Toucasia hancockensis

Overview

Toucasia hancockensis is a Cretaceous bivalve with two valves of different sizes. The large left valve anchors the animal, while the smaller right valve covers it. Assuming that both valves have the same shape would miss a distinctive feature.^[1]

Key facts

The large left valve has two long, narrow, curved grooves. The smaller right valve is described as having a rounded outline and a keel-like elevation. Examining left and right separately makes the body’s asymmetry clearer.^[1]

Time and place

A 1995 study examined Toucasia material from present-day southwestern Mexico. It is a Cretaceous bivalve; the Cretaceous spanned approximately 140–66 million years ago. Species-level descriptions and material with both valves joined provide clues to its body.^[1][2]

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

Size, form and skeleton

The large attached left valve differs greatly from the small right valve covering it. Narrow curved grooves occur on the large valve, while the small valve is rounded with a raised ridge. The umbo at the shell tip curves upward, and growth folds continue across the surface.^[1]

Habitat and reproduction

Of the two valves enclosing the body, the large left valve forms the attachment. Distinguishing the fixed valve from the covering valve reveals the variety of ways bivalves house their bodies. Features preserved on the hard shell help imagine the position of the soft body inside.^[1]

Fossil discovery

García-Barrera’s 1995 study described both valves of this species and presented articulated shell material. Matching morphological terms in the text with material preserving both valves clarifies the location of each feature. This is morphological research reading one bivalve from several directions.^[1]

References

1. García-Barrera 1995 — Toucasia hancockensis (Hippuritacea-Requienidae) in southwestern Mexico, pp191–194
2. International Commission on Stratigraphy (2024) — Geological time chart
3. Chrząstek (2012) — Turonian Inoceramus from Poland

Research questions

Where do similarly named bivalves differ? In this species, the two grooves on the large valve and the keel shape of the small valve are compared together. Comparing corresponding parts is more important than simply adopting the overall appearance of another Toucasia.^[1]

Observation

Find the two long grooves on the large left valve, then move your gaze to the small right valve. Following the upward-curving umbo and successive growth folds reveals differences between the valves.^[1]