I suppose I won't be wrong saying that people, even if interested in evolution and palaeontology, are rarely aware of the fact how two Devonian mass extinctions shaped the way our current world of land vertebrates looks. I have to confess I wasn’t. Until I have read the book by George R McGhee Jr. ‘When the Invasion on Land Failed. The Legacy of the Devonian Extinctions.’ It's one of the best popular science books I have been read last year and I strongly recommend you to try it. But for the goal of this entry I can summarise it's content in only one, my favourite, passage:
‘In the world where the Devonian extinctions had never occurred, perhaps we, with our characteristic modesty, might have called ourselves the Eutetrapoda, the “true four-limbed vertebrates”. Our cousins the Elpistostegalian tetrapods we might have called the Paratetrapoda, the “near, or similar four-limbed vertebrates”, and our more distant Tristichopterid tetrapod relatives the Pseudotetrapoda, the “false four-limbed vertebrates”’
So I have a proposition for you all - let's try to invent what kind of organisms would inherit the land with us if tetrapodomorph evolutionary tree was not that severely depleted in Devonian with more than one lineage of land vertebrates surviving.