Of Horse-Caterpillars and Homologies: Evolution of the Hippocampus and Its Name

Abstract

The hippocampus was first named in mammals based on the appearance of its gross morphological features, one end of it being fancied to resemble the head of a horse and the rest of it a silkworm, or caterpillar. A hippocampus, occupying the most medial part of the telencephalic pallium, has subsequently been identified in diverse nonmammalian taxa, but in which the “horse-caterpillar” morphology is lacking. While some strikingly similar functional similarities have been identified, questions of its homology (“sameness”) across these taxa and about the very fundamental relationship of structure to function in central nervous system structures remain open. The hippocampal formation of amniotes participates in allocentric (external landmark) spatial navigation, memory, and attention to novel stimuli, and these functions generally are shared across amniotes despite variation in its morphological features. Substantially greater deviation in its morphology occurs in anamniotes, including amphibians and ray-finned fishes (actinopterygians), but its functions of allocentric spatial navigation and/or memory have been found to be preserved by studies in these taxa. Its shared functional roles cannot be used as evidence of structural homology, but given that other criteria indicate homology of the medial pallial derivative across these clades, the similar functions themselves may be regarded as homologous functions if they are based on the same cellular mechanisms and connections. The question arises as to whether the similar functions are performed by as yet undiscovered, shared morphological features or by different features that accomplish the same results via different mechanisms of neural function.

Comparative Contemplations on the Hippocampus

“…2 a2, 3), the hippocampus indeed comprises a folded dentate gyrus and a separated CA field emerging close to the hilar region of the dentate, at more anterior levels ( Fig. 2 a1), the two structures become aligned such as to show a striking similarity to what is found in some reptilian species [Striedter, 2016;Butler, 2017]. A similar arrangement has been described in monotremes, such as Echidna [Hassiotis et al, 2004].…”

The neural basis of tadpole transport in poison frogs

“…Indeed, spatial cognition is an important, but rarely examined, component of parental care [29,30], and motherhood is associated with changes in hippocampal plasticity in rodents (reviewed in [31]). Comparisons of hippocampal involvement in parental care across species may yield particularly interesting results given the functional-but not anatomical-conservation of this structure across vertebrates [32].…”

Ecology, Cognition, and the Hippocampus: A Tale of Two Frogs

“…While there is widespread agreement that the amphibian medial pallium is homologous to the amniote hippocampus, there are notable differences. For example, the amphibian medial pallium lacks the gross cytoarchitecture