Gaiman Formation

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]

The basal stratum of the Gaiman Formation is a thin transgressive lag with some gravels, bones and teeth from marine vertebrates.^[5] The unit displays a concentration of phosphatic concretions, ooids, bones and teeth.^[7] The marine sediments overlying this basal stratum are composed of white, tuffaceous, thoroughly bioturbated mudstones and fine sandstones with occasional mollusk molds and thin oyster horizons, deposited in a shallow shelf environment.^[5]

The marine transgression leading to the deposition of the Gaiman Formation is the first of two major South Atlantic transgressions of the Miocene, the second causing the deposition of the overlying Puerto Madryn Formation.^[8]

Facies analysis shows that the formation represents a transgressive-regressive stratigraphic cycle, with palaeoenvironments including coastal, storm-dominated shoreface, inner shelf embayment and open inner shelf.^[9]

The age of the Gaiman Formation is established on stratigraphic correlations to other absolutely-dated sections in Patagonia and biostratigraphic data and dates to the Early Miocene (Burdigalian, or Colhuehuapian in the SALMA classification).^[5]

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 [es].^[18]

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

Taxa	Species	Locality	Material	Description	Images	Notes
Aglaocetus	A. moreni	Cerro del Castillo and Bahia Solano	A skull	An early baleen Whale		[24]
Aondelphis	A. talen	Bryn Gwyn Locality	A partial skull	An early river dolphin		[25]
Argyrocetus	A. patagonicus	Bryn Gwyn Locality	A skull	An early toothed whale		[12]
Crisocetus	C. lydekkeri	Estancia El Crisol	A partial skull	An early toothed whale		[26]
Dolgopolis	D. kinchikafiforo	Estancia El Crisol	A partial skull	An early toothed whale		[27]
Eurhinodelphinidae	indet.					[20]
Idiorophus	I. patagonicus	Bryn Gwyn Locality	A complete skull	An early toothed whale		[24]
Kentriodon	K. sp.	Bryn Gwyn Locality	A specimen	An early toothed whale		[12]
Morenocetus	M. parvus	Cerro del Castillo, Trelew	A partial cranium	An early baleen whale		[24]
Phoberodon	P. arctirostris	Playa Magagna and Cerro del Castillo, Trelew	A partial skeleton with skull	An early river dolphin		[12]
Plesiocetus	P. dyticus	Cerro del Castillo	A specimen	An early rorquals		[24]
Prosqualodon	P. australis	Playa Magagna and Cerro del Castillo	A preserved skeleton	An early toothed whale		[12][28]
Squalodontidae	indet.					[20]

Taxa	Species	Locality	Material	Description	Images	Notes
Anseriformes	indet.					[20]
Apterodytes	A. ictus	Cerro del Castillo		An early penguin		[21]
Arthrodytes	A. sp.					[20]
Cayaoa	C. bruneti	Cerro Castillo	A specimen	An early anatid water bird		[4][29]
Eretiscus	E. tonnii			An early prenguin		[30]
Palaeospheniscus	P. bergi	Cerro del Castillo, Trelew	A set of limb elements (humerus, radius, tarsometatarsus)	An early penguin		[30][19]
	P. bilocular	Cerro del Castillo	Two characteristic humeri (AMNH 3341 and 3346)	A early penguin, formally named Chubutodyptes biloculata		[30]
	P. gracilis	Gran Bajo de San Julian and Cerro del Castillo	A specimen	An early penguin		[20]
	P. patagonicus	Cerro del Castillo		An early penguin		[19][31]
	P. wimani	Cerro del Castillo		An early penguin		[22]
Paraptenodytes	P. antarcticus		A partial skeleton			[30]
	P. robustus					[19][31]
Sphenisciformes	indet.					[20]

• 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