The La Meseta Formation is a sedimentary sequence deposited during much of the Paleogene on Seymour Island off the coast of the Antarctic Peninsula. It is noted for its fossils, which include both marine organisms and the only terrestrial vertebrate fossils from the Cenozoic of Antarctica.
In some treatments, the La Meseta Formation is restricted to just the older Thanetian to Lutetian-aged strata, with the younger Bartonian to Rupelian strata treated as the overlying Submeseta Formation. However, other papers instead treat the Submeseta Formation as an allomember of the La Meseta Formation.
Description La Meseta Formation lies unconformably on the Cretaceous López de Bertodano Formation. It is an approximately thick sequence of poorly consolidated sandstones and siltstones. The depositional environment was probably coastal, deltaic or estuarine in character. The top of the sequence is an erosional unconformity to Pleistocene glacial gravels. La Meseta Formation is one of the sequences that make up the fill of the Late Jurassic to Paleogene James Ross Basin.
Paleoenvironment The terrestrial environment surrounding the deposition area is thought to have been a temperate polar forest, including podocarp and araucarian conifers, as well as Nothofagus. Most of the fossilized woods and flowers discovered on Seymour Islands consist of extinct species of conifer trees and lilies during warm climate.Pujana, R. R., Wilf, P., & Gandolfo, M. A. (2020). Conifer wood assemblage dominated by Podocarpaceae, early Eocene of Laguna del Hunco, central Argentinean Patagonia. PhytoKeys, 156, 81–102. https://doi.org/10.3897/phytokeys.156.54175
Paleobiota La Meseta Formation is extremely rich in fossils. Among mammals, the meridiungulata Antarctodon and Trigonostylops have been found in the formation.Antarctodon at Fossilworks.orgTrigonostylops at Fossilworks.org as well as marsupial Derorhynchidae, Microbiotheria, and polydolopimorphia.Chornogubsky, L. (2009). A Reassessment of Antarctic Polydolopid Marsupials (Middle Eocene, La Meseta Formation). Antarctic Science. It is famous for its penguin fossils, for example the two genera Archaeospheniscus and Palaeeudyptes.Tambussi, Claudia & Acosta Hospitaleche, Carolina & Reguero, Marcelo & Marenssi, Sergio. (2006). Late Eocene penguins from West Antarctica: Systematics and biostratigraphy. Geological Society London Special Publications. 258. 145-161. 10.1144/GSL.SP.2006.258.01.11. Other bird fossils include Dasornis, a genus of pseudotooth birds. There is also an abundance of trace fossils. Diplocraterion, Helminthopsis, Muensteria, Oichnus, Ophiomorpha, Skolithos, Teredolites and Zapfella have been described. Over 35 species and 26 families of fish, which includes sharks, have been described from the Ypresian Cucullaea bed.
Mammals
Meridiungulata
Astrapotheria Taxa Species Locality Stratigraphic position Material Notes Images Antarctodon A. sobrali Locality DVP 2/84 and IAA 1/90. Cucullaea I allomember (Telm 5). MLP 08-XI-30-1, an isolated right p4 or m1. An astrapothere Trigonostylops T. sp. Cucullaea I allomember (Telm 5). An astrapothere
Litopterna Taxa Species Locality Stratigraphic position Material Notes ImagesNotiolofosN. arquinotiensis Locality IAA 1/13 and DPV 16/84. Acantilados II, Campamento, Cucullaea I Allomember (Telm 4) and Submeseta Member (Telm 7). A Brachyodont lower right molar fragment, probably m1 or m2, preserves a portion of the talonid with most of the lingual side (MLP 13-I-25-1) and left M3 incomplete (MLP 95-I-10-6). A sparnotheriodontid litoptern.N. regueroi Locality IAA 2/16 Lower coquina bed of Cucullaea I Allomember. IAA-PV 173, Isolated complete left m3.VictorlemoineaV. sp. Cucullaea I allomember (Telm 5).A sparnotheriodontid litoptern.
Cetaceans Taxa Species Locality Stratigraphic position Material Notes ImagesBasilosauridae IndeterminateLlanocetusL. denticrenatus Locality DPV 10/84 and DVP 13/24. Unit III Member and Telm 7. A partial mandible with two teeth (specimen USNM 183022) and an endocast of the braincase. A toothed baleen whale.
