The computer-generated model used for the Dimetrodon hatchlings is actually drastically smaller version of the fully grown Dimetrodon animation, with apparently the same body proportions and color-scheme. So when one of them (the newly hatched animals) is shown climbing up a tree, the viewer is treated to a rather awkward sight, since it looks just like an adult, to which of course it wouldn't be possible to climb up a tree.
The giant spider that appears in the first segment of the episode is referred to as "Mesothelae", which is although not a proper name of a specific genus or species, the series tends to refer to a number of animals by the name of the animal group they belong to. Originally, this Mesothelae was meant to be a Megarachne, possibly the biggest spider ever discovered, until during the series' production, it turned out that "Megarachne" wasn't a spider at all, but an eurypterid (or sea scorpion). By this time, though, since the production was already underway, all they could do was to change the name of the spider. The error was not fixed completely, though, as (now that Megarachne has been re-classified) there is no evidence of any spider being as large as the "Mesothelae" depicted in this program.