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Ancient Amphibians Discovered in Iowa

Ancient Amphibians Discovered in Iowa

Diagram of amphibian and reptile relationships
The evolutionary relationships of early land-living amphibians and reptiles are not well known. The Iowa discovery will add significantly to an otherwise scanty worldwide fossil record. Illustration by Patricia J. Lohmann.

Amphibians are the most primitive and earliest known tetrapods (four-footed animals), and are the basal stock from which all other land vertebrates, including reptiles, birds, and mammals, have derived. They represent the first successful attempt by vertebrate animals to migrate from the aquatic realm and colonize the land. It is generally accepted that amphibians evolved from bony crossopterygian (lobe-finned) fish (see diagram), but the early history and subsequent radiation of early tetrapods during the Carboniferous is still poorly known, principally because of the sparse number of fossil localities and specimens worldwide. In May 1985, an important new fossil amphibian locality was discovered in a quarry in southeast Iowa. The discovery will certainly rank high on the list of important fossil finds, not only for the state, but for the North American continent as well. This limestone quarry, in the upper St. Louis Formation, contains a 12 to 20-inch thick bone bed, which has yielded abundant and well-preserved fossil amphibian bone and fish material during preliminary collecting. The quarry is owned by Mr. Jasper Hiemstra, a landowner and farmer in the area. This discovery marks the first known occurrence of Paleozoic amphibians in Iowa.

 

Limestone/shale quarry wall
Abundant Mississippian-age amphibian bone was discovered within this unusually shaped limestone and shale-filled depression exposed in a quarry in southeastern Iowa. Photo by Bob McKay.

 

The bone bed occurs in the middle of a unique sequence of rocks which were deposited in a depression formed within the flat-lying limestones and shales of the upper St. Louis Formation. The unusual dish-shaped configuration of the deposit led to closer inspection and to discovery of the fossils. The basal half of the deposit consists of angular-to-rounded blocks and boulders of St. Louis limestone in a shale matrix containing scattered bone. Overlying this is the bone bed, a semi-continuous to lenticular, bone-rich limestone conglomerate with thin interlayered shales, also rich in bone. Above the bone bed is a sequence of bedded limestones and minor shales containing fish remains, ostracodes and snails, but lacking fossils suggestive of normal marine conditions. At the edge of the depression, these limestones overlap and rest on top of the St. Louis and represent the top of the rock sequence in the quarry. Pleistocene glacial till overlies the rock in the quarry.

The significance of the discovery lies not only in the apparent abundance of very rare fossils amphibians, but also that the deposit is of probable Mississippian age. Most fossil amphibians have been found in rocks of younger Pennsylvanian or Permian age, while only a dozen localities worldwide contain Mississippian specimens, most of these from the British Isles, and two from North America. The Iowa amphibians, though younger than the oldest known amphibians from the Devonian of East Greenland and Australia, are probably older than other Mississippian finds in North America. The antiquity of the Iowa site is important because primitive representatives of early amphibian groups might be present, and it is much closer in time to the important evolutionary transition between fish and amphibians. The Iowa site provides important fossil evidence from a portion of the Mississippian previously lacking tetrapod fossils. As such, the Iowa discovery helps fill in part of a long gap in the fossil record of ancient amphibians and potentially will provide significant insights into the evolution of the early tetrapods. Discovery of this site in Iowa, with its significant quantities of well-preserved fossil amphibian material, is a major paleontological event in the scientific quest to track the evolution of land-living vertebrates. This locality promises to be an unusually revealing "window on the past."

 

Amphibian skull fossil  
Rare fossils of early land-dwelling vertebrates found in Keokuk County rank high among North American fossil discoveries. This 340-million-year-old amphibian skull (approx. 9" x 7 1/2") shows exceptional preservation. Photo courtesy, The Field Museum, Chicago. Neg# GEO85147c.

 

Collections from the bone bed include disarticulated and semi-articulated cranial and post-cranial remains. These include: jaws with teeth, pelvic girdles, shoulder girdle elements, limb bones (femur, humerus, ulna, radius, and smaller carpal and phalangeal elements), vertebrae and ribs. Bone from the shale is typically compressed and flattened, while bone in the conglomerate has retained its original shape. Bones of several amphibians of varying size are present. The largest pelvis probably came from an individual whose total length exceeded six feet.

Many of the bones clearly represent amphibians from the subclass Labyrinthodontia, an important group of primitive amphibians. Described remains of early labyrinthodonts from the Devonian of Greenland and the Carboniferous of Scotland illustrate the skeletal anatomy of similar primitive amphibians. Early amphibians superficially resembled salamanders; they possessed elongate bodies, short limbs, and required water in which to lay their non-amniotic eggs to complete their reproductive cycle. Many primitive amphibians were quite large, however, some measuring six to ten feet in length. They were covered with bony scales, and the size and shape of their teeth demonstrate that they were voracious predators whose diet probably consisted mainly of fish. The environment in which they thrived probably was one of shallow, fresh- to brackish-water pools, ponds, and streams in lowlands marginal to the sea.

 

Lungfish fossil  
Skeletal remains of the rare lungfish include skull plates and scattered ribs. Photo by Brian Witzke.

 

The bone bed and associated sediments were deposited during a time interval after the regression of the St. Louis seas, but prior to the transgression of the seas which deposited the younger Pella Formation. During this time period an unconformity, or surface of erosion, was developed upon the St. Louis limestones. Formation of karst from the dissolution of limestones and perhaps deeper evaporite (gypsum) beds, plus erosion by flowing water sculpted subtle relief into an otherwise flat landscape. Streams and water-filled depressions on this surface were probably excellent locations for amphibians to feed and reproduce. More importantly though, these same depressions served as sites where skeletons and bones would be deposited, buried, and preserved from the destructive effects of weathering and decay.

The Field Museum of Natural History in Chicago secured a research grant from the National Geographic Society of Washington, D.C. to excavate the site, study the specimens, and learn what type of amphibians, fish, and other animals are present, as well as the subtleties of their habitat and mode of preservation.

 

Adapted from Iowa Geology 1986, No. 11, Iowa Department of Natural Resources