Gaiman Formation

Outcrop map and stratigraphic column of the Gaiman Formation in the Peninsula Valdes Basin

The Gaiman Formation was first defined by Mendía & Bayarsky in 1981,[1] and further described by Mendía in 1983, taking its name from Gaiman in Chubut Province, eastern Patagonia.[2] Gaiman in the local language of the native Tehuelche people means "rocky point".[3] The formation crops out in the lower course of the Chubut River,[4] and overlies the continental Trelew Member of the Sarmiento Formation and is overlain by the Late Miocene Puerto Madryn Formation. The formation is 70 metres (230 ft) thick,[5] and comprises marine tuffs, tuffaceous mudstones, sandstones and coquinas.[1]

The Gaiman Formation is correlated with the Chenque Formation of the Golfo San Jorge Basin and the Monte León Formation of the Austral Basin to the south, the Saladar Member of the lower Gran Bajo del Gualicho Formation of the Colorado Basin to the north and the Vaca Mahuida Formation of the Neuquén Basin to the northwest.[6]

Fossils of the Gaiman Formation in the Bryn Gwyn Paleontological Park

The Gaiman Formation has provided several fossil penguins, of which nine are assigned to specific species and include five species of the genus Palaeospheniscus. Also, shark teeth, fossils of indeterminate seals and turtles, and several dolphins, baleen and toothed whales have been found in the formation. Argentine paleontologist Florentino Ameghino described an isolated tooth from the formation first as an ichthyodectiform fish, Portheus patagonicus in 1901, however this fossil was assigned to a delphinoid cetacean of the family Kentriodontidae by Cione and Cozzuol in 1990.[10] Fish recorded from the formation are hexanchiform hexanchids, lamniform isurids, odontaspidids, cetorhinids, carcharhiniform carcharhinids, heterodontiform heterodontids, squatiniform squatinids myliobatiform myliobatids, perciform oplegnathids and labrids, and tetraodontiform molids.[11] Rays are abundant in the formation.[12] The lack of somatic remains of most molluscs, bryozoans, polychaetes and cirripeds in the formation is attributed to the high-energy and corrosive environment at time of deposition, as well as diagenesis after the formation was deposited.[13]

The bird species Eretiscus tonnii solely occurs in the Gaiman Formation.[14] Bite marks on several of the penguin bones are attributed to terrestrial mammals such as didelphid or hathliacyniid sparassodont marsupials, common in Patagonia during the Miocene.[15] Other ichnofossils found on the bones are presumably caused by dental erosion by regular echinoids,[16] and bite marks by sharks, as Galeocerdo aduncus.[17]

The paleontological richness of the formation, as well as the more fossiliferous underlying Sarmiento Formation, led to the establishment of the Bryn Gwyn Paleontological Park.[18]

Fossil content

The formation has provided the following fossils:[19][20][21][22][23]

Class	Group	Fossils	Image	Notes
Mammals	Dolphins	Aondelphis talen		[24]
		Dolgopolis kinchikafiforo		[25]
		Kentriodon sp.		[12]
		Eurhinodelphinidae indet.		[20]
	Toothed whales	Argyrocetus patagonicus		[12]
		Idiorophus patagonicus		[26]
		Phoberodon arctirostris		[12]
		Prosqualodon australis		[12][27]
		Squalodontidae indet.		[20]
	Baleen whales	Aglaocetus moreni		[26]
		Morenocetus parvus		[26]
		Plesiocetus dyticus		[26]
	Seals	Phocidae indet.		[22]
Birds	Penguins	Apterodytes ictus		[21]
		Eretiscus tonnii		[28]
		Palaeospheniscus bergi syn Palaeospheniscus planus, P. rothi		[28][19]
		Palaeospheniscus bilocular		[28]
		Palaeospheniscus gracilis		[20]
		Palaeospheniscus patagonicus syn Palaeospheniscus affinis, P. intermedius, P. interruptus		[19][29]
		Palaeospheniscus wimani		[22]
		Paraptenodytes antarcticus		[28]
		Paraptenodytes robustus syn. Treleudytes crassa		[19][29]
		Arthrodytes sp.		[20]
		Sphenisciformes indet.		[20]
	Anseriformes	Cayaoa bruneti		[4][30]
		Anseriformes indet.		[20]
Reptiles	Turtles	Dermochelyidae indet.		[20]
Fish	Sharks	Carcharoides totuserratus		[17]
		Echinorhinus pozzii		[12]
		Galeocerdo aduncus		[17]
		Megascyliorhinus trelewensis		[12]
		Heterodontus sp.		[12]
		Odontaspis sp.		[12]
		Pristiophorus sp.		[12]
		Squalus sp.		[12]
	Oplegnathidae	Oplegnathus sp.		[12]
Mollusks	Bivalves	Crassostrea ?hatcheri		[23]
		Balanus sp.		[31]
	Gastropods	Turritella sp.		[31]
Ichnofossils	Bioturbation	Chondrites, Ophiomorpha, Planolites, Skolithos		[11]
	Bite marks	Gnathichnus pentax		[16]
		Entobia		[16]

• South American land mammal ages
• Uitpa Formation, contemporaneous fossiliferous formation of the Cocinetas Basin, Colombia
• Castillo Formation, contemporaneous fossiliferous formation of the Falcón Basin, Venezuela
• Biblián Formation, contemporaneous fossiliferous formation of Ecuador
• Bahía Inglesa Formation, contemporaneous fossiliferous formation of Chile

• Cabrera, Ángel. 1926. Cetaceos fósiles del Museo de La Plata. Revista del Museo de La Plata 29. 363–411.
• Lydekker, Richard. 1894. Cetacean skulls from Patagonia. Anales del Museo de la Plata II. 1–13.
• Simpson, George Gaylord. 1946. Fossil Penguins. Bulletin of the American Museum of Natural History 87. 1–100.