# La Meseta Formation

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 Lopez de Bertodano Formation. It is an approximately 557 metres (1,827 ft) 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.

## Paleobiota

La Meseta Formation is extremely rich in fossils. Among mammals, the meridiungulata Antarctodon and Trigonostylops have been found in the formation. as well as marsupial Derorhynchidae, Microbiotheria, and polydolopimorphia. It is famous for its penguin fossils, for example the two genera Archaeospheniscus and Palaeeudyptes. 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

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |

#### Meriduingulata
##### 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 |
| Notiolofos | N. 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. |
| Notiolofos | N. regueroi | Locality IAA 2/16 | Lower coquina bed of Cucullaea I Allomember. | IAA-PV 173, Isolated complete left m3. | A sparnotheriodontid litoptern. |
| Victorlemoinea | V. sp. | | Cucullaea I allomember (Telm 5). | | A sparnotheriodontid litoptern. |

#### Cetaceans

| Taxa | Species | Locality | Stratigraphic position | Material | Notes | Images |
| Basilosauridae | Indeterminate | | | | | |
| Llanocetus | L. 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 |
| Derorhynchus | D. 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 | 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). |
| Xenostylos | X. 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 | Images |
| Marambiotherium | M. 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. | |
| Woodburnodon | W. 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 |
| Antarctodolops | A. 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. |
| Antarctodolops | A. mesetaense | Locality IAA 1/90 and DPV 6/84. | Cucullaea I allomember (Telm 5). | Right dentary fragments. | A polydolopimorphian metatherian. |
| Perrodelphys | P. coquinense | Locality IAA 1/90. | Cucullaea I allomember (Telm 5). | MLP 96-1-5-11, an isolated left lower molar. | A polydolopimorphian metatherian. |
| Polydolops | P. dailyi | Locality IAA 1/90 & DPV 3/84. | Cucullaea I allomember (Telm 5). | | A polydolopimorphian metatherian. |
| Polydolops | P. seymouriensis | Locality IAA 1/90 & DPV 3/84. | Cucullaea I allomember (Telm 5). | | A polydolopimorphian metatherian. |
| Polydolops | P. thomasi | Locality IAA 1/90 & DPV 3/84. | Cucullaea I allomember (Telm 5). | | A polydolopimorphian metatherian. |
| Pujatodon | P. 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 |
| 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 gondwanathere |
| Meridiolestida? | 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 |
| Anthropornis | A. 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. | Anthropornis Palaeeudyptes |
| Anthropornis | 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). | A giant penguin. | Anthropornis Palaeeudyptes |
| Archaeospheniscus | A. lopdelli | | Submeseta Allomember. | | A giant penguin. | Anthropornis Palaeeudyptes |
| Archaeospheniscus | A. wimani | | Submeseta Allomember. | | A giant penguin. | Anthropornis Palaeeudyptes |
| Delphinornis | D. arctowskii | DPV 14/84 | Cucullaea I Allomember (Telm 7). | | A penguin. | Anthropornis Palaeeudyptes |
| Delphinornis | D. graclis | DPV 14/84 | Cucullaea I Allomember (Telm 7). | | A penguin. | Anthropornis Palaeeudyptes |
| Delphinornis | D. larseni | DPV 13/84 and DPV 14/84. | Cucullaea I Allomember (Telm 5 and Telm 7) and Submeseta Allomember. | | A penguin. | Anthropornis Palaeeudyptes |
| Marambiornis | M. exilis | DVP 2/84, IAA 1/12, and ZPAL 4 | Telm 7 Member. | A limb element (right tarsometatarsus). | A penguin. | Anthropornis Palaeeudyptes |
| Mesetaornis | M. polaris | | Telm 7 member. | A nearly complete right femur and two distal left tibiotarsi. | A penguin. | Anthropornis Palaeeudyptes |
| Orthopteryx | O. gigas | | | | A penguin. | Anthropornis Palaeeudyptes |
| Palaeeudyptes | P. antarcticus | | Cucullaea I Allomember (Telm 3, Telm 4, Telm 5, and Telm 7) and Submeseta Allomember. | | A giant penguin. | Anthropornis Palaeeudyptes |
| Palaeeudyptes | 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. | A giant penguin. | Anthropornis Palaeeudyptes |
| Palaeeudyptes | 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. | A giant penguin. | Anthropornis Palaeeudyptes |
| Wimanornis | W. seymourensis | DPV 14/84. | Cucullaea I Allomember (Telm 7) and Submeseta Allomember. | | A penguin. | Anthropornis Palaeeudyptes |
| Tonniornis | T. mesetaensis | DPV 14/84. | Cucullaea I Allomember (Telm 7) and Submeseta Allomember. | | A penguin. | Anthropornis Palaeeudyptes |
| Tonniornis | T. minimum | DPV 14/84. | Cucullaea I Allomember (Telm 7) and Submeseta Allomember. | | A penguin. | Anthropornis Palaeeudyptes |

