So I was reading about hackled orb spiders
http://en.wikipedia.org/wiki/Uloboridae
They’re apparently not venomous which it states is a secondary evolved trait. What does that mean exactly? (I think I’ve heard the cartilage of sharks/rays is a secondary evolved trait as well. So what does that mean?)
The Wiki article has it wrong. The term is “secondarily,” not “secondary.” It means that the trait was found in an ancestral form, but has been lost in the descendant form; in other words that the ancestors of the spider (like almost all spiders) were venomous.
For another example, ostriches are secondarily flightless; that is, they are descended from birds that flew. This is unlike, say, crocodiles, which are primitively flightless; their ancestors never flew.
Likewise, sharks are thought be descended from fish with bony skeletons. The cartilagineous skeleton is a secondarily evolved trait, rather than being a primitive (ancestral) condition.
Oh, so that means for example sharks and rays used to have regular skeletons and evolved to have cartilage?
Technically, it means that the ancestors of sharks and rays had regular skeletons.
The cartilaginous fishes are often depicted as being more “primitive” than bony fishes because of their skeletons. However, both groups appear to be descended from either the placoderms or other early groups of fish that had bony skeletons. In vertebrates with bony skeletons, in the embryo/juvenile the skeleton is first formed of cartilage, which is later replaced by bone as the animal grows. Sharks and rays have retained this juvenile condition into adulthood.