Cretaceous Period

Cretaceousn depiction - artwork

End of an Era

G. Paselk
Massive Volcanism at the end of the Paleozoic Era forms a backdrop for a scene including Hadrosaurs, a Tyrannosaur, Quezalcoatlus, Tricerotops, and an Ankylosaur. Plants include firs, cycads and an early magnolia tree in flower.

145.5 to 65.5 Million years ago

Richard Paselk

early cretaceous plate tectonic reconstruction map

Early Cretaceous

Mid Cretaceous Plate Tectonic Reconstruction map

Late Cretaceous Plate tectonic reconstruction map

Late Cretaceous 

These maps of major tectonic elements (plates, oceans, ridges, subduction zones, mountain belts) are used with permission from Dr. Ron Blakey at Northern Arizona University. Explanation of map symbols.

The Cretaceous* saw the first appearance and initial diversification of flowering plants (Angiosperms). Insects and other organisms soon evolve to take advantage of the new food sources and opportunities these plants provide. Conifers continued replacing seed ferns, ginkgos and cycads. Marine life flourished, with many groups achieving their peak abundances and diversity. Rudist mollusks build new reefs rivaling today's coral reefs. This Period saw the emergence of the largest of all known land predators, such as Tyrannosaurus Rex, and the largest flying animal, Quetzalcoatlus.Reptiles dominated the land (dinosaurs), the sea (mososaurs and the giant turtle, archelon) and the air (pterosaurs). Duck-billed dinosaurs are the commonest ornithischians.

Dinosaurs, pterosaurs, mososaurs and ammonoids, just to name a few, . The now emptied ecosystems allowed the evolution of a largely new fauna in the coming Cenozoic Era.

The sea level rise begun in the Jurassic Period due to rapid plate movements continued. Ocean basin volumes diminished and the seas reached their highest levels during the Cretaceous Period, resulting in vast shallow continental seas. Huge deposits of chalk left from the skeletal remains of marine organisms, give the period its name. Gases from undersea volcanoes and spreading mid-ocean ridges enhanced middle–late Cretaceous super-greenhouse conditions.

The Cretaceous Period ends with one of the greatest known extinction events, so severe it also marks the end of the Mesozoic Era. Dinosaurs, pterosaurs, mosasaurs, and ammonoids, to name a few, were among the groups lost at this time. The Cretaceous extinction event is marked by the famous K-T boundary and asteroid impact on what is now the Yucatan peninsula.

Artists image of K-T impact

Many believe this impact caused dinosaur and other extinctions. Other possible causes, including extensive volcanic eruptions (the Deccan Traps in India) occur at this time. Over two million years pass after the K-T extinction before Earth’s ecosystems recover in the Paleogene Period, with largely new fauna and flora dominated by mammals, small feathered-dinosaurs (birds), and flowering plants.

*Cretaceous comes from the “Terrain Crétacé” established by d’Omalius d’Halloy in 1822 for the chalk [Latin creta] deposits of the Paris basin.

Cretaceous Animal (Metazoan) Fossils

Crustacea (ToL: Arthropoda<Ecdysozoa<Bilateria<Metazoa<Eukaryota)

lobster engraving


Lobsters (Decapoda) fossils, for example Hoploparia browni, are relatively rare in Cretaceous deposits.

fossil image icon Hoploparia browni

crab engraving


Crabs (Decapoda), such as Avitelmessus grapsoideus, which had appeared in the Jurassic, became more abundant.

fossil image icon Avitelmessus grapsoideus


Echinoderms (ToL: Echinodermata<Deuterostomia<Bilateria<Metazoa<Eukaryota)

echinoid engraving


Echinoids (Echinoidea), sea urchin/sand dollar-like echinoderms, such as Epiaster whitei and Micraster sp. were common.

fossil image icon Epiaster whitei

fossil image icon Micraster sp.

Sharks (ToL: Chodrichthyes<Vertebrata<Chordata<Deuterostomia<Bilateria<Metazoa<Eukaryota)

shark tooth engraving


The sharks, one of the two major modern fish families, are represented here by a group of fossil sharks teeth.

fossil image icon sharks teeth

Bony Fish (ToL: Osteichthyes<Vertebrata<Chordata<Deuterostomia<Bilateria<Metazoa<Eukaryota)

fish engraving

Ray-finned fish

Ray-finned fish (Actinopterigii), The teleost fish (teleosti), for example Rhacholepis buccalis, which dominate modern groups, appeared first in the Jurassic. In the Cretaceous they first outnumber the earlier fish types.

fossil image icon Rhacholepis buccalis

Reptilia (ToL: Vertebrata<Chordata<Deuterostomia<Bilateria<Metazoa<Eukaryota)

