All right. I've asked the same question on the Ichnology Facebook page, and Dr. Emma Rainforth, who wrote her doctoral dissertation on fossil footprints of the Connecticut River valley, answered with what is likely to be the oldest mention of fossil vertebrate footprints in speculative fiction. This was originally published in The Knickerbocker in December 1836, not long after Edward Hitchcock described the tracks scientifically. In fact, Hitchcock wrote both the scientific article and the work of fiction, the latter under the pseudonym Poetaster. It is a dramatic poem later rewritten and called "The Sandstone Bird" because he thought that the three-toed tracks were those of giant primordial birds rather than dinosaurs, though he was aware of the Iguanodon.
In this rather turgid work, a geologist (evidently Hitchcock himself) evokes a sorceress who wakes the Sandstone Bird, Ornithichnites Giganteus or "giant ancient bird track". The bird, rising from the ground like a "sawyer" (i.e., a log bobbing up and down in a river), roundly criticizes the modern Connecticut River valley as lacking its old friends, now extinct, and too chilly besides; moreover, the bird is unhappy with present company. The bird therefore returns to the earth, leaving the geologist irritated but with his pride properly chastened. Here follows the original version, which I have transcribed from The Knickerbocker. If anyone can find an earlier example of fossil footprints in speculative fiction, please let me know; otherwise this must stand as the oldest:
The writer supposes a geologist, solus, examining traces of the Ornithichnites giganteus on the sand-stone, whose shade he apostrophizes thus:
A thousand pyramids have mouldered down,
Since on this rock thy foot-print was impress'd;
Yet here it stands unalter'd: though since then
Earth’s crust has been upheav'd, and fractur'd oft:
And deluge after deluge o’er her driven,
Has swept organic life from off her face.
Bird of a former world! -- would that thy form
Might reappear in these they former haunts!
O for a sorceress nigh, to call thee up
From thy deep sandstone-grave, as erst of old
She broke the prophet’s slumbers! But her arts
She may not practice in this age of light.
'Let the light of science shine!
I will show that power is mine.
Skeptic, cease my art to mock,
When the dead starts from the rock.
Bird of sandstone era, wake!
From thy deep, dark prison, break!
Bird of sandstone era, wake!
From thy deep dark prison break.
Spread thy wings upon our air --
Show thy huge, strong talons here:
Let them print the muddy shore
As they did in days of yore.
Praeadamic bird, whose sway
Ruled creation in thy day,
Come, obedient to my word:
Stand before creation’s lord.”
The sorceress vanish'd; but the earth around,
As when an earthquake swells her bosom, rock'd;
And stifled groans, with sounds ne’er heard before,
Broke on the startled ear. The placid stream
Began to heave and dash billows on the shore;
Till soon, as when Balaena spouts the deep,
The waters suddenly leap'd toward the sky;
And up flew swiftly, what a sawyer seemed,
But proved a bird’s neck, with a frightful beak.
A huge-shaped body follow'd; stilted high,
As if two mainmasts propp'd it up. The bird
Of sandstone fame was truly come again;
And shaking his enormous plumes and wings,
And rolling his broad eye around, amaz'd,
He gave a yell so loud and savage too --
Though to Iguanodons and kindred tribes,
Music it might have seem'd, on human ear
It grated harshly, like the quivering roar
That rushes wildly through the mountain gorge,
When storms beat heavy on its brow. Anon,
On wings like mainsails flapping on the air,
The feather'd giant sought the shore where stood,
Confounded, he who called the sorceress’ aid.
Awhile surveying all the monster paused,
The mountain, valley, plain -- the woods, the fields,
The quiet stream, the village on its banks,
Each beast and bird. Next the geologist
Was scann'd, and scann'd again with piercing glance.
Then arching up his neck, as if in scorn,
His bitter taunting plaint he thus began.
'Creation’s lord! The magic of these words
My iron slumbers broke: for in my day
I stood acknowledged as creation’s head,
In stature and in mind surpassing all:
But now -- O strange degeneracy! one,
Scarce six feet high, is styled creation’s lord!
If such the lord, what must the servants be!
Oh how unlike Iguanodon, next me
In dignity, yet moving at my nod.
The Mega, Plesi, Hylae, Saurian tribes,
Rank'd next along the grand descending scale:
Testudo next: below, the Nautilus,
The curious Ammonite, and kindred forms,
All giants to these puny races here,
Scarce seen except by Ichthyosaurian eye,
Gone, too, the noble palms, the lofty ferns,
The Calamite, Stigmaria, Voltzia -- all:
And O, what dwarfs, unworthy of a name,
(Iguanodon could scarce find here a meal,)
Grow o'er their graves! Here, too, where ocean roll'd,
Where coral groves the bright green waters grac'd,
Which glorious monsters made their frolic haunts;
Where the long sea-weed strewed its very bed,
And fish, of splendid forms and hues, rang'd free,
A shallow brook, (where only creatures live,
Which in my day were Sauroscopic called,
Scarce visible, now creeps along the waste.
And ah! this chilling wind! -- a contrast sad
To those soft balmy airs, from fragrant groves,
Which fann'd the never-varying summer once.
E’en he, who now is called creation’s lord,
(I call him rather nture’s blasted slave,)
Must smother in these structures dwellings call'd,
(Creation’s noble palace was my home.)
Or these inclement skies would cut him off.
The sun himself shines but with glimmering light --
And all proclaims the world well nigh worn out:
Her vital warmth departing and her tribes
Organic, all degenerate, puny, soon
In nature’s icy grave to sink forever.
Sure’t is a place for punishment designed,
And not the beauteous, happy spot I loved.
These creatures here seem discontented, sad:
They hate each other, and they hate the world,
O who would live in such a dismal spot?
I freeze, I starve, I die! -- with joy I sink
To my sweet slumbers with the noble dead.'
Strangely and suddenly the monster sank,
Earth oped and closed her jaws -- and all was still.
The vex’d geologist now called aloud --
Reach'd forth his hand to seize the sinking form --
But empty air alone he grasp'd. Chagrinned,
That he could solve no geologic doubts,
Nor learn the history of sandstone days,
He pour'd out bitter words ’gainst sorcery’s arts:
Forgetting that the lesson taught his pride,
Was better than new knowledge of lost worlds.