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EXHIBIT 267

Polycotylus

Polycotylus latipinnis

Marine reptile

A short-necked plesiosaur whose pregnant fossil provides direct evidence of live birth. [3]

Evidence profile

TWO WAYS TO PICTURE THE PAST

Compare the evidence with the familiar movie image. The illustrations are not to a shared scale.

01 / SOURCE-INFORMED RECONSTRUCTION

A life reconstruction from the evidence

Illustrative life reconstruction of Polycotylus in a period-appropriate environment; coloration and missing anatomy are interpretations.

An illustrative reconstruction. Compare its anatomy with the evidence and limitations in the dossier. [2]

Source-informed reconstruction. Appearance evidence [2]. Original illustration; missing anatomy, coloration and scenery include interpretation.

Fossils constrain this life reconstruction. Missing anatomy, coloration and the environmental scene include interpretation; the dossier explains the evidence limits.

02 / MOVIE-STYLE INTERPRETATION

A cinematic imagination

Illustrative life reconstruction of Polycotylus in its interpreted aquatic environment.

An illustrative reconstruction. Compare its anatomy with the evidence and limitations in the dossier. Lighting, coloration and scenery are artistic choices. This image does not document a fossil locality or behavior.

Original artwork · artistic interpretation.

Scientific view: source-informed reconstruction, credited paleoart or specimen-based imagery. Movie-style view: Original artistic interpretation. Reconstructions involve uncertainty.

PERIODLate Cretaceous[5]
DIETCarnivore (animal prey inferred)Evidence and uncertainty in the dossier
TOTAL LENGTHSee the dossierEstimate scope matters
FOSSILS FOUNDUnited States[5]
CompareRead the dossierView the sources

MEET THE ANIMAL

Meet Polycotylus

Polycotylus was a marine reptile with long jaws, a relatively short neck and four swimming flippers. It belonged to the plesiosaurs. Comparing its skull and neck bones with other polycotylids helps researchers distinguish similar-looking animals instead of giving them all the same name. [D2]

One remarkable Kansas fossil preserves a mother and her developing young. The region was part of an ancient sea during the Late Cretaceous. This pregnancy provides direct evidence that this animal gave birth to live young rather than relying on a guessed nest. [D5]

Researchers also looked inside fossil bones. Microscopic tissue records rapid growth before birth. By comparing bones at different stages, they estimated how large the youngster might have been when born. An estimated newborn size is different from simply measuring the fetus preserved beside its mother. [D3]

That research inspired the idea that a mother may have protected a vulnerable youngster. The museum explains that the young animal’s fast-growing bones may have made swimming difficult. It is an interesting possibility, but the fossil does not show a family traveling together or a parent caring for its baby. [D4]

PREHISTORIC ANIMAL DOSSIER

A life in profile.

The animal, the evidence, and the questions still open.

The preserved mother and fetus support live birth. Bone tissue adds growth evidence; proposed parental care remains an interpretation. [D3]

Profile scope: One genus exhibit anchored to P. latipinnis; pregnancy refers to LACM129639, while size uses a separate YPM1125 reconstruction. [D3] [D1]

Time
Late Cretaceous
Fossil locations in this exhibit
United States
How to read the evidence

Source supported identifies a description documented by the cited source. Inferred marks a reconstruction or functional estimate. Debated marks competing interpretations. Not established means the sources reviewed for this dossier do not support a firm conclusion; it does not mean a behavior never occurred.

A size reference is an estimate, not a population range. Nesting evidence does not by itself establish courtship, mate choice, or lifelong pairing.

Length

Inferred

5.074 m (individual model)

YPM1125 reconstructed length; not an adult range. [D1]

Mass

Inferred

1,039 kg (individual model)

YPM1125 volumetric estimate; not measured body weight. [D1]

Diet

Inferred

Animal prey

Elongate jaws and pointed teeth support predation; a complete menu is not established here. [D2]

Habitat

Source supported

Western Interior Seaway

The museum pregnancy fossil comes from Late Cretaceous Kansas, then covered by a sea; exact preferred depth is unknown. [D5]

Locomotion

Inferred

Four-flipper swimming

Plesiosaur limb anatomy supports swimming; fetal bone-strength estimates do not measure swimming speed. [D4]

Anatomy

Source supported

Long jaws, short neck and four flippers

Polycotylid comparisons distinguish its neck and skull proportions from Dolichorhynchops; the animals are not interchangeable. [D2]

Reproduction & life together

Reproduction

Source supported

Live birth

LACM129639 preserves a mother and one fetus. Estimated birth size comes from a growth comparison, not a measured newborn. [D3]

Courtship and mating

Not established

Not established here

The reviewed pregnancy and histology study supplies no observed mating display or courtship sequence. [D3]

Social behavior

Debated

Parental care proposed

Large offspring and fetal bone growth prompted a care hypothesis. The fossils do not directly show nursing, bonds or group living. [D4]

Dossier research & sources

Sources and inspection notes for the claims in this profile.

