Introduction:

The family Profundulidae is a lineage of freshwater fish, with a limited geographical distribution extending from southern Mexico to Central America, composed of two genera Profundulus and Tlaloc; the latter comprised only four species; however, studies based on the analysis of osteological characters are scarce for this family of fish.

Objective:

To describe the anatomy of the caudal bones and backbone of all species of the genera Tlaloc.

Methods:

A comparative analysis was performed on 44 cleared and stained specimens to identify the differences and similarities of backbone elements and the organization of the caudal skeleton among the species.

Results:

The comparison of the caudal skeleton in the species of the genus Tlaloc, revealed a very similar pattern in the morphology of the species; however, some characteristics were detected that may be useful for discrimination of species of the genus; the triangular shaped hipural plate is a characteristic that has only appeared in T. labialis, while in the rest of the species (T. candalarius, T. portillorum and T. hildebrandi), the hypural plate is fan-shaped. The total number of vertebrae, including the vertebral centrum, in the four species of the genus Tlaloc ranged from 33 to 38, of which 15-17 were abdominal vertebrae and 17-21 caudal vertebrae; T. portillorum had the lowest number of total vertebrae (33); while T. labialis had the highest number of total vertebrae (37-38).

Conclusions:

The consistent relationship to these skeletal elements suggests that the vertebrae and caudal fin ray count is a character of fundamental nature for the profundulid fishes, giving it particular taxonomic significance in that group.