Arroyo Formation facts for kids
Quick facts for kids Arroyo FormationStratigraphic range: Kungurian |
|
|---|---|
| Type | Formation |
| Unit of | Clear Fork Group |
| Underlies | Vale Formation |
| Overlies | Waggoner Ranch Formation |
| Thickness | 250 ft (76 m) |
| Lithology | |
| Primary | Limestone, mudstone |
| Other | Gypsum |
| Location | |
| Coordinates | 33°48′N 99°12′W / 33.8°N 99.2°W |
| Approximate paleocoordinates | 0°24′N 29°36′W / 0.4°N 29.6°W |
| Region |  Texas |
| Country |  United States |
| Type section | |
| Named for | Los Arroyos, Runnels County, Texas |
| Named by | Beede & Waithe |
| Year defined | 1918 |
The Arroyo Formation is a special type of rock layer found in Texas. It holds many fossils from a very long time ago, specifically from the Kungurian stage of the Permian period. This was about 270 million years ago!
The Arroyo Formation is the lowest part of the Clear Fork Group. This group is famous for its "red beds" in the south-central United States. These red beds are known for having lots of fossils.
Contents
What is the Arroyo Formation?
The Arroyo Formation is the oldest and easternmost part of the Clear Fork Group. It stretches northeast from Concho County all the way to Wilbarger County in Texas. In Oklahoma, similar rock layers are found in the upper Garber Formation and at Richards Spur.
Southern Texas Area
The Arroyo Formation was first named in 1918 by scientists Beede and Waite. They found its main location, called the type locality, near Los Arroyos in Runnels County, Texas. Here, they saw layers of limestone, shale, and gypsum.
In this southern area, the Arroyo Formation has layers of mudstone mixed with several distinct layers of marine limestone. Four of these limestone layers even have names: Rainey, Lytle, Kirby Lake, and Standpipe Limestones. These limestone layers don't appear everywhere in the southern area.
These limestone layers often contain many fossils. You can find teeth and scales from ancient fish, and shells from clams. Worm burrows and tiny creatures called ostracods are also very common.
Fossils of land animals (called tetrapods) are also found. These include teeth from Dimetrodon, small bones from Lysorophus and Diplocaulus, and parts from Eryops. Most of these bones are from small animals. This might be because smaller bones were easier to move around by water long ago.
Northern Texas ("Classic") Area
North of the Salt Fork Brazos River, the Arroyo Formation shows what an ancient floodplain looked like. This area, called the "Classic area," is in Baylor, Wilbarger, and Wichita counties. Scientists have studied it a lot because it has so many plant and animal fossils from the Permian period.
The rocks here are mostly red clays, shales, and mudstones. Sometimes, you can find layers of sandstone or conglomerate (rock made of pebbles). There are also small lumps of carbonate rock.
It can be tricky to tell the Arroyo Formation apart from the Vale Formation above it in this northern area. This is because the special limestone layers found in the south are missing here. Because of this, some scientists just call these northern red beds the "Clear Fork Formation" as a whole. The Arroyo Formation would be like the "Lower Clear Fork Formation" in this system.
The "Classic area" of the Arroyo Formation is one of the best places to find fossils in the Texas Red Beds. It's known for its amazing collection of early Permian land animals. These include many types of amphibians, pelycosaurs (which were early relatives of mammals), sharks, and early reptiles.
One important fossil found here is Edaphosaurus pogonias, the last and largest known species of its kind. Other pelycosaurs like Secodontosaurus, Varanosaurus, and several types of Dimetrodon were also common. Early reptiles like Captorhinus and Labidosaurus were the most common reptiles.
Many aquatic amphibians, such as Diplocaulus, Trimerorhachis, and Eryops, are found here. Land-dwelling amphibians like Seymouria and Diadectes were also present. Some of these land amphibians did not survive into the next rock layer, the Vale Formation.
Scientists have found burrows in the middle part of the Arroyo Formation. These burrows contain fossils of animals like Brachydectes, Gnathorhiza (a lungfish), and Diplocaulus. These burrows suggest that the climate might have been drier during that time.
The most common shark fossils found are teeth from Orthacanthus platypternus. Teeth from Xenacanthus luederensis are also known from some early sites.
Ancient Animals of the Arroyo Formation
The Arroyo Formation is famous for its incredible collection of ancient animals. Here are some of the types of creatures that lived there:
Amphibians
Many different amphibians lived in and around the waters of the Arroyo Formation.
- Lepospondyls: These were a group of small, often snake-like or salamander-like amphibians.
- Brachydectes: A common worm-like amphibian.
- Diplocaulus: A very distinctive amphibian with a boomerang-shaped head.
- Pelodosotis: Another type of small amphibian.
- Temnospondyls: These were a diverse group of amphibians, some very large.
- Acheloma: An amphibian with a broad, flat head.
- Aspidosaurus: An amphibian with bony armor on its back.
- Broiliellus: Another armored amphibian.
- Cacops: A well-known amphibian with a short, wide skull and armor.
- Eryops: A large, crocodile-like amphibian that was very common.
- Trimerorhachis: A common aquatic amphibian.
Reptile Relatives
These animals were early relatives of reptiles and mammals.
- Seymouriamorphs: These were a group of animals that shared features of both amphibians and early reptiles.
- Seymouria: A well-known animal that looked like a sturdy lizard.
- Diadectomorphs: These were among the first large plant-eating land vertebrates.
- Diadectes: A large, heavily built animal that ate plants.
Synapsids (Mammal Relatives)
Synapsids are a group of animals that includes mammals and their ancient relatives. Many important synapsids lived in the Arroyo Formation.
- Casea: A large, barrel-bodied plant-eater.
- Dimetrodon: A famous predator with a large sail on its back. Several species are found here.
- Edaphosaurus: A plant-eater also known for its sail, but with bony knobs on it.
- Secodontosaurus: A sail-backed predator, similar to Dimetrodon but with a longer, narrower skull.
- Varanosaurus: A smaller, lizard-like predator.
Eureptiles (Early Reptiles)
These were some of the earliest true reptiles.
- Araeoscelis: A slender, lizard-like reptile.
- Captorhinus: A small, common reptile.
- Labidosaurus: Another type of early reptile.
Images for kids
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Brachydectes
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Pelodosotis
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Acheloma cumminsi
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Cacops aspidephorus
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Diadectes
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Dimetrodon grandis
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Edaphosaurus pogonias
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Captorhinus