Metatherians
Derorhynchidae Taxa Species Locality Stratigraphic position Material Notes ImagesDerorhynchusD. minutus Locality IAA 1/90. Allomember Cucullaea I (Telm 5). MLP 96-1-5-44, incomplete right horizontal ramus of the dentary fragment with m2-3. Pauladelphys"Antarctic Paleontological Heritage: Late Cretaceous–Paleogene vertebrates from Seymour (Marambio) Island, Antarctic Peninsula". Advances in Polar Science. 30. 20 August 2019. doi:10.13679/j.advps.2019.0015.P. juanjoi Locality IAA 2/95 and IAA 1/90. Allomember Cucullaea I (Telm 5). An isolated lower molar (MLP 95-1-10-2) and an upper left molar (MLP 96-1-5-44).XenostylosX. peninsularis Locality IAA 1/90. Allomember Cucullaea I (Telm 5). MLP 94-111-15-10, an isolated upper right molar.
Microbiotheria Taxa Species Locality Stratigraphic position Material Notes ImagesMarambiotheriumM. glacialis Locality IAA 1/90 and RV-8200. Cucullaea I allomember (Telm 5). MLP 95-1-10-1, a fragment of a right mandibular ramus with a complete m4 and the posterior alveolus of m3 and MLP 88-1-1-1, an edentulous left mandibular ramus with the alveoli for p3-m4. A microbiothere, related to the modern monito del monte.WoodburnodonW. casei Locality IAA 1/95. Cucullaea I allomember (Telm 5). MLP 04-III-1-2, an isolated, worn upper right molar (M2 or M3).A microbiothere, related to the modern monito del monte.
Polydolopimorphia Taxa Species Locality Stratigraphic position Material Notes ImagesAntarctodolopsA. dailyi Locality IAA 2/95, DPV 2/84, and DPV 6/84. Cucullaea I allomember (Telm 5). UCR 20910, a left dentary with p3-m2. A polydolopimorphian metatherian. A. mesetaense Locality IAA 1/90 and DPV 6/84. Cucullaea I allomember (Telm 5). Right dentary fragments.PerrodelphysP. coquinense Locality IAA 1/90. Cucullaea I allomember (Telm 5). MLP 96-1-5-11, an isolated left lower molar.A polydolopimorphian metatherian. PolydolopsP. dailyi Locality IAA 1/90 & DPV 3/84. Cucullaea I allomember (Telm 5). A polydolopimorphian metatherian. P. seymouriensis Locality IAA 1/90 & DPV 3/84. Cucullaea I allomember (Telm 5).P. thomasi Locality IAA 1/90 & DPV 3/84. Cucullaea I allomember (Telm 5).PujatodonP. ektopos Locality IAA 1/90. Cucullaea I Allomember (Telm 5). Specimen MLP 14-I-10-20, a lower left molar (m2 or m3) partially worn. A polydolopimorphian metatherian.
Other mammals Taxa Species Locality Stratigraphic position Material Notes Images Sudamerica S. ameghinoi Locality IAA 1/90 upper level. Cucullaea I Allomember. MLP 95-I-10-5, anterior part of a left dentary with the rodent-like incisor partially preserved.A gondwanathereMeridiolestida?Indeterminate Locality IAA 1/90 upper level. Cucullaea I Allomember.A single tooth, now lost.Xenarthra? Indeterminate. Locality S124. Cucullaea I Allomember/ Telm 4.TMM 44190-1, left metacarpal II and A phalanx and a partial tooth
Birds Sphenisciformes Taxa Species Locality Stratigraphic position Material Notes Images AnthropornisA. grandis Locality IAA 4/12, IAA 1/90, DPV 13/84, and DVP 2/84. Telm 4 member, Telm 7 member, and Submeseta Allomember. Humerus: MLP 93-X-1-4 (proximal epiphysis), MLP 82-IV-23-4 (proximal epiphysis), MLP 83-I-1-190(proximal epiphysis) and MLP 88-I-1-463(proximal epiphysis). A giant penguin. A. nordenskjoldi Submeseta Allomember. Humerus: MLP CX-60-25 (proximal epiphysis), MLP 83-V-30-5 (diaphysis) and MLP 93-X-1-104(complete humerus).ArchaeospheniscusA. lopdelli Submeseta Allomember. A giant penguin.A. wimani Submeseta