#### Other birds

| Taxa | Species | Locality | Stratigraphic position | Material | Notes |
| Antarctoboenus | A. carlinii | IAA 2/95 locality | Cucullaea I Allomember | MLP 95-I-10-8, distal end of left tarsometatarsus | A stem falconiform. |
| Dasornis | D. sp. | | | | A pseudotooth bird. |
| ?Diomedeidae | Indeterminate | | Cucullaea I Allomember | | A potential albatross. |
| Notoleptos | N. giglii | | Submeseta III Allomember | Left tarsometatarsus (MLP 12-I-20-305) | An early albatross. |
| ?Phorusrhacidae | Indeterminate | Locality IAA 2/13 | Cucullaea I Allomember | A 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. |
| ?Procellariidae | Indeterminate | | Cucullaea I Allomember | Distal end of ulna (MLP 91-II-4-6) | A potential procellarid. |
| ?Threskiornithidae | Indeterminate | | Cucullaea 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 4 | Platelets, shell fragment | A dermochelyid sea turtle related to modern leatherback turtles. | |
| Testudines indet. | | | Cucullaea I Allomember (Telm 5) | Carapace plates | A non-dermochelyid turtle with a bony carapace. | |

### Amphibians

| Taxa | Species | Locality | Stratigraphic position | Material | Notes | Images |
| Calyptocephalella | 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 | Stratigraphic position | Material | Notes |
| Callorhinchus | C. stahli | | | A ploughnose chimaera. |
| Chimaera | C. seymourensis | | | A rabbitfish. |
| Ischyodus | I. dolloi | Upper | Tooth plates | A relative of ploughnose chimaeras. |

#### Sharks

| Taxa | Species | Locality | Stratigraphic position | Material | Notes |
| Abdounia | A. mesetae | | Cucullaea I Allomember (Telm 5) | | A requiem shark. |
| Abdounia | A. richteri | | Cucullaea I Allomember (Telm 5) | | A requiem shark. |
| Anomotodon | A. multidenticulata | | | | An extinct relative of goblin shark. |
| Carcharhinus | C. sp. | | | | A requiem shark. |
| Centrophorus | C. sp. | | | | A gulper shark. |
| Cetorhinus | C. sp. | | Middle | Gill raker | A relative of the basking shark. |
| Ceolometlaouia | C. pannucae | | | | A carpet shark. |
| Dalatias | D. licha | | | | The modern kitefin shark. |
| Deania | D. sp. | | | | A longnose dogfish. |
| Eodalatias | E. austrinalis | | | | A dalatiid. |
| Galeorhinus | G. mesetaensis | | | | A relative of the school shark. |
| Galeorhinus | G. sp. | | Cucullaea I Allomember (Telm 5), Telm 6 | | A relative of the school shark. |
| Heptranchias | H. howellii | | | Upper lateral teeth. | A relative of the sevengill shark. |
| Hexanchus | H. sp. | | | | A relative of the sixgill shark. |
| Kallodentis | K. rhytistemma | | Cucullaea I Allomember (Telm 5), Telm 6 | | A houndshark. |
| Lamna | L. cf. nasus | | | | A relative of the porbeagle. |
| Macrorhizodus | M. praecursor | | | | A lamnid. |
| Meridiogaleus | M. cristatus | | Cucullaea I Allomember (Telm 5), Telm 6 | | A houndshark. |
| Mustelus | M. sp. | | Cucullaea I Allomember (Telm 5), Telm 6 | | A smooth-hound. |
| Notoramphoscyllium | N. woodwardi | | | | A carpet shark. Initially thought to be teeth of the modern zebra shark. |
| Odontaspis | O. winkleri | | | | A relative of the sand shark. |
| Otodus | O. auriculatus | | | | A megatooth shark. |
| Otodus | O. sokolovi | | Upper | | A megatooth shark. |
| Palaeohypotodus | P. cf. rutoti | | | | A sand shark. |
| Paraorthacodus | P. sp. | | | | A paraorthacodontid shark. |
| Pristiophorus | P. lanceolatus | | Upper | | A sawshark. |
| Pristiophorus | P. laevis | | | | A sawshark. |
| Pseudoginglymostoma | P. cf. P. brevicaudatum | | | | Nomen dubium. |
| Scoliodon | S. sp. | | | Multiple teeth. | A requiem shark. |
| Squalus | S. weltoni | UCR RV-8200, Telm 4. | Lower | A complete lateral tooth. | A dogfish. |
| Squalus | S. woodburnei | UCR RV-8200, Telm 4. | Lower. | A complete lateral tooth. | A dogfish. |
| Squatina | S. sp. | | Upper | | An angelshark. |
| Striatolamia | S. cf. macrota | | Upper | | A sand shark. |