The display includes an unidentified reptile egg.

fossil image icon reptile egg

Mosasaur (Mosasauridae<Squamata<Reptilia)

A museum-quality model of a Tylosaurus, a huge predatory marine lizard, chasing a large ammonite, sits on top of the case. Tylosaurus was one of the largest of the mosasaurs , reaching lengths of 50 ft or more. It was a dominant predator of the Western Interior Seaway of the U.S. in the Late Cretaceous.

model image icon Tylosaurus modelmodel image icon fossil of a complete 29 ft specimen (AMNH, 1899)

Dinosaurs (Dinosauria [Birds and Dinosaurs]<Reptilia)

dinosaur engraving


The Ornithischia (bird-hipped plant eaters) are represented by a Hadrosaurus tooth plate and Hadrosaurus teeth.

fossil image icon Hadrosaurus tooth plateimage icon life reconstruction

fossil image icon Hadrosaurus teeth

dinosaur engraving


The Theropods (predatory bipedal dinosaurs, such as Tyrannosaurus and Birds) are represented by a tooth from an Albertosaurus sp. (note the serrated edges).

fossil image icon Albertosaurus sp. image icon life reconstruction

Mollusks (ToL: Mollusca<Lophotrochozoa<Bilateria<Metazoa<Eukaryota)

bivalve engraving


Bivalves were very common and important in this Period. One group, the rudist bivalves have an unusual form in which one valve is cone shaped, much like rugose corals of the Paleocene. During the Cretaceous they built huge reefs, cementing themselves together as they grew upward. The rudists became extinct at the end of the Mesozoic.

fossil image icon rudist bivalve

Other bivilves also flourished including the oysters, such as Actinostrea travisana , and extinct oyster relatives. Extinct oysters sometimes had unusual forms with one valve snail-shaped (pyritized fossil), or twisted into a spiral. In each case a second valve covered the opening as seen (partial) in this second spiral example.

fossil image icon Actinostrea travisana

fossil image icon oyster relative

fossil image icon snail-shaped oyster valve

fossil image icon spiral oyster valve

fossil image icon second spiral oyster valve

gastropod engraving


Gastropods are represented by two assemblages: a slab with Goniobasis chrysalis and a rock with freshwater snails exposed.

fossil image icon Goniobasis chrysalis

fossil image icon freshwater snails

The neogastropods, the most advanced of the marine gastropods, appeared in the Cretaceous, while the older mesogastropods remained numerous.

ammonite engraving


Cephalopods: The ammonoids, for example Scaphites nodosus were diverse and abundant during the Cretaceous, to be completely lost at the end of this Period. A cephalopod, Exiteloceras jennyi, is also displayed.

fossil image icon Scaphites nodosus

fossil image icon Exiteloceras jennyi

Two life reconstruction models of cephalopods are displayed hanging from the ceiling, a coiled ammonite (Planticeras meeki) with a 24" dia shell cast from a fossil specimen from the Pierre shale of South Dakota and reconstructed body, and a straight ammonite (Baculites grandis) with the 74" long shell cast from an authentic fossil from the Pierre shale of Wyoming and reconstructed body. Note that in each of these models the animal has directed its siphon backwards so that is is swimming in a forward direction.

image icon Planticeras meeki

image icon Baculites grandis

Sponges (ToL: Porifera<Metazoa<Eukaryota)

sponge engraving


The museum displayes two cup-shaped Rhaphidonema farringdonense sponges in this case.

fossil image icon Rhaphidonema farringdonense 1

fossil image icon Rhaphidonema farringdonense 2

Cretaceous Plant Fossils

Vascular Plants (ToL: Embryophytes [land plants] <Green Plants<Eukaryota)

cycad engraving


Cycads are a largely unbranched, woody plant resembling a pine. Cycads flourished during this Period, though they are nearly extinct today.  A trunk showing obvious leaf-scars is on display.

fossil image icon Cycad trunk (Mural Specimen)

conifer cone engraving


Conifers, represented here by a Spruce cone (Picea), continued to dominate the flora of the Cretaceous.

fossil image icon Picea cone

leaf engraving


The angiosperms, or flowering plants, first appeared and became abundant in the Cretaceous. Fossils of the cones" of an alder, Alnus, and the fruit of a fig, Ficus ceratops, are shown.

fossil image icon Alnus

fossil image icon Ficus ceratops


The engravings are from Dana, James D. (1870) Manual of Geology, Le Conte, Joseph (1898) A Compend of Geology, Shimer, Harvey Woodburn (1914) An Introduction to the Study of Fossils,or McMurrich (1894) Invertebrate Morphology.

©1998, Cal Poly Humboldt NHM | Last modified 2 October 2012

Associated Exhibit : 
Life Through Time