Megarachne servinei

Overview

Megarachne servinei is an arthropod once known as a giant spider and later reidentified as a eurypterid, or sea scorpion. Different readings of rounded structures and appendages in the same fossil produced entirely different accounts of its body.^[1]

Key facts

The type fossil was once kept in a bank vault. After it moved to a museum and a new specimen was found at the same locality, researchers could reexamine the two together.^[1]

Time and place

It came from the Bajo de Véliz Formation in present-day San Luis Province, Argentina. The 2005 paper described the fossil-bearing layer in the context of the Carboniferous–Permian boundary, placing the material at roughly 300 million years ago.^[1][2]

The time range describes source context, not the complete duration of this species.

Size, form and skeleton

The anterior carapace, eye-bearing region, and a greatly broadened tergite are known. An anterior projection once regarded as a spider’s chelicera was also reinterpreted.^[1]

The 2005 study reinterpreting Megarachne as a eurypterid reconstructed a body length of about 54cm. This estimate filled in missing parts by comparing two fossils with the body proportions of related forms.^[5]

Habitat and reproduction

The study associated thinly layered mud deposits with alluvial and lake environments. Plants, insects, and other remains from the same strata were also discussed, helping us consider the environment in which the fossil was preserved.^[1]

Fossil discovery

It was described as a giant spider in 1980, and replicas were used in museum displays. A 2005 reexamination compared the type specimen with the new specimen and proposed an interpretation as a distinctive eurypterid.^[1]

References

1. Selden et al. (2005) — The identity of Megarachne, once thought to be a giant spider
2. International Commission on Stratigraphy (2024) — Geological time chart
3. Selden et al. (2005) — The identity of Megarachne, once thought to be a giant spider
4. Royal Ontario Museum — Reinterpreting the body of Hallucigenia
5. Selden et al. (2005) · The true identity of the supposed giant fossil spider Megarachne

Research questions

Is the rounded part of the fossil a spider’s abdomen or a tergite of another arthropod? Comparisons with a new specimen and the structures of related eurypterids changed the old interpretation.^[1]

Observation

Move your gaze from the anterior carapace to the greatly broadened tergite. Consider how important it is for interpreting an organism’s identity to decide which body part the same outline represents.^[1]