Dinosaur Hit Dice

I’ll soon start dealing with dinosaur stats for my barbarian Conqueror King setting. However, the question arises whether the high hit dice of the ACKS core-book dinosaurs are justified. After all, a Stegosaurus has more HD (11) than an Adult dragon (10), and a T-Rex has the same HD (20) as a Venerable dragon. While this was one of the largest land predators to walk the Earth, should it have as much HD has a massive fantastic dragon?

I ask myself this question every single day. This is true of a large number of creatures in ACKS, actually. Another example is sperm whales. The HD of all of these creatures were based on values from B/X D&D.

When I began working on Lairs & Encounters I derived a formula that provides the “hit dice from weight” of a creature.

Consider that in ACKS, HD represent 2 things: 1) sheer mass, with commensurate ability to absorb damage; 2) fighting prowess. Contrasting an 8HD hill giant with a 9HD elephant, it seems like more of the hill giant’s HD represent fighting prowess while more of the elephant’s HD represent mass. But how much?

With that in mind, here’s the formula:

HD from Weight = Weight ^ (0.4) / 10
For these purposes each HD = 4 hp, so 1+1HD = 1.25, 1-1HD=0.75, etc.

Let’s consider two creatures of different mass but the same HD, a man and a goblin.
Goblin (1-1HD, 50lb): 50^.4 / 10 = 0.48. A goblin gets 1/2HD from its mass and the other 1/4HD from its ferocity.
Human (1-1HD, 150lb): 150^.4 / 10 = 0.75. A normal man gets all of his HD from his mass.

Now let’s consider two creatures that weigh the same but have very different HD, a donkey and a tiger.
Tiger (6HD, 500lb): 500^.4 / 10 = 1.2. A tiger gets 1+1 HD from its mass. The other 4+3HD are from its ferocity.
Donkey (1.5HD, 500lb): 500^.4/10 = 1.2. A donkey gets 1+1 HD from its mass, and the last +1 (1hp) are from its ferocity.

So far so good. But now consider:

Venerable Dragon (20HD, 36,000lb): 36,000^0.4 /10 = 6.65. A venerable dragon gets 6+3 HD from its mass, and the other 13+1 HD from its ferocity.
Sperm Whale (36HD, 120,000lb): 120,000^0.4 / 10 = 10.76. A sperm whale gets 10+3 HD from its mass, and the other 25 HD from its ferocity.

It seems reasonable that a dragon gets twice as many ferocity HD as it does weight HD. Does it make sense for a sperm whale? We’d be forced to conclude that a sperm whale is far more ferocious than a dragon!

Another way of thinking about the same problem is to calculate what a creature’s weight exponent is, relative to its HD, such that weight = (HD*10)^ Weight Exponent.

A normal man has a weight exponent of ~ 2.5.
A dragon, horse, and a sperm whale have a weight exponent of ~ 2.0.
A tyrannosaurus rex has a weight exponent of ~ 1.8.
A tiger has a weight exponent of ~1.5.

We could, of course, make dragons weigh less while keeping their HD the same (they are fictional creatures of course!) but most people seem to want and envision Smaug-sized dragons at the high end.

If dragons were as tough, pound-for-pound, as T-Rexes, their HD would look like this:
Spawn: 2+3
Very Young: 5+3
Young: 9
Juvenile: 12+2
Old: 23
Very Old: 26+2
Ancient: 30
Venerable: 34
Hugest Venerable: 40

If dinosaurs were only as tough, pound-for-pound, as dragons, then a t-rex would have 12+1 HD.

Thanks for the VERY detailed answer! I will use this formula for my dinosaurs.

So far in my new dinosaurs:

Alamosaurus: 8+1 HD from its mass alone (60,000lb). Total 8+1 HD.
Ankylosaurus: 4+2 HD from its mass (13,000lb), 3+2 HD from its ferocity (angry herbivore with massive club-tail). Total 8 HD.
Carnotaurus: 2+3 HD from its mass (4,000lb), 4+2 HD from its ferocity (nasty predator with a massive bite). Total 7+1 HD.

I came back to this because I've been looking at using some of the ideas from Goodman Games' Broncosaurus Rex for one of the undescribed continents, and the last bit lined up so closely with some math I ran that it's a bit scary.

There are five dinosaurs in the ACKS core book - the 1 HD pterodactyl, 5 HD pteranodon, 11 HD Stegosaurus and Triceratops, and 20 HD Tyrannosaurus Rex. In Broncosaurus Rex, those dinosaurs respectively have 1, 8, 14, 16, and 18 HD. Trying to map out a logical conversion, what I came up with was that 1 ACKS hit die (HDA) was equivalent to 1 Broncosaurus Rex hit die (1 HDBR), and that above that, 3 HDBR = 2 HDA. This would give a 1 HD pterodactyl, 5 HD pteranodon, 9 HD Stegosaurus, 11 HD Triceratops, and...13 HD Tyrannosaurus, compared to the 12+1 for "as tough as dragons." As a side note for those of us who grew up on Harryhausen stop-motion animation, the 11 HD Trike and 13 HD T-Rex make for a better battle than 11 and 20.

I'm using dinosaurs in my upcoming campaign (first session scheduled for January). I went for size-based hit dice:

Man-sized: 2 HD
Large: 4 HD
Huge: 8 HD
Gigantic: 12 HD
Colossal: 16 HD

My dinos also use d12 for hit dice instead of d8. That makes them damage sponges without directly increasing their offensive abilities.

Pedatory dinosaurs gain a +3 bonus (a Large predatory dinosaur has 4+3 HD); giving them a combat edge until they are just too big for it to matter.

Ya'll want to waste an afternoon, start digging through the various Wikipedia classification pages on dinosaurs and such.

Of the ACKS types:

Pterosaurs are a completely different branch than Dinosauria, which I did not know

Stegosauruses fall under Ornithischia, "bird hipped" a family holding bipedal and quadrapedal herbivores, and that also includes the Triceratops (and anklyosaurs)

The lizard-hipped dinosaurs are firstly the Theropods, variably sized carnivorous bipeds, which is the Tyrannosaur, and everything else that keeps Sam Neill up at night.

The second group in that is the Sauropods, the largely quadroped, long tailed and long necked types like brachiosaurus, with no ACKS entry.

If I'd run a game with dinos I'd probably start with 4 main types:

Theropods - vary in size and speed, but ferocious carnivores with high bite damage

Pterosaurs - the flying ones

Orinthisaurs - variably sized herbivores, either quadropeds with varying bonuses to AC (anklyosaurs), tail attacks (stegosaurs) or charge attacks (triceratops), then bipeds with headbutts or lighter bites.

Sauropods - self explanatory, probably. Step on you or hit you with a head or tail. Earlier forms were smaller and bipedal.

The various aquatics are apparently classified as reptiles, but a big swimming monster that'll eat you is relatively simple to put into service.

I would expect the Orinthisaurs to be more like very dangerous buffalo, rhinos, hippos, and the like, perhaps the bipedal ones move more like the faster antelope-types. Sauropods would move like elephants. Pterosaurs and Theropods are already well-covered.