#### Rays

| Taxa | Species | Stratigraphic position | Material | Notes |
| Marambioraja | M. leiostemma | Lower | | A skate. |
| Mesetaraja | M. maleficapelli | Lower | | A skate. |
| Myliobatis | M. sp. | | | An eagle ray. |
| Myliobatoidea indet. | | | Partial spine | A stingray. |
| Pristis | P. sp. | | | A sawfish. |
| Raja | R. amphitrita | Lower | | A skate. |
| Raja | R. manitaria | Lower | | A skate. |

### Ray-finned fish

| Taxa | Species | Material | Notes |
| Labridae indet. | | Large lower pharyngeal tooth plate | A wrasse. |
| Macrouridae indet. | | Well-preserved skull with otoliths | A grenadier. |
| Marambionella | M. andreae | Articulated skeleton | A clupeioid herring relative. |
| Mesetaichthys | M. jerzmanskae | Fragmentary skull bones and vertebra. | An early member of the Notothenioidei, the dominant marine fish of Antarctica today. Initially assigned to the hakes. |
| Oplegnathus | O. sp. | Jaws with teeth | A knifejaw, earliest record of this family. |
| Proeleginops | P. grandeastmanorum | Neurocranium | An early member of the Notothenioidei. |
| Siluriformes indet. | | | A catfish. |
| Trichiurus | T. sp. | Teeth | A cutlassfish. |
| Xiphiorhynchus | X. cf. sp. | | A relative of the swordfish. |

### Cephalopods

| Taxa | Species | Locality | Stratigraphic position | Material | Notes | Images |
| Antarcticeras | A. nordenskjoeldi | NRM 8, below IAA 1/90 (Ungulate Site). | Cucullaea I shell bed, Telm 4. | A fragmentary preserved straight shell. | An enigmatic cephalopod that is either considered a descendant of the orthocerids or an oegopsid squid. | |
| Euciphoceras | E. sp. | NRM 8, below IAA 1/90 (Ungulate Site). | Cucullaea I shell bed, Telm 4. | A fragmentary preserved straight shell. | A nautiloid. | |

### Plants

| Taxa | Species | Locality | Stratigraphic position | Material | Notes | Images |
| Agathoxylon | A. pseudoparenchymatosum | Locality 5 | Cucullaea I allomember | Fossilized wood. | | fossilized seed of Notonuphar |
| Cupressinoxylon | C. hallei | Localities 4, 5, 6 and 7. | Campamento and Cucullaea I allomember. | Fossilized wood. | | fossilized seed of Notonuphar |
| Notonuphar | N. antarctica | IAA 2/95. | Cucullaea I Allomember (Telm 5). | Fossilized seeds. | A water lily. | fossilized seed of Notonuphar |
| Nelumbo | ? Nelumbo sp. | | | | | fossilized seed of Notonuphar |
| Protophyllocladoxylon | P. francisiae | | | | | fossilized seed of Notonuphar |
| Phyllocladoxylon | P. antarcticum | | | | | fossilized seed of Notonuphar |
| Phyllocladoxylon | P. pooleae | | | | | fossilized seed of Notonuphar |