Gizzard Group

The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including archosaurs ( pterosaurs, crocodiles, alligators, dinosaurs, birds), earthworms, some gastropods, some fish, and some crustaceans. This specialized stomach constructed of thick muscular walls is used for grinding up food, often aided by particles of stone or grit. In certain insects and molluscs, the gizzard features chitinous plates or teeth.

Etymology

The word ''gizzard'' comes from the Middle English ''giser'', which derives from a similar word in Old French ''gésier'', which itself evolved from the Latin ''gigeria'', meaning giblets.

Structure

In birds

Birds swallow food and store it in their crop if necessary. Then the food passes into their glandular stomach, also called the proventriculus, which is also sometimes referred to as the true stomach. This is the secretory part of the stomach. Then the food passes into the gizzard (also known as the ''muscular stomach'' or ''ventriculus''). The gizzard can grind the food with previously swallowed grit and pass it back to the true stomach, and vice versa. In layman's terms, the gizzard 'chews' the food for the bird because it does not have teeth to chew food the way humans and other mammals do.

By comparison, although in birds the stomach occurs in the digestive tract prior to the gizzard, in grasshoppers the gizzard occurs prior to the stomach, while in earthworms there is only a gizzard, and no stomach.

Koilin lining

In order to protect the muscles of the gizzard, the organ has a multi-layered membrane - or gastric cuticle - made of koilin, a carbohydrate-protein complex (and not keratin as once believed) to protect the muscles. The thickness of this membrane varies with the types of food the animal eats, with diets heavier in grains, seeds, and insects creating thicker membranes than those consisting of fruits, worms, meat, and other softer edibles. In some animals the membrane is slowly worn and replaced over time, while others will discard the worn lining in its entirety periodically.

The lining is critical to the proper functioning of the gizzard, but in some animals it can play an additional role as well. The male hornbill, for example, will fill its gizzard with fruit and then slough off the entire membrane to present it like a 'bag of fruit' to its mate during the nesting season.

Gizzard stones

Some animals that lack teeth will swallow stones or grit to aid in fragmenting hard foods. All birds have gizzards, but not all will swallow stones or grit. Those that do employ the following method of chewing:

These stones are called ''gizzard stones'' or '' gastroliths'' and usually become round and smooth from the polishing action in the animal's stomach. When too smooth to do their required work, they may be excreted or regurgitated.

Animals with gizzards

Mammals

Pangolins lack teeth and grind their food in a gizzard-like structure.

Birds and other archosaurs

All birds have gizzards. The gizzards of emus, turkeys, chickens, parrots, and ducks are most notable in cuisine (see below). Crocodilians such as alligators and crocodiles also have gizzards.

Dinosaurs that are believed to have had gizzards based on the discovery of gizzard stones recovered near fossils include:

* '' Psittacosaurus''
* '' Massospondylus''
* ''Sellosaurus''
* '' Omeisaurus''
* '' Apatosaurus''
* ''Barosaurus''
* '' Dicraeosaurus''
* ''Seismosaurus''

The belief that ''Claosaurus'' had a gizzard has been discredited on the grounds that the fossil remains this claim was based on were another species and the stones merely from a stream.

At least some pterosaurs seemingly had gizzards. The most notable cases are '' Pterodaustro'' (inferred via gastroliths) and '' Rhamphorhynchus'' (directly preserved).

Conversely, it is thought that some extinct birds like Enantiornithes didn't have gizzards.

Fish

The mullet ( Mugilidae) found in estuarine waters worldwide, and the gizzard or mud shad

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