It took me a while to figure this out but it's not a horse skeleton...
...it's a melted floor walkway panel.
Zahn does the "running down a tunnel away from flame blast" thing and leaps to safety...from the walkway.
In your original image you can see that the melted walkway connects to the walkway in the tunnel.
An unnamed Tropical Disease
The novel and movie cover some of the same ground but differ in a number of ways.
The ship, its contents and the mortal remains of its crew, they having succumbed to a tropical disease, are a century later discovered buried in an old, dried up riverbed of the Niger River.