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
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.
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)
Lobsters
Lobsters (Decapoda) fossils, for example Hoploparia browni, are relatively rare in Cretaceous deposits.
Hoploparia browni
Crabs
Crabs (Decapoda), such as Avitelmessus grapsoideus, which had appeared in the Jurassic, became more abundant.
Avitelmessus grapsoideus
Echinoderms (ToL: Echinodermata<Deuterostomia<Bilateria<Metazoa<Eukaryota)
Echinoids
Echinoids (Echinoidea), sea urchin/sand dollar-like echinoderms, such as Epiaster whitei and Micraster sp. were common.
Epiaster whitei
Micraster sp.Sharks (ToL: Chodrichthyes<Vertebrata<Chordata<Deuterostomia<Bilateria<Metazoa<Eukaryota)
Sharks
The sharks, one of the two major modern fish families, are represented here by a group of fossil sharks teeth.
sharks teethBony Fish (ToL: Osteichthyes<Vertebrata<Chordata<Deuterostomia<Bilateria<Metazoa<Eukaryota)
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.
Rhacholepis buccalisReptilia (ToL: Vertebrata<Chordata<Deuterostomia<Bilateria<Metazoa<Eukaryota)
The display includes an unidentified reptile egg.
reptile eggMosasaur (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.
Tylosaurus model; 
fossil of a complete 29 ft specimen (AMNH, 1899)
Dinosaurs (Dinosauria [Birds and Dinosaurs]<Reptilia)
Ornithischia
The Ornithischia (bird-hipped plant eaters) are represented by a Hadrosaurus tooth plate and Hadrosaurus teeth.
Hadrosaurus tooth plate
life reconstruction
Hadrosaurus teeth
Theropods
The Theropods (predatory bipedal dinosaurs, such as Tyrannosaurus and Birds) are represented by a tooth from an Albertosaurus sp. (note the serrated edges).
Albertosaurus sp. 
life reconstruction
Mollusks (ToL: Mollusca<Lophotrochozoa<Bilateria<Metazoa<Eukaryota)
Bivalves
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.
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.
Actinostrea travisana
oyster relative
snail-shaped oyster valve
spiral oyster valve
second spiral oyster valve
Gastropods
Gastropods are represented by two assemblages: a slab with Goniobasis chrysalis and a rock with freshwater snails exposed.
Goniobasis chrysalis
freshwater snailsThe neogastropods, the most advanced of the marine gastropods, appeared in the Cretaceous, while the older mesogastropods remained numerous.
Cephalopods
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.
Scaphites nodosus
Exiteloceras jennyiTwo 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.
Planticeras meeki
Baculites grandisSponges (ToL: Porifera<Metazoa<Eukaryota)
Sponges
The museum displayes two cup-shaped Rhaphidonema farringdonense sponges in this case.
Rhaphidonema farringdonense 
Rhaphidonema farringdonense Cretaceous Plant Fossils
Vascular Plants (ToL: Embryophytes [land plants] <Green Plants<Eukaryota)
Cycads
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.
Cycad trunk (Mural Specimen)
Conifers
Conifers, represented here by a Spruce cone (Picea), continued to dominate the flora of the Cretaceous.
Picea cone
Angiosperms
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.
Alnus
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