UPPER CRETACEOUS

Turonian

SERIES

Stages

Informal substages

Western Interior ammonite age

spans and radiometric ages (m.y.)

Santonian Coniacian

Lower Upper Middle Lower Upper Middle

Informal zone nos.

Scaphites ferronensis 33

Prionocyclus macombi 31

Collignoniceras praecox 29

Collignoniceras woollgari 28

Clioscaphites saxitonianus Cladoceramus undulatoplicatus

 87.14 44 Magadiceramus crenelatus

Scaphites ventricosus 43 Volviceramus involutus

Western Interior inoceramid 

age spans

Cremnoceramus crassus inconstans

39 Cremnoceramus deformis erectus

Scaphites mariasensis 38 Cremnoceramus walt. waltersdorfensis

42 Volviceramus koeneni

 88.55

41

Prionocyclus germari 37 Mytiloides scupini

Scaphites nigricollensis 36 Mytiloides incertus

35 Inoceramus dakotensis

Scaphites whitfieldi

34 Inoceramus perplexus

Scaphites warreni 32 Inoceramus dimidius

Prionocyclus hyatti 92.46 30

Inoceramus howelli

Mytiloides hercynicus

Mammites nodosoides 27

Lower

Vascoceras birchbyi 93.48 26

25 Pseudaspidoceras 

 flexuosum

Mytiloides subhercynicus

Mytiloides mytiloides

Mytiloides kossmati

Watinoceras devonense 24 Mytiloides puebloensis

Nigericeras scotti 23 Mytiloides hattini

Upper

Cenomanian 

Neocardioceras juddii 22

21

20

19

18

17

16

15

14

13

12

11 

10 

 9

Burroceras clydense

Euomphaloceras 

 septemseriatum

Inoceramus pictus

Vascoceras diartianum 

Dunveganoceras conditum

Dunveganoceras albertense

Dunveganoceras problematicum

Dunveganoceras pondi 94.71

Inoceramus ginterensis

Plesiacanthoceras wyomingense

Acanthoceras amphibolum 94.96

Acanthoceras bellense

Plesiacanthoceras muldoonense

Acanthoceras granerosense 

Conlinoceras tarrantense 95.73

Inoceramus prefragilis

Inoceramus rutherfordi

Inoceramus macconnelii

Middle

 

90.60

0.39

0.59

0.46

0.58

0.58

93.19 0.42

93.99 0.57

0.49

0.50

0.51

Magadiceramus subquadratus

Cremnoceramus crassus crassus

40

Inoceramus arvanus

Lower

(Gap in biostratigraphic record)

Neogastroplites maclearni 

 8

 7

 6

Neogastroplites americanus 

Neogastroplites muelleri 

Scaphites preventricosus 

93.82 0.30

93.32 0.38

Neogastroplites cornutus 

Neogastroplites haasi

99.6 0.9 

Inoceramus maclearni

98.88 

 0.35 

1

2

3

4

5

45

Middle

Clioscaphites vermiformis Cordiceramus cordiformis

Cordiceramus bueltenensis

Upper

46

Clioscaphites choteauensis

Desmoscaphites erdmanni

Desmoscaphites bassleri 84.30 

Cordiceramus muelleri

Sphenoceramus lundbreckensis

47

48

49

Scaphites leei III 50

Campanian

Lower

Scaphites hippocrepis I 51

Scaphites hippocrepis II 81.86 0.36 52

Scaphites hippocrepis III 

Baculites sp. (smooth) 

Baculites sp. (weak flank ribs) 

Middle

53

54

55

Cataceramus subcompressus

Cataceramus balticus

Baculites obtusus 80.58 0.55

Baculites maclearni 

56 “Inoceramus”azerbaydjanensis

Baculites asperiformis 

57

58

Baculites sp. (smooth) 

Baculites perplexus 

59

60

Baculites gregoryensis 61

Baculites reduncus 62

Baculites scotti 75.77 0.34 63

Scaphites depressus 

Didymoceras nebrascense 75.95 0.41 64

“Inoceramus” tenuilineatus

Didymoceras stevensoni 

Exiteloceras jenneyi 75.05 0.32 Sphaeroceramus pertenuiformis 65

66

Didymoceras cheyennense 74.74 0.23 67

Baculites compressus 73.52 0.39 

Baculites cuneatus 

“Inoceramus” altus

Baculites reesidei 

Baculites jenseni “Inoceramus” oblongus

Baculites eliasi 71.98 0.31 “Inoceramus” redbirdensis

68

69

70

71

72

Endocostea typica

“Inoceramus” incurvus Baculites baculus 73

Baculites grandis 70.00 0.45 74 Trochoceramus” radiosus

Upper

Baculites clinolobatus 

Maastrichtian

Lower Upper

 69.59 0.36 75 “Inoceramus” balchii

Hoploscaphites birkelundae

Hoploscaphites nicolletii

Jeletzkytes nebrascensis 

K-T boundary = 65.50 0.10 

76

77

78

Inoceramus aff. dimidius

Inoceramids present

0.34

Cremnoceramus walt. hannovrensis

“Inoceramus” gibbosus

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

Figure 2. Molluscan fossil zones, informal zone numbers, fossil 

ranges, and radiometric ages for marine strata of Cenomanian, 

Turonian, Coniacian, Santonian, Campanian, and Maastrichtian 

Stages (Upper Cretaceous) in the Western Interior of the United 

States [modified after Ireneusz Walaszczyk (written commun., 

2011); Cobban and others, 2006; Kennedy and others, 1996; 

Cobban, 1977]. m.y., millions of years.