Gruiform Morphology

As you can see from the pictures in the first post, the Gruiformes are very diverse in the way they look.  Faiq posted a great question: how are the rails related to the cranes?  They look nothing alike.  According to Gill, the order Gruiformes has few unifying characteristics.  The Wikipedia site states that some of the birds in this order were placed here simply out of necessity to place them somewhere, and hence, the order lacks apomorphies (derived characters found only in that group that would tie them together evolutionarily).  However, this statement, while plausible, is uncited.   The following tree, from Harshman, shows current Gruiform phylogeny:



As far as common morphology, the the similarities are:

• No member of the order has a crop, a digestive organ that serves to store food for later consumption or regurgitation; however, many birds, such as owls, also do not have crops, so this is not necessarily an apomorphy that evolved strictly in the Gruiformes.  In fact, since, as Gill states, the order is old, it may be the primitive character (no crop). (Gill)
• Most of the Gruiformes share "certain skeletal and palatal features". (Gill)  Livezey exhaustively covers these minute and numerous similarities in a 1998 paper.
• Most of the birds in this order have an oil gland. (Gill)
• Most of the birds in this order have "pervious nostrils", which are nostrils that, lacking a septum, are basically "see-through". (Gill)
• The Limpkin, which is the only species in the family Aramidae, is a larger wading bird which is the most crane-like in habit of the non-cranes in the order.  Harshman has the Limpkin most-closely related to the Cranes in his phylogenetic tree (see above).

Sandhill Crane (above) and Limpkin (below)

A great example of pervious nostrils in a Vulture - NOTE: Vultures ARE NOT Gruiformes!

An illustration detailing the crop.  None of the Gruiformes have a crop.

The morphological differences, however abound in these birds.  Rails (family Rallidae), such as the Black Rail, Virginia Rail, and King Rail, look much different than the, the American Coot (also in the family Rallidae), which superficially resembles a duck, dives for aquatic plants, and has lobed toes for swimming.  Many rails are also flightless, while their Gruiform cousin, the Cranes, undertake agonizingly long migrations.  Cranes are also very tall, gracile birds, while the Rails are shorter and rounder, some being almost grouse-like in shape and size.

A Coot and a Rail together.  One would not immediately leap to the conclusion that these birds are both in the same family.

A Whooping Crane, which superficially resembles an ibis or a heron more than a Rail or a Coot.

The foot of the American Coot, lobed for swimming, sets it apart not just from the other Gruiformes, but also of the other birds in its family, Rallidae.

As one can imagine, the systematics for this order have been difficult to sort out.  Harshman states that the Gruiformes are not a "natural group", and thus, many bird family have been plucked out of this order recently and put into other orders.  The Plains Wanderer (Pedionomidae) and Buttonquail (Turnicidae) have been removed from Gruiformes and placed in Charadriiformes.  Also the Seriemas (Cariamidae), Bustards (Otididae), Mesites (Mesitornithidae), Kagu (Rhynochetidae), and Sunbittern (Eurypygidae) have recently been removed from Gruiformes.

The Barred Buttonquail, which may superficially resemble a rail, has been removed from the order recently.

Sources:

• Gill, F. 1994. Ornithology, pp 641-643. W.H. Freeman and Company, New York, NY.
• Harshman, J. Gruiform Phylogeny. http://tolweb.org/Gruiformes/26307. Retrieved June 12, 2012.
• Livezey, B. C. 1998. A phylogenetic analysis of the Gruiformes (Aves) based on morphological characters with an emphasis on the rails (Rallidae). The Royal Society 2077-2151.
• Livezey, B. C. and R. L. Zusi. 2007. Higher order phylogeny of modern birds (Therapoda, Aves: Neornithes) based on comparitive anatomy. II. Analysis and discussion. Zoological Journal of the Linnean Society 149:1-95. 
• Ramel, Gordon. Gruiformes. http://www.earthlife.net/birds/gruiformes.html. Retrieved June 12, 2012.
• Wikipedia. Gruiformes. http://en.wikipedia.org/wiki/Gruiformes. Retrieved June 12, 2012.