I was having a bath and thinking about the intelligence of animals. In particular, I was thinking about how intelligence had evolved in diverse evolutionary lineages, such as dogs, crows and octopuses. Then I wondered what the common ancestor of the dog, crow and octopus was.
So what is it? Also, what animals around today didn’t evolve from the common ancestor of the dog, crow and octopus?
I think the most restrictive clade including all three would be Nephrozoa.
I’m basing this on a very recent (like just now) degree in biology from the university of Wikipedia.
Nephrozoa encompasses almost all animals, and the vast majority of those with bilateral symmetry.
Bilateral exceptions appear to be Xenacoelomorpha
Non bilaterals include sponges, ctenophores, placozoans, and cnidarians.
Dogs and crows are deuterostomes, while octopuses are protostomes. From that latter page: “The common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal. The two clades diverged about 600 million years ago.”
Dogs and crows are very much more closely related than Octopuses. Both dogs and crows are vertebrates, while octopuses are mollusks. There are dozens of iterations of the “tree of life” over the last decade alone. Some stem species which produced mollusks would be the likely source for all the vertebrates. Jellyfish would probably not be descendants of that species. Maybe sponges, but I am not sure about that.
Tris
Here’s a pretty good (and up to date, I think) tree of life diagram that allows you to drill down.
You will find dog & crow under
\Nephrozoa\Deuterostomia\Chordata
You will find the octopus under
\Nephrozoa\Protostomia.…\Mollusca
The MRCA of dog, crow & octopus is (I think) the ancestor of all animals with any kind of central nervous system.
Neurons evolved earlier than this, however. The outgroups Cnidaria and Ctenophora have a decentralized nervous system. Porifera do not.
This has more:
Thanks for the replies.
So, “a worm-like aquatic animal” looks to be the common ancestor of the dog, crow and octopus then. I suspected it might be something like a jellyfish for the living non-dog/crow/octopus ancestor.
These names made me think of the alien species in Star Trek. 
From the first article:
The common ancestor of dogs and crows is going to be some kind of dinosaur. Birds came from dinosaurs. Mammals came from therapsids which came from dinosaurs.
Molluscs are a whole other ballgame. They are protostomes, so you’ll have to go way back to find that common ancestor.
No, a jellyfish is not a bilaterian. It’s in the phylum cnidaria. The last common ancestor between the dog, crow and octopus is still a bilaterian. I think the urbilaterian is how it’s referred to as.
Incorrect.
Synapsids (including Therapsids and mammals) and reptiles (including dinosaurs and birds) split about 100 million years before the dinosaurs appeared; mammals themselves predate the dinosaurs. In fact, nether is descended from the other - they both evolved from the basal amniotes, which were the first vertebrates that could lay eggs on land.
This is a cool visualization:
http://lifemap-ncbi.univ-lyon1.fr/
In the lower left, where it says “imput[sic] multiple taxid”, type this in and click “view”:
28725 6643 9608
(That’s just the codes for octopus, corvids, and canines).
There’s also this text-based tool using a similar dataset:
https://www.ncbi.nlm.nih.gov/Taxonomy/CommonTree/wwwcmt.cgi
It returns this:
Metazoa
+-Chordata
| ++Aves
| | \-Corvus
| \+Mammalia
| \-Canidae
\-Mollusca
\-Cephalopoda
\-Octopoda
\-Octopodidae
\-Octopus
-----------------------------------
Or phylogenetically:
Bilateria
+-Deuterostomia
| \-Chordata
| \-Craniata
| \-Vertebrata
| \-Gnathostomata
| \-Teleostomi
| \-Euteleostomi
| \-Sarcopterygii
| \-Dipnotetrapodomorpha
| \-Tetrapoda
| \-Amniota
| ++Sauropsida
| | \-Corvus
| \+Mammalia
| \-Canidae
\-Protostomia
\-Lophotrochozoa
\-Mollusca
\-Cephalopoda
\-Coleoidea
\-Neocoleoidea
\-Octopodiformes
\-Octopoda
\-Incirrata
\-Octopodidae
\-Octopus
-----------------------------------
But none of these are as exciting as “worm-like aquatic animal” 
So is there any suggestion what the common ancestor would have looked like? (Maybe something more exciting than aquatic worm-like animal? He said hopefully )
The first amniote (the group including the synapsids and reptiles) was probably small lizard-like thing called Casineria. Wiki for Casinaria.
And the worm-like animal was probably similar to a planarian. Superficially, at least-- modern planaria are just as highly evolved as any other creature on the planet, and have probably developed all sorts of differences “under the hood”.
The book
Other Minds: The Octopus, the Sea, and the Deep Origins of Consciousness
by Peter Godfrey-Smith
is well worth a read. The author spends some time talking about exactly your question.
It would probably be something like the previously mentioned Xenocoelomorpha, including “flatworms” like Xenoturbella. They are bilaterally symmetric, but have a simple digestive system with a single opening that serves as both mouth and anus. They are regarded as the “sister group” to the group that includes the ancestors of both vertebrates and mollusks, so the common ancestor of all these groups most likely looked something like this. (The distant common ancestor of most animals above sponges and jellyfish is going to be some kind of “aquatic worm-like animal.”)
As mentioned, mollusks and vertebrates differ fundamentally in their early embryology in the way the digestive system acquires a second opening. In mollusks, the first opening becomes the mouth, and the anus forms later. In vertebrates, the anus is the first opening, and the mouth forms second.
Everyone’s just an asshole when they’re young.
Or a big mouth, apparently.