Allomember.Delphinornis D. arctowskii DPV 14/84 Cucullaea I Allomember (Telm 7).A penguin.D. graclis DPV 14/84 Cucullaea I Allomember (Telm 7).D. larseni DPV 13/84 and DPV 14/84. Cucullaea I Allomember (Telm 5 and Telm 7) and Submeseta Allomember. Marambiornis M. exilis DVP 2/84, IAA 1/12, and ZPAL 4 Telm 7 Member. A limb element (right tarsometatarsus). A penguin.MesetaornisM. polaris Telm 7 member.A nearly complete right femur and two distal left tibiotarsi.A penguin.OrthopteryxO. gigasA penguin.PalaeeudyptesP. antarcticus Cucullaea I Allomember (Telm 3, Telm 4, Telm 5, and Telm 7) and Submeseta Allomember. A giant penguin.P. gunnari DVP 2/84, DVP 10/84, DVP 13/84, DVP 14/84, DVP 15/84, and ZPAL 4. Cucullaea I Allomember (Telm 3, Telm 5, and Telm 7) and Submeseta Allomember. Multiple specimens consist of coracoids, ulna, and humerus.P. klekowskii DVP 2/84, DVP 10/84, DVP 13/84, DVP 14/84, DVP 16/84, and ZPAL 4. Cucullaea I Allomember (Telm 3, Telm 5, and Telm 7) and Submeseta Allomember. Several specimens consist of humerus, coracoid, ulna, and tibiotarsus.WimanornisW. seymourensis DPV 14/84. Cucullaea I Allomember (Telm 7) and Submeseta Allomember.A penguin.TonniornisT. mesetaensis DPV 14/84. Cucullaea I Allomember (Telm 7) and Submeseta Allomember.A penguin.T. minimum DPV 14/84. Cucullaea I Allomember (Telm 7) and Submeseta Allomember.
Other birds Taxa Species Locality Stratigraphic position Material Notes ImagesAntarctoboenusA. carliniiIAA 2/95 localityCucullaea I AllomemberMLP 95-I-10-8, distal end of left tarsometatarsusA stem falconiform.DasornisD. sp. An articular fragment of lower jaw.Tonni, Eduardo Pedro & Tambussi, Claudia Patricia (1985): Nuevos restos de Odontopterygia (Aves: Pelecaniformes) del Terciario temprano de Antártida ["New pseudotooth bird remains from the Early Tertiary of Antarctica"]. Ameghiniana 21(2-4): 121-124 [Spanish with English abstract]. HTML abstractA pseudotooth bird.?DiomedeidaeIndeterminate DPV 6/84Cucullaea I Allomember Tarsometatarsus with missing distal end.A potential albatross.NotoleptosN. giglii DPV 16/84Submeseta III AllomemberLeft tarsometatarsus (MLP 12-I-20-305)An early albatross.?Phorusrhacidae Indeterminate Locality IAA 2/13 Cucullaea I AllomemberA right ungual phalanx (MLP-PV 13-XI-28-546) and an incomplete ungual phalanx (MLP-PV 14-I-10-199)A large-sized potential terror bird.?ProcellariidaeIndeterminateCucullaea I AllomemberDistal end of ulna (MLP 91-II-4-6)A potential procellarid.?ThreskiornithidaeIndeterminateCucullaea II Allomember (Telm 6)A fragmentary, curved bill (IB/P/B-0698).A potential ibis.
Reptiles Taxa Species Locality Stratigraphic position Material Notes Images"Psephophorus" cf. P. terrypratchetti Telm 4Platelets, shell fragmentA dermochelyid sea turtle related to modern leatherback turtles.Testudines indet.Cucullaea I Allomember (Telm 5)Carapace platesA non-dermochelyid turtle with a bony carapace.Crocodilia IndeterminateTelm 6/7Isolated Teeth and mandibleA crocodilian.
Amphibians Taxa Species Locality Stratigraphic position Material Notes ImagesCalyptocephalella C. sp.Locality IAA 2/95. Cucullaea I Allomember (Telm 5).The fragmentary right ilium (NRM-PZ B282) and a skull bone (NRM-PZ B281).A calyptocephalellid frog
Cartilaginous fish
Chimaeras Taxa Species Locality Stratigraphic position Material Notes ImagesCallorhinchusC. stahliA ploughnose chimaera.ChimaeraC. seymourensisA rabbitfish.IschyodusI. dolloiUpperTooth platesA relative of ploughnose chimaeras.
Sharks