Yup. Stats look about done, though.
It needs a value for its Damage Reduction.
DR 3/— or DR 4/—?
It's description and tactics need some work, the "These "tusks" are really tentacular appendages with additional grasping arms on them, making it a strong swimmer that aggressively chases its prey." bit doesn't make any sense, for a start.
Also, 80 feet is very big for Gargantuan, which is why I put a "40 to 80 feet" in the working draft. If you'd rather only mention the original's 80' in the entry, I'd go for:
An anomalocatis's body is up to 80 feet long, they weigh from 20 tons to 150 tons or more.
We don't need to list a weight if we don't want to.
How's this:
An enormous creature swims into view. Big as a longship, its flattened torpedo-shaped body tapers to a fanlike tail. A row of overlapping finlike lobes run down each flank, undulating in concert with its tail to drive the beast through the water. Its broad head sports two semi-globular eyes on short stalks and what appear to be a pair of giant, walrus-like tusks. A second look reveals these "tusks" to be tentacle-like grasping appendages lined with spiky protrusions. These appendages flex inwards towards a circular mouth on the underside of the creature's head. Said mouth irises in and out, clashing together a ring of jagged teeth; more rings of teeth lie farther down the creature’s throat, ready to grind up anything it swallows.
Anomalocaris, or "odd shrimp", somewhat resembles a giant squid with curved walrus tusks. By far the largest creature in the primeval oceans it calls home, anomalocaris is a free-swimming predator that had no competitors apart from other anomalocaris.
An anomalocatis's body is up to 80 feet long, their weight ranges from 20 tons to 150 tons or more.
COMBAT
An anomalocaris views any creature small enough to swallow as a potential meal. Anomalocaris aggressively chase prey, seizing victims in its pseudo-tusks and transferring them to its mouth to be swallowed and ground up by its toothy gullet.
An anomalocaris will attack prey on the surface as well as underwater. They may mistake ships for rival anomalocaris and attack them, which can result in vessels being capsized by an anomalocaris surfacing beneath them. Any creatures thrown in the water when their craft capsizes will usually be attacked by the anomalocaris that "defeated" its perceived rival.