The Branching Tree

Many existing bird orders trace back to Enantiornithes niche subgroups.

Quote:

http://en.wikipedia.org/wiki/Enantiornithes so that it is possible that enantiornithines may actually represent successive outgroups on the lineage leading to modern birds. AND http://en.wikipedia.org/wiki/Apsaravis This would mean that, rather than a radiation of primitive birds separate from the radiation that led to modern birds, "enantiornithines" would actually be steps along the way to becoming modern birds.

AND

Quote:

http://en.wikipedia.org/wiki/Enantiornithes Enantiornithine fossils appear to include waders, swimmers, fish-catchers, and hook-beaked raptors. And http://en.wikipedia.org/wiki/Bird Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters.

Let us say for the sake of simplicity that there are 3 niche groups within enantiornithes (waders, seabirds and landbirds). Think of these as 3 major branches. From each of these major branches, a number of smaller subbranches branch off. For example, the subbranches off the wader branch lead to the variety of modern wader orders. The subbranches off the seabird branch lead to the variety of modern seabird orders. The subbranches off the landbird branch lead to the variety of modern landbird orders.
It is a branching tree. In this picture there is increasing specializations and expansion within the niche and expansion across the globe exploiting that niche..
For example, the wader groups of Enantiornithes developed into a variety of specialized wader bird orders and expanded across the globe in their niche.