The Styxosaurus

Once within the times Cretacean, Leviathan, a grand creation,
Moved with grace and speed above the fractured ocean floor—
Presently her gentle gliding mirrored the fishes, all residing
In the depths while wafting, sliding to avoid the predator.
The styxosaurus, shard-like teeth, the reefs she did explore,
For a languid floating, noting nothing more.

But deep into the water peering was the Tylosaur, still leering
with his toothy grin endearing in an open silent roar.
Eagerly the beast had waited in the shadows with breath baited
Hoping soon to be well-sated,
sated on the lengthy styxosaur.

Though her world was spacious and her appetite predacious
there is no safety in Cretaceous and in time her fate would find her.
Soon the Tylosaur was turning, adenine triphosphate burning,
all his soul within him yearning, as he pulled up straight behind her,
He was, perhaps a favor as a fatal great reminder.

Ennervation took its token and with force her neck was broken
And her prayers remained unspoken as the giant maw enclosed.
And she struggled in delirium, the great impressive therium,
Then fell, ensanguined, weary and to permanent repose.
(Soon she’d start to decompose.)

The story’s grim rhetoric on the victim prehistoric
Will forever represent the fate that every one of us sustains.
And the beast, while waiting lonely will surely be beholden only
to whoever should discover what remains of her remains.

But bones do not grow weary, fossils don’t feel faintly teary,
And she’s waiting—with no organs she will not be sick-at-heart.
And what’s left has left no token, slowly she becomes more broken,
when for her not a soul has spoken since so long ago she did depart. 
And the Tylosaur was slated with the same fate that was fated,
And still their souls so long have waited till they both will fall apart. 

(For ohplesiosaur's dinosaur or ocean creature contest)