Elasmobranchii
Trygonidae
_Xiphotrygon_ (sting ray)
Chondrostei
Polyodontidae
_Polyodon_ (paddle fish)
Holostei
Lepisosteidae
_Lepisosteus_ (gar)
Amiidae
_Amia_ (bowfin)
Isospondyli
Osteoglossidae
_Phareodus_
Gonorhynchidae
_Notogoneus_ (sand fish)
Clupeidae
_Diplomystus_ (herrings, shad)
_Knightia_
Ostariophysi
Siluridae
_Ameiurus_ (catfish)
Xenarchi
Aphredoderidae (pirate perches)
_Erismatopterus_
Asineopidae
_Asineops_
Acanthoptergii
Percidae
_Mioplosus_ (perches)
Serronidae
_Priscacara_
More than 40 species of fish have been described from the Green River
shales, but authorities do not agree on how many of these are valid.
Because of this, only super specific groups will be considered in the
following discussion.
[Illustration: Fig. 14. _Xiphotrygon_, a fossil ray. These forms are
present but rather rare in the fish deposits of Fossil Butte. Length
of original, 39 cm. _Collection of University of Wyoming._]
One of the strangest forms to come from Fossil Butte is that of
_Xiphotrygon_ (Fig. 14). This is not a true fish, but is a sting ray,
related to sharks. Closely related forms still survive, with little
change, in coastal waters in many parts of the world. Like sharks, sting
rays have a skeleton composed of cartilage. Normally, cartilage is not
preserved as a fossil as it disintegrates readily. The excellent
skeletons of _Xiphotrygon_ are good evidence of how well the shales of
the Green River Formation preserve the fossils. _Xiphotrygon_ is
characterized by being shortened from back to front and flattened from
top to bottom. It possesses a long, whip-like tail and bears enormous,
flat pectoral (anterior) fins. The mouth is on the underside of the body
indicating that _Xiphotrygon_, like its modern relatives, fed along the
bottom of the lake. The strong flattened teeth form something like a
pavement in the mouth, suggesting that it fed on clams and other
hard-shelled invertebrates. Close relatives of _Xiphotrygon_ live mostly
in marine waters, but occasionally enter fresh water.
The paddlefish, _Polyodon_, is rare but present at Fossil Butte. Similar
forms still survive in China and the United States. The prominent
features of _Polyodon_ include a virtually scaleless body, a long,
depressed snout, and long gill rakers. The skeleton, unlike that of most
modern fish, is composed of cartilage, hence skeletons of _Polyodon_ are
rare. The difference between the modern and Eocene paddlefish are so
slight that the two probably filled similar or the same ecologic niches.
This consists of feeding on plankton and other small organisms which can
be obtained from the water. The long gill rakers would provide a large
surface area on which food could be trapped as the water passed out
through the gill slits. The large mouth would also provide a large
surface area for catching and trapping food.
Quite common to streams and rivers of North America from the Cretaceous
to the Recent is the garfish, _Lepisosteus_ (Fig. 15). The most striking
feature of the gar is its diamond-shaped scales, which are extremely
hard and shiny. The body is long, essentially of the same depth
throughout, and ending in a broad-based tail that is slightly
asymmetrical. The mouth is armed with many small, sharp teeth for
catching prey. One specimen over 5 ft long has been found in the Fossil
Butte area.
[Illustration: Fig. 15. The fossil gar _Lepisosteus_. Only a few gar
have been found at Fossil Butte itself. Length of original, 70 cm.
Across the highway from the butte, one specimen over 5 ft long was
found by Carl Ulrich. _Collection of University of Wyoming._]
Fossils of the bowfin _Amia_ (Fig. 16) are recorded from the Green River
shales at Fossil Butte and in stream deposits from Cretaceous to Recent.
This fish is still another “living fossil.” _Amia_ is a nocturnal
predator. Presumably, the Eocene _Amia_ had similar habits. The body in
_Amia_ has become elongate, as has the dorsal fin. The thickness of the
scales has been reduced, but they are still rather heavy and cover the
body. Specimens are rather large and deep-bodied. The tail fin is nearly
symmetrical above and below. _Amia_ is rare in the Fossil Butte fish
quarries.
By far the most abundant fish from Fossil Butte is the genus _Knightia_
(Fig. 17, 18). This fish is a member of the Clupeidae, a family that
includes modern herrings, shad, and sardines. The tail fin is small
relative to the rest of the body. A row of modified scales extends on
the back from the skull to the dorsal fin. Possibly they aided in
streamlining the fish. _Knightia_ apparently fed on the large amounts of
plankton, especially algae, which lived in the waters of Fossil Lake.
Fossils of _Knightia_ are also known from Eocene lake sediments in South
America.
_Knightia_ appears to have been susceptible to mass mortalities. Some
layers of shale at Fossil Butte contain literally hundreds of thousands
of these fish that must have suffered catastrophic mass mortality.
Possible reasons for the obvious sudden death of so many fish will be
discussed in the section on paleoenvironment and taphonomy.
Another Eocene relative of the herring found at Fossil Butte is
_Diplomystus_ (Fig. 19, 20). Like _Knightia_, _Diplomystus_ bears a row
of modified scales on its back. _Diplomystus_ is the second most
abundant fish at Fossil Butte. The jaw in _Diplomystus_ has a rather
pronounced oblique angle to it. The deepest portion of the body is
directly behind the gill region, with a continuous narrowing of the body
toward the tail region. _Diplomystus_ is known from Cretaceous rocks of
Brazil and Syria and from Tertiary sediments in Brazil and West Africa
(Schaeffer and Mangus 1965). Close relatives of _Diplomystus_ now
inhabit the waters off the coasts of Peru and eastern Australia.
[Illustration: Fig. 16. _Amia_, the bowfin. This too is rare but
present at Fossil Butte. Length of original, 67 cm. _Collection of
American Museum of Natural History._]
[Illustration: Fig. 17. The fossil herring, _Knightia_. This fish is
by far the most abundant in the fish beds at Fossil Butte and at
least in one layer was involved in a catastrophic mass mortality.
Length of each fish, 13 cm. _Collection of University of Wyoming._]
[Illustration: Fig. 18. The herring, _Knightia_.]
[Illustration: Fig. 19. _Diplomystus_, another type of herring, is
the second most abundant fish at Fossil Butte. Length of original,
12 cm. _Collection of University of Wyoming._]
[Illustration: Fig. 20. A large _Diplomystus_ with four small bass,
_Priscacara_. Length of _Diplomystus_, 45 cm. _Collection of
University of Wyoming._]
_Phareodus_ (Fig. 21) belongs to a family of fish, the Osteoglossidae,
which began as marine forms and has since become a fresh-water family.
At present, they are restricted to tropical rivers and lakes of South
America, Africa, and Australia. In the past they had a much greater
distribution, as its occurrence at Fossil Butte indicates, including
North America. It is a deep-bodied fish with a large head. The anal and
dorsal fins are close to the tail fins. The many sharp teeth in the
mouth attest to the carnivorous habits of _Phareodus_.
The family Gonorhynchidae, the living sandfish of the Indo-Pacific area,
is represented at Fossil Butte by _Notogoneus_ (Fig. 22). _Notogoneus_
is very long-bodied and quite slender. The body width does not vary much
from head to tail, although narrowing does occur in the tail region. The
tail fin is symmetrical, forming a good rudder. The anal, dorsal,
pectoral, and pelvic fins are all small. The body is covered with small,
elongate scales. _Notogoneus_ was apparently a bottom feeder, living on
small organisms picked off or out of the bottom of Fossil Lake.
The Recent catfish, _Ameuirus_, has also been found at Fossil Butte in
the Green River shales. The form of the Eocene catfish was very much
like the Recent one. The habits of the Eocene catfish were probably like
those of the modern _Ameuirus_, i.e., adaptations to a scavenger-type
existence.
Rather common in the sediments of Lake Gosiute, but not yet known from
the Fossil Basin, is _Erismatopterus_, a member of the family
Aphredederidae or pirate perches. This small, elongate fish has few
distinguishing characters, the rounded front edge of the skull being
most distinguishable.
_Asineops_ is an Eocene relative of _Erismatopterus_ and also one of the
pirate perches. _Asineops_ is, however, placed in a different family,
the Asineopidae. _Asineops_, like _Erismatopterus_, is a rather
plain-looking fish. The dorsal fin is long relative to the rest of the
body. The body is slightly deeper behind the gills than elsewhere,
giving _Asineops_ a rather common appearance.
The Percidae, or perches, are represented at Fossil Butte by _Mioplosus_
(Fig. 23). _Mioplosus_, as indicated by its well-developed teeth, was a
carnivore that probably preyed on its piscine relatives in Fossil Lake.
The large head blends in well with a strongly built, long body. The anal
and dorsal fins are subequal in size and positioned opposite each other.
The tail fin is large and fan-shaped. It is easily identified by the
presence of two dorsal fins.
[Illustration: Fig. 21. A large _Phareodus_, fairly common in the
Fossil Butte fish beds. Length of original, 54 cm. _Collection of
University of Wyoming._]
[Illustration: Fig. 22. _Notogoneus_, a long, slender bottom-feeder,
occasionally found at Fossil Butte. Length of original, 56 cm.
_Collection of University of Wyoming._]
[Illustration: Fig. 23. _Mioplosus_, a fairly common perch from
Fossil Butte. Length of original, 30 cm. _Collection of American
Museum of Natural History._]
[Illustration: Fig. 24. _Priscacara_, a small bass that is common in
the Fossil Butte fish beds. Length of original, 13 cm. _Collection
of University of Wyoming._]
The one genus for which no close living relatives can be found is
_Priscacara_ (Fig. 24). The family Priscacaridae is thought to be
related to the bass. The family is known only from the Eocene. The size
of _Priscacara_ is extremely variable. The body is deep and nearly round
like that of the common sunfish, the head blending in well with the body
and its contours. The most distinguishing feature of _Priscacara_ is the
series of strong, stout spines supporting the anal and dorsal fins.
These spines may have acted as cut-waters for better swimming and/or
they may have protected _Priscacara_ from its more voracious relatives.