper into that, though. They have appeared in the Cambrian and still live today. We'd expect them in the Cretaceos, so they're fine!

Corals exist since the Ordovician (~490 m.y. ago). The corals used here are two groups from the Paleozoic era: The colonial tabulate corals and the solitary rugose corals. Both groups built reefs during the Silurian and Devonian, then shrinked in amount and eventually died out at the end of the Permian together with the trilobites. Though, modern forms of corals do look pretty similar, so I guess these could've appeared in a similar way in the Cretaceous.

Stromatopores, hardly recognizable on this picture, are probably spongue-like beings. They've died out at the end of the Cretaceous, so they should be fine as well!

Dunkleosteus is a terrifying hunter indeed! He belongs to the group of placoderms. I think he is presented very realistically in the game! He had armoured plates and a powerful jaw and was most likely very fast with his strong tail fin. Though he lived only during the Late Devonian (~360 m.y. ago), placoderms got extinct in the Carboniferous at the latest. So, unfortunately this guy would not have appeared in the game either...

We also see some plants in the game. These ones are called horsetails. They are some of the oldest forms of plants and appeared in the Upper Devonian (~375 m.y. ago). They all look pretty similar and we still have some forms today, so it wouldn't be uncommon to find these in the Cretaceous I guess. One thing I'd note though, I doubt they would grow in the deep salt water like they are presented in the game. I'd rather expect them near some flat sweet water ponds, like depicted by Burian above.

Sigillaria belong to the lycophytes. While today they appear rather herbaceous, back in the Carboniferous (~350 m.y. ago) they could grow trees up to 40 metres in hight. They got extinct during the Permian, so we wouldn't have seen them in the game's set actually either.

Cycads are seed plants which appeared during the Early Permian (~280 m.y. ago) and haven't changed a lot since then. They are a pretty old group of gymnosperms still living today. They should've been like that in the Cretaceous as well!

The pteranodon lived only during the Cretaceous - practically the only one from this game who truly belonged to this time! I doubt he was strong enough to lift up a whole dolphin in the sky, but truly this is the least thing we should suspect right now anyhow ;-) He is a very nice feature, and very remarkable for this game.

As a last gimmick I'd like to add the well-known seahorse, which appears at least 2 times in the prehistoric part of the game. This has not been taken by Burian's drawings, but I thought I should add it, to complete this list of the prehistoric creatures. Interestingly these have appeared a lot later in Earth's history, in the Miocene (~13 m.y. ago). And neither do I believe would they have gotten such huge sizes. A truly weird yet beautiful monster the developers have added to the game. Weird humor, too!
Let me also mention the worm, or tentacle, which I don't have a picture of at hand right now. It's not part of Burian's pictures either, and I don't think anyone really knows what it is. But if it's something real, it most came from those rough ancient times of Earth! :-P
In the stage Origin Beach there are two messages hidden in the cliffs on the far left and right ends. You have to jump in the air and sing towards the cliffs to read them. (Actually they are two hidden glyphs, which have been covered by the rock, but you can make them visible only by manipulating the game with emulators, so that's out of interest.) Anyhow, here's what they say:
We hear song in the ocean!
We hear our kind in the sea!
Never have we heard songs in the sea!
You sing our song under the waves - who are you?
Could we sing in the sea? Could we live in the sea?
Perhaps we will try!
Ecco creator, Ed Annunziata, revealed that during the developement it was once planned that Ecco would see his ancestors at this point. Ecco always asked himself where his kind originated and why them, having to breathe air, must live in the oceans. So when he would sing to those creatures and they'd reply with the above he'd become the answer to that question himself. He'd be the reason why his kind lives in the sea.
I was a bit curious what those creatures, would've looked like, and where the cetaceans have their roots. So I googled around the web and found some interesting results! Cetaceans have evolved from land-living mammals, obviously. They have to breathe air, like previously said, and their skeleton resembles that of a mammal. Those land-living ancestors may have searched for food alternatives or hid from their natural enemies in the sea, and then could've adapted to live there (kind of like seals do today). Interestingly due to latest research the possibly earliest cetacean, Pakicetus has been spotted. It has lived 53 million years ago - that's pretty close to the Asterite's predictions now, isn't it?
A reconstruction of Pakicetus, the possibly earliest cetacean
What'd be of most interest for the Ecco game is, of course, what the ancestors of Ecco would've looked like. When I first read those messages in Origin Beach, I imagined something like the long-necked plesiosaurs sitting on the cliffs, and then jumping into the water after Ecco talked to them. Plesiosaurs are reptiles though, and they got extinct together with the dinosaurs.
Pakicetus was a hoofed-mammal, about the size of a wolf. Sure, his ancestors wouldn't have jump into the water and magically grow fins and swim with Ecco in the game. Ecco giving them the idea to use the sea as a place for living would be enough uniqueness. Though, Ecco talking to a wolve-like creature, that would be weird! Especially since, how could Ecco actually communicate with them? Sonar for land-living mammals is pretty much useless, cetaceans developed their sonar functions most likely when they transitioned to the sea already. Hence the realism of Ecco being able to communicate with his ancestors shrinks a lot.
I guess these are the main reasons why this idea got canned eventually, even though it is pretty nice one after all!
Following I will add two more steps in cetacean evolution. On the first picture Kutchicetus has been reconstructed, a crocodile-like mammal which might've lived like todays seals already. The second picture shows a reconstruction of Basilosaurus, not being a dinosaur of course, but one of the first true whales.
Summing up everything above we can safely assume that the game developers did not try to recreate a realistic scenario of the end of the Cretacious time period of Earth's history. Back in the days of Ecco's release dinosaurs and related things became quite popular. My guess is that the developers of Ecco were very fascinated by this topic and wanted to implement it in a way that would impress the players. They probably never expected anyone to double check if the scenario they created is actually realistic. Maybe they didn't even care about it themselves. What counts for most people is the sheer distance this old world produces. By distance I don't only mean in time, but also in nature. Like in many similar presentations of the ancient past it is presented in a very dramatic way in Ecco the Dolphin. You see volcanoes, seemingly fresh rocks, terrifying and obscure monsters and creatures, roots of evolution and many dangers. A very alien world noone would dare to enter. The world may have been dangerous at the time Ecco is supposed to be at, but it definitely did not look like that. But of course most people don't want or need to know all about this, which is totally fine. They enjoy what they're given and feel satisfied with it. Of course, except those who want to know more about it, like you and me! :-)
-Hauke
Evolution of Cetaceans on Wikipedia
Dedicated Website to Zdenek Burian
Zdenek Burian on Wikipedia