Monster Monday: Ammonites - Spiral-Shelled Cephalopods of Prehistoric Seas

Today's Monster Monday is the ammonite, a hard-shelled nautilus-like creature that floated through the prehistoric seas for some 350 million years before being wiped out in the same extinction that ended the age of the dinosaurs.

Ammonites typically had a spiral shell made up of successively smaller chambers that they could flood with gas or water to control their buoyancy. The animal itself was a cephalopod, probably ten-armed, which protruded from the end of the shell. Some ammonites may have been able to withdraw into their shells like a modern nautilus or a snail. Some may have had ink they could release in a cloud like modern squid and octopuses. But when we are talking about ammonites, we are talking about thousands of species across hundreds of millions of years - there was room for plenty of variation. Some - like the nostoceratidae - had irregularly unwound shells and probably floated through the sea like plankton or jellyfish. Some just crawled along the sea floor. Most ammonites were small, with shell spirals reaching no larger than 9 inches in diameter. But the two types of ammonites we will be looking at today were big.

photo by Ghedoghedo, via Wikimedia

Titanites giganteus - Natural History Museum, London

Titanites giganteus did not let the ocean currents decide where it would go, as if it were some kind of overgrown plankton. No, titanites was a swift hunter, jetting through the ocean in search of prey it could wrap its probing tentacles around and gnaw with its sharp beak. Their shells reached 2-3 feet in diameter, much larger than the typical ammonite of its day. But even the bigly named titanites giganteus was dwarfed by:

photo via Wikimedia

German zoologist Hermann Landois sitting next to his fossil of Parapuzosia seppenradensis,
with a wire frame showing the projected size if the specimen were complete. 

Parapuzosia seppenradensis may be a mouthful of a name, but you would be hard pressed to find a predator capable of making this ammonite into a mouthful. The 5.9 foot diameter fossil seen above is broken - with its intact living chamber, estimates put this specimen at 8.4 to 11 feet in diameter, making it by far the largest ammonite species. In life, the creature may have weighed 3,200 pounds, fully half of which comes from its massive shell. If parapuzosia was a hunter like titanites was, it must have been an impressive and terrifying sight to behold.

The following text in gold is available as Open Game Content under the OGL. Open Game Content is ©2018 Jonah Bomgaars.

A spread of ten tentacles around a beaked maw emerge from this spiral shell

Ammonite, Titanites   CR 1

XP 400

N Small animal (aquatic)

Init +1; Senses low-light vision; Perception +4

DEFENSE

AC 17, touch 12, flat-footed 16 (+1 Dex, +5 natural, +1 size)

hp 13 (2d8+4)

Fort +4, Ref +3, Will +1

Defensive Abilities ink cloud, shell (AC 20)

OFFENSE

Speed 5 ft., swim 30 ft., jet 120 ft.

Melee tentacles +1 (2d4 plus grab), bite +1 (1d4)

STATISTICS

Str 11, Dex 12, Con 14, Int 2, Wis 13, Cha 3

Base Atk +0; CMB +0 (+4 grapple); CMD 11

Feats Skill Focus (Perception)

Skills Perception +4, Stealth +4, Swim +8

SPECIAL ABILITIES

Ink Cloud (Ex)

An ammonite can emit a 10-foot radius sphere of ink once per minute as a free action. The ink provides total concealment in water and lasts for 1 minute.

Shell (Ex)

As a move action, an ammonite can withdraw its extremities into its shell, sealing the shell by sliding a pair of hard plates into place as a lid. Its natural armor bonus to AC increases by 4 while it is so protected, but it loses its Dexterity bonus to AC. It cannot see, smell, or attack creatures outside of its shell as long as it remains in this state, and it cannot move except to swim vertically by altering the pressure of internal chambers. The ammonite can emerge from its shell as a free action.

Tentacles (Ex)

An ammonite’s tentacles all strike as a single primary attack.

ECOLOGY

Environment any ocean

Organization solitary, pair, or school (3-12)

Treasure none

This tremendous ammonite has a massive spiral shell 10 feet in diameter

Ammonite, Giant           CR 5

XP 1,600

N Large animal (aquatic)

Init +4; Senses low-light vision; Perception +11

DEFENSE

AC 21, touch 9, flat-footed 21 (+12 natural, -1 size)

hp 52 (7d8+21)

Fort +9, Ref +5, Will +4

Defensive Abilities ink cloud, shell (AC 27)

OFFENSE

Speed 5 ft., swim 20 ft., jet 90 ft.

Melee tentacles +10 (4d6+6 plus grab), bite +10 (1d8+6)

Space 10 ft.; Reach 10 ft.

Special Attacks constrict (4d6+9)

STATISTICS

Str 22, Dex 10, Con 17, Int 2, Wis 15, Cha 5

Base Atk +5; CMB +12 (+16 grapple); CMD 22

Feats Blind Fight^B, Improved Initiative, Lunge, Skill Focus (Perception), Skill Focus (Stealth)

Skills Perception +11, Stealth +6, Swim +14

SPECIAL ABILITIES

Ink Cloud (Ex)

An ammonite can emit a 30-foot radius sphere of ink once per 1d3 minutes as a free action. The ink provides total concealment in water and lasts for 1 minute.

Shell (Ex)

As a move action, an ammonite can withdraw its extremities into its shell, sealing the shell by sliding a pair of hard plates into place as a lid. Its natural armor bonus to AC increases by 6 while it is so protected, but it loses its Dexterity bonus – if any – to AC. It cannot see, smell, or attack creatures outside of its shell as long as it remains in this state, and it cannot move except to swim vertically by altering the pressure of internal chambers. The ammonite can emerge from its shell as a free action.

Tentacles (Ex)

An ammonite’s tentacles all strike as a single primary attack.

ECOLOGY

Environment any ocean

Organization solitary, pair

Treasure none

Ammonites are cephalopods, like squid and octopuses, that live in hard, spiral shells. The front of their body – the tentacles, eyes, and beak – are usually exposed at the mouth of the shell, but these creatures can withdraw their soft parts into their shells for protection. Most species of ammonite are less than one foot in length, but the titanites ammonites have shells two to three feet in diameter and are capable swimmers and deadly hunters. The rare giant ammonite is a true monster of the deep, with shells up to ten feet in diameter.

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Plot Hooks and Encounter Ideas

• While diving for a lost treasure, the adventurers are assaulted by a a school of swift ammonites, lashing out with tentacles and beaks at the soft-skinned swimmers.
• Before the mer-king will grant his assistance to the party, they must bring him tribute - the shell of a giant ammonite to adorn his feast-hall. 

Okay, before you go, you need to see this ridiculous reconstruction of Parapuzosia from 1895, when I guess they didn't think ammonites could swim:

via Wikimedia

'Real human skeleton for scale' was the 'banana for scale' of its day. 

-your nektonic d20 despot