What does the dinosaur record show?

Only a few spots on Earth have late Maastrichtian dinosaur record:

• China: only moderately known
• Europe: good for Spain, southern France, Transylvania
• South America: maybe… Great fossils, but correlations VERY poor at present
• Australasia, Antarctica: only very few fossils
• Africa: Sadly, no dinosaur-bearing units from Late Cretaceous
• India & Madagascar: fairly good Maastrichtian fossils, but best are from early Maastrichtian
• Eastern North America: very few good dinosaur fossils

Only good, continuous record for late Maastrichtian and earliest Tertiary, and also earlier in the late Late Cretaceous is western North America.

The last two Ages in the Late Cretaceous Epoch are the Campanian and the Maastrichtian:

Age	Absolute time range(Ma)
Maastricthian	71.3-65.51
Campanian	83.5-71.3

The Montana Group (late Late Cretaceous dinosaur-bearing rocks of western North America) spans the Campanian and Maastrichtian, and has earliest Tertiary rocks right above it. What does the dinosaur record of the Montana Group show us?

Throughout the Montana Group are the same basic groups of dinosaurs:

• Tyrannosaurids: top predators
• Dromaeosaurids: smaller predators
• Ornithomimosaurs, Oviraptorosaurs, Troodontids, Alvarezsaurids: omnivores
• Titanosaurs: rare in the Montana
• Nodosaurids, Ankylosaurines: last of the thyreophorans
• Hypsilophodontians
• Hadrosaurids: one of the two most common herbivore groups
• Pachycephalosaurs
• Basal Neoceratopsians
• Ceratopsids: the other most common herbivore group

Currently, changes in these are best seen in hadrosaurids and ceratopsids; definite changes in tyrannosaurids, ankylosaurines, and pachycephalosaurs; other taxa too poorly sorted out at species level to be certain.

The Montana Group is often divided into four Land Mammal Ages (based on the land mammals, naturally). These are arranged stratigraphically one after the other, as follows:

Age	"land mammal age"
late Maastrichtian	Lancian
early Maastrichtian	Edmontonian
middle-late Campanian	Judithian
early Campanian	Aqulian

The following describes some of the changes in the dinosaur community:
Aqulian:

• Named after Eagle Sandstone (aquila = Latin for eagle)
• Early Campanian
• Only poorly known at present
• Both hadrosaurines and lambeosaurines present
• Both centrosaurines and ceratopsines present

Judithian:

• Named after Judith River Formation
• Late Campanian
• Single most diverse dinosaur-bearing unit of all time (Dinosaur Park Formation of Alberta, Canada) is Judithian
• Both short- and long-snouted hadrosaurines present, former very diverse
• Both tube- and helmet-crested lambeosaurines present, former very diverse
• Both centrosaurines and ceratopsines present and diverse

Edmontonian:

• Named after “Edmonton Formation” (now Horseshoe Canyon Formation)
• Early Maastrichtian
• Short-snouted hadrosaurines extinct, but long-snouted hadrosaurines present and diverse
• Tube-crested lambeosaurines extinct; helmeted-crested lambeosaurines present, but down to only one species
• Centrosaurines down to one species, but ceratopsines common and diverse

Lancian:

• Named after Lance Formation
• Late Maastrichtian: the last nonavian dinosaurs of North America
• Long-snouted hadrosaurines present
• Lambeosaurines extinct
• Centrosaurines extinct, but ceratopsines present and diverse
• Last and largest of almost every major clade: Tyrannosaurus (largest tyrannosaurid, arctometatarsalian, coelurosaur); Ankylosaurus (largest ankylosaurian, thyreophoran); Pachycephalosaurus (largest pachycephalosaur); Triceratops and Torosaurus (last and largest ceratopsines, ceratopsids, marginocephalians)

Pattern among big ornithischians:
Long snouted forms preferentially survive: link to vegetation change?

This pattern consistent with long-term (millions of year scale) change associated with Maastrichtian Regression (and possible vegetation change). However, no evidence that Lancian dinosaurs were declining WITHIN Lancian: might well have continued on to live in post-Lancian if not for Deccan Traps &/or Chicxulub impact.

So, what caused the dinosaurs to die out?

Three equally valid answers:

• Dinosaurs are NOT extinct: toothless birds (Aves) are dinosaurs, and survive today
• Some latest Cretaceous dinosaurs were being killed off by long-term climatic changes due to regression (and volcanism?)
• Latest Maastrichtian non-avian dinosaurs (including toothed birds) were probably finished off by asteroid impact

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Last modified: 14 July 2006