Slimehead Size Through Time: Testing the Temperature-Size Relationship in Late Cretaceous Trachichthyidae.

Area of Science:

• Paleontology
• Climate Science
• Ichthyology

Background:

• Global temperature rise is predicted to decrease fish body size.
• Understanding fish size response to climate change is crucial, especially for commercially important species.
• The deep-dwelling, slow-reproducing slimeheads (Trachichthyidae) present challenges for direct temperature-size relationship studies.

Purpose of the Study:

• To investigate the temperature-size relationship in the fossil slimehead genus *Hoplopteryx* from the Late Cretaceous.
• To determine if fish size declined in response to higher paleotemperatures.
• To explore other environmental factors potentially influencing fish body size.

Main Methods:

• Analysis of the fossil record of *Hoplopteryx* from the British Chalk Group.
• Measurement of Standard Lengths from complete and partial fossil fish remains.
• Estimation of paleotemperatures using oxygen stable isotope analysis (δ¹⁸O) of surrounding chalk rock.

Main Results:

• A significant negative correlation was found between estimated seawater temperature and Standard Length in *Hoplopteryx lewesiensis*.
• Higher estimated temperatures (19.5°C–27.1°C) corresponded to smaller fish sizes (56.3–262.6 mm).
• Carbon stable isotope values (δ¹³C) also negatively predicted Standard Length, indicating other environmental influences.

Conclusions:

• The study supports the prediction that increased temperatures lead to smaller fish body size.
• The fossil record of *Hoplopteryx* provides empirical evidence for this temperature-size relationship.
• Environmental factors like primary productivity may have also impacted fish body size during the Late Cretaceous.