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Acanthostega is an extinct tetrapod genus, among the first vertebrate animals to have recognizable limbs. It appeared in the Upper Devonian (Famennian) about 360 million years ago, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto land.
Description
It had eight digits on each hand and foot linked by webbing, it lacked wrists, and was generally poorly adapted to come onto land. Acanthostega also had a remarkably fish-like shoulder and forelimb. The front foot of Acanthostega could not bend forward at the elbow, and thus could not be brought into a weight-bearing position, appearing to be more suitable for paddling or for holding on to aquatic plants. It had lungs, but its ribs were too short to give support to its chest cavity out of water, and it also had gills which were internal and covered like those of fish, not external and naked like those of some modern amphibians which are almost wholly aquatic.
Therefore, paleontologists surmise that it probably lived in shallow, weed-choked swamps, the legs having evolved for some other purpose than walking on land. Jennifer A. Clack interprets this as showing that this was primarily an aquatic creature descended from fish that had never left the sea, and that tetrapods had evolved features which later proved useful for terrestrial life, rather than crawling onto land and then gaining legs and feet as had previously been surmised. At that period, for the first time, deciduous plants were flourishing and annually shedding leaves into the water, attracting small prey into warm oxygen-poor shallows that were difficult for larger fish to swim in. Clack remarks on how the lower jaw of Acanthostega shows a change from the jaws of fish which have two rows of teeth, with a large number of small teeth in the outer row, and two large fangs and some small teeth in the inner row. It differs, having a small number of larger teeth in the outer row and smaller teeth in the inner row, and she suggests that this change probably went with a shift in early tetrapods from feeding exclusively in water to feeding with the head above water or on land.
Research based on analysis of the suture morphology in its skull indicates that the species may have bitten directly on prey at or near the water's edge. Markey and Marshall compared the skull with the skulls of fish, which use suction feeding as the primary method of prey capture, and creatures known to have used the direct biting on prey typical of terrestrial animals. Their results indicate that Acanthostega was adapted for what they call terrestrial-style feeding, strongly supporting the hypothesis that the terrestrial mode of feeding first emerged in aquatic animals. If correct, this shows an animal specialized for hunting and living in shallow waters in the line between land and water.
Roleplaying
Originally Posted by
Raptorial of the Wizards Community forums.
* D&D Fluff: Acanthostega are some of the most primitive tetrapods you'll find in the murky swamps, eight-toed on each wristless limb, lunged yet not carrying a ribcage that could support it for much time on land, as well as gilled like a fish. Acanthostega are indeed rarely seen, other than the times the pop up to the surface of the weedy murk to take a gulp of air or snatch a bug. With two sharp rows of teeth, picking up an acanthostega from his weed-filled home in the oxygen-poor shallow swamps is generally not recommended. Compounding this is another, more unusual defense. Like some salamanders, acanthostega can secrete a foul and quite sticky slime. This is used by some of the swamp-dwellers as makeshift glue.
| Acanthostega | |
| Small Animal (Aquatic) | |
| Hit Dice | 1/2d8+3 (5 hp) |
| Initiative | +0 |
| Speed | 5 ft. (1 square), swim 30 ft. (6 squares) |
| Armor Class | 12 (+1 size, +1 natural), touch 11, flat-footed 12 |
| Base Attack/Grapple | 0/-7 |
| Attack | Bite -2 melee (1d4-3) |
| Full Attack | Bite -2 melee (1d4-3) |
| Space/Reach | 5 ft./0 ft. |
| Special Attacks | - |
| Special Qualities | Blindsense, low-light vision, sticky secretion |
| Saves | Fort +3, Ref +2, Will +0 |
| Abilities | Str 5, Dex 11, Con 12, Int 1, Wis 11, Cha 2 |
| Skills | Hide +5, Listen +1, Spot +2, Swim +5 |
| Feats | Toughness |
| Environment | Warm marshes |
| Organization | Solitary or cluster (2-5) |
| Challenge Rating | 1/4 |
| Treasure | None |
| Alignment | Always neutral |
| Advancement | 1 HD (Small); 2 HD (Medium) |
| Level Adjustment | - |
* Sticky Secretion (Ex): As a move action, twice per day, an acanthostega grappled by an opponent may release a sticky secretion. If the target is holding the acanthostega with his hands, he has a -6 penalty to Dexterity as his fingers are glued together. The glue either wears off in 2d6 hours or can be broken with a Strength check (DC 16). A creature that has bitten onto the acanthostega and received the secretion cannot bite, and the same wearing off and breaking rules apply. A biter, however, has a 35% of having some of the glue dislodge into its throat after breaking it, dealing 1d4 points of damage as it rasps into the throat.