  1. Body reconstruction and size estimation of plesiosaursPeerJ / Ruizhe Jackevan Zhao (2026) · Accessed 2026-10-09Table 1, reconstruction methods and uncertainty discussion inspected. Separate reconstructed individuals; not adult species ranges. · Inspected: page
  2. The evolutionary history of polycotylid plesiosauriansRoyal Society Open Science / Fischer et al. (2018) · Accessed 2026-10-09Introduction, phylogenetic results and comparative Polycotylus/Dolichorhynchops anatomy inspected. Dolichorhynchops nomenclature is updated using the 2024 revision. · Inspected: page
  3. Ontogeny of Polycotylid Long Bone Microanatomy and HistologyIntegrative Organismal Biology / O’Keefe et al. (2018) · Accessed 2026-10-09Abstract, Introduction, gravid LACM129639 specimen, comparative histology and Discussion inspected. Historical D. bonneri material is not treated as current Dolichorhynchops evidence. · Inspected: page
  4. Plesiosaur: Baby plesiosaurs may have been weak swimmersNatural History Museum of Los Angeles County (2019) · Accessed 2026-10-09Museum account inspected: rapid fetal growth, bone-strength interpretation and explicitly hypothesized maternal care. Display width and height are not body measurements. · Inspected: page
  5. Prehistoric Planet Self-Guided TourNatural History Museum of Los Angeles County · Accessed 2026-10-09Polycotylus exhibit section inspected: Kansas marine fossil context, live birth and marine-reptile classification. · Inspected: page
  6. The Dino ArtistNatural History Museum of Los Angeles County · Accessed 2026-10-09Polycotylus skeletal reconstruction and artist account inspected; reconstruction history rather than proof of color or behavior. · Inspected: page

THE STORY IN THE FOSSILS

Reading the evidence

A preserved fetus and a reconstructed birth-size estimate are different evidence. The pregnancy directly supports live-bearing; histological comparisons estimate growth and the probable size at birth. [3]

The museum’s reconstructed skeleton also required anatomical judgment. Fossil bones guide the shape, while an artist must supply contours that were not preserved. [6]

A note on scale and time

The dossier identifies the scope of each reference estimate. A mount, an individual fossil and a species-wide adult range are different measurements. Comparison metrics remain unavailable where adult scope has not been established. Unknown does not mean zero.

Sources & editorial notes

Sources support the stated claims and scopes. Refer to the evidence notes for uncertainty.

  1. Body reconstruction and size estimation of plesiosaursPeerJ / Ruizhe Jackevan Zhao (2026) · Accessed 2026-10-09Table 1, reconstruction methods and uncertainty discussion inspected. Separate reconstructed individuals; not adult species ranges. · Inspected: page
  2. The evolutionary history of polycotylid plesiosauriansRoyal Society Open Science / Fischer et al. (2018) · Accessed 2026-10-09Introduction, phylogenetic results and comparative Polycotylus/Dolichorhynchops anatomy inspected. Dolichorhynchops nomenclature is updated using the 2024 revision. · Inspected: page
  3. Ontogeny of Polycotylid Long Bone Microanatomy and HistologyIntegrative Organismal Biology / O’Keefe et al. (2018) · Accessed 2026-10-09Abstract, Introduction, gravid LACM129639 specimen, comparative histology and Discussion inspected. Historical D. bonneri material is not treated as current Dolichorhynchops evidence. · Inspected: page
  4. Plesiosaur: Baby plesiosaurs may have been weak swimmersNatural History Museum of Los Angeles County (2019) · Accessed 2026-10-09Museum account inspected: rapid fetal growth, bone-strength interpretation and explicitly hypothesized maternal care. Display width and height are not body measurements. · Inspected: page
  5. Prehistoric Planet Self-Guided TourNatural History Museum of Los Angeles County · Accessed 2026-10-09Polycotylus exhibit section inspected: Kansas marine fossil context, live birth and marine-reptile classification. · Inspected: page
  6. The Dino ArtistNatural History Museum of Los Angeles County · Accessed 2026-10-09Polycotylus skeletal reconstruction and artist account inspected; reconstruction history rather than proof of color or behavior. · Inspected: page

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