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Araripe fish fossils

The fish fossils of Araripe are perfectly conserved fish bodies, found and excavated at the Chapada do Araripe in northeastern Brazil. The fossils, encapsulated by scales of a brownish color, were sold at the Sunday morning market in São Paulo's Praça da República in the 1970s.

Sao Paolo's Sunday market

Fig. 1 – Four juvenile Tharrhias araripis in each others company.

To the casual tourist strolling along the various stalls of the popular Sunday morning market in São Paulo’s oldest square, the Praça da República, they looked like cheap fakes intended to bamboozle the innocent visitor: perfect fish bodies with fins, scales of a brownish colour, gills and even sharply outlined eyes, held in some sort of cocoon that looked like a sandy coloured cement. Surely, these could not be the real fossilized fish for which the sellers in three of these stalls were trying to sell them?

Most tourists walked on without a further look, convinced that these fakes were concocted from snake skin and heaven knows what else; others bought a few broken its for a few Cruzeiro, as the Brazilian currency was called in the 1970s. However, some people, like Jaap Schut, were fascinated by these perfectly conserved fish bodies, and started a collection of the best specimens. After regular visits to the market, he discovered that the three stalls belonged to a tightly-knit family group, which had established a monopoly of the excavation, splitting of cocoons, and marketing in São Paulo, of the fossils of Araripe.

Geological research

In 1841, the Swiss-American geologist Luis Agassiz published a monograph on the fossils found by Mr. Gardner on the plateau of Araripe, situated in the Brazilian state of Ceará, near the border of Pernambuco. For a long time, nothing else was heard of these findings, until Brazilian geologists Karl Beurlen, Rubens da Silva Santos and Joel Gomes Valença further defined and classified a whole population of some of the earliest bone fishes, respectively in 1963 and 1968, and provided a pictorial panorama in which the entire fish fauna was depicted.

Fig. 2 – Tharrhias araripis, seemingly jumping like a fish in the fisherman’s net.

The geological period to which the Araripe fossils belong is the Lower Cretaceous, which dates them to about 140 million years old. In the same period, a large section of a vast continental mass called Gondwanaland began to drift away from the main body to become the new continent of South America. This period of separation was characterized by shallow seas, strong volcanic activity and the emergence of the first species of bone fishes.

It is notable that the species portrayed in fig. 1 all belong to a marine environment. However, the geological formation in which the cocoons are embedded belongs to a fresh water environment.

Hypothesis

Interestingly, all Araripe fossils are encapsulates in a cocoon of a thickness varying from a few millimetres to about 40 mm. The cocoons have a cement-like appearance of a light, yellow and sandy colour. There are no cocoons without a fossil fish and there are no fossil fish without a cocoon or its remnants. Moreover, the fish bodies inside the cocoons are extraordinarily well preserved. There are no traces of fish bodies partly eaten by predators. Presumably, an entire fish population died en masse and in a very short period of time.

Possibly, a very large volcanic eruption of mostly ash occurred, filling a shallow sea with hot ashes and leading to death by suffocation, poisoning and overheating. Was this the catastrophe that struck the shallow sea at what is now the Chapada do Araripe? That such a large volcanic eruption is conceivable, is proven by the Pinatubo volcano in the Philippines, on the island of Luzon. On the 15^th of June 1991, this volcano erupted in an explosion which threw an ash column of 24 kilometres up in the air. Two American Air Force bases, at Clarke Field, 40 kilometres distant from the eruption, and at Subic Bay, 75 kilometres distant, had to be abandoned because of the damage by heavy ash rains. The 1991 eruption blew about 10 cubic kilometres of material up in the air.

Studies of the history of the Pinatubo volcano have, however, established that an earlier eruption, 35.000 years ago, produced about 25 square kilometres of ashes. Spread over the surface equal to the size of the Araripe plateau, this would have resulted in a layer of about 10 meters thick.

Fossilisation

Fig. 3 – Enneles audax with the front of an Aspidorhynchus comptoni alongside of it. 

The hypothesis of a volcanic catastrophe explains some other puzzles. The first of these concerns the formation of cocoons around each fish body. In the absence of oxygen, and under conditions of pressure and heat, the slow process of fossilisation or petrification would begin. Fossilisation consists of an exchange of atoms which have equal valency (potential bonds); in this case between atoms of carbon in the fish protein and silica in the surrounding ash.

The quantity of silica atoms would have been unlimited, so the fossilisation of the fish bodies continues to completion. On the other hand, the availability of carbon atoms would have been limited by the volume of the fish body. This explains that the penetration of carbon into the surrounding ash is limited to 30-40 millimetres. Dispersion of carbon atoms into other crystalline material is known to have a hardening and toughening effect due to a localised distortion of the crystalline grid. In metallurgy, it is used to make iron into a vastly stronger and tougher steel.

During the millions of years following the mass extinction of the Araripe fish population, erosion by fresh water streams caused the gradual washing away of the layer of volcanic ash. Only the toughened and carbon-reinforced cocoons of the fossilised fish bodies would resist the erosion, leaving the encapsulated fossils clearly visible in what had gradually become a fresh water environment.

Exceptional specimen

Fossils which contain extra information about the dramatic events that took place some 120-130 million years ago are rare. See the following images for some examples.

Fig. 1 – Four juvenile Tharrhias araripis in each others company. Could there be another cause of their joint death and fossilisation than a sudden environmental catastrophe?

Fig. 2 – Tharrhias araripis, seemingly jumping like a fish in the fisherman’s net. 

Fig. 3 – Non-identified shallow water fish. One of its vertebrae has been crushed, and its spine has been dislocated. Being a large specimen, this specimen would not have had many attackers. Furthermore, should it not have been eaten by its attacker? Possibly, the fish was hit by a heavy object, falling from the sky.

Fig. 4 – This fossil was a real puzzle, until a CT scan in the Haga Hospital in The Hague, made in March 2016, resulted in the following conclusion: the Enneles audax, a known predator, met an Aspidohynchus componi, which is floating belly-up, and begins to swallow this far too big prey. Then the head of the Aspidorhynchus comptoni is expelled through one of the gills, followed by about half the length of the prey. In the process, the normally very smooth skin of the Aspidorhynchus comptoni gets badly rumpled. Then the Enneles audax dies, leaving the tail of its prey still sticking out of its mouth.    

References

Bibliography

• Agassiz, L., "On the fossil found by Mr. Garner in the province of Ceará, in the north of Brazil", Edinburgh New Philosophical Journal 30, pp. 82-84
• Beurlen, K., "Geologia e estratigrafia da Chapada do Araripe", XVII Congresso Nacional de Geologia, Recife-Pernambuco (1963), pp. 1-47.
• Silva Santos, R. da and Valença, J.G., "A Formação Santana e sua paleoictiofauna", Anais da Academia Brasileira de Ciências  vol. 40, no. 3 (1968), pp. 339-360.