Charadriomorpha: Overview

The Charadriomorphs

The Charadriomorphs are perhaps the original radiation of neornithine shore birds, which may make them very ancient indeed. Like everything in neornithine phylogeny, the idea is vague and controversial. We are tentatively following the path of Feduccia (1999). Many charadriomorphs have remained shorebirds, with some spectacular exceptions in the vultures, doves and parrots. However, their feeding habits differ greatly. The group includes filter feeders (flamingos), wading birds (storks), and divers, both arial (gulls) and aquatic (penguins). Except for the parrots, they have little vocal ability. All communicate by means of complex behavioral rituals, as might be expected in a group that is frequently colonial. Sexual dimorphism is uncommon, and the young are usually independent at a relatively young age.

Phylogeny and Origins

There are probably as many bird phylogenies as there are avian taxonomists. The neornithines seem to have radiated either very quickly or very quietly. Anatomically, birds tend to be very much alike. However, this doesn't necessarilly tell us a great deal. Sauropods, for example, are all very similar, probably because of the engineering constraints imposed by their great size. Similarly, birds may be unable to depart far from the basic plan without losing the ability to fly. Their similarity doesn't have to result from a very quick, short period of explosive radiation. In fact, its hard to see how a group with reasonable flying abilities could  radiate explosively. The individuals all have the potential to come in contact with sexual partners over a very wide radius, making genetic isolation difficult to accomplish.

Then, do we assume that the neornithine radiation was a slow and gradual matter which happened to leave a poor fossil record?  Unfortunately, we can't jump to that conclusion either. Many birds have lost the ability to fly, some groups (ostriches, for example) for tens of millions of years. Yet they have remained anatomically very bird-like. The general avian anatomical plan may simply be an adaptive one-way street, or even a dead end. As to the problem of genetic isolation, one only needs to look at Darwin's finches on the Galapagos. These birds, probably descended from a single pregnant female only a few millions of years ago, have diverged considerably in a geologically short period without much physical separation. Instead, selection has favored groups who specialize in different food sources or, more correctly, specialization in how food is obtained. The unspecialized generalists were less succesful than any of the specialists, so that the genetic link between the food specialists was quickly lost.

So why is the fossil record so bad?  Why can't we see a long fossil history of neornithines gradually acheiving the diversity of Eocene times when most of the modern orders were well-established? Bird bones are thin, small and hollow and do not fossilize well; but we are gradually accumulating a good many enantiornithine -- and perhaps even archaeornithine -- fossils. Lake and ocean margins are great for preservation of even the most delicate fossils. What happened to the neornithines? Were they simply very rare?

These are all questions without anything close to reasonable answers. One day, perhaps rather soon, we will be able to hazard some guesses. At the moment, we can only say -- and even this with considerable uncertainty -- that the charadriomorphs are one of at least four neornithine groups which were likely present around the end of the Cretaceous, the others being the gruimorphs, palaeognaths, and the closely-related chicken-duck group. It is likely that they were shorebirds, not too distantly related to the Eocene filter feeder Presbyornis probably a duck). Like Presbyornis, they may have been colonial filter feeders; but, at least in light of their later diversity, it is more likely that they were wading hunters of fishes, crustaceans and even insects and shoreline carrion. They were, and often remain, rather unspecialized feeders.

This is the sort of lifestyle that might easily develop on the genetic, as well as physical, edges of a  group of wading filter-feeders, as some groups sought out food sources that were a little too big, or too deep or too active for the colony as a whole. Just as likely, the filter-feeders may have evolved from something closer to a charadriomoprh as populations expanded close to lakes rich in algae and organic detritus. In either case, both the origin and later history of the group is understandable in terms of the same dynamic that created the finches of the Galapagos Islands: speciation by feeding method, without fundamental anatomical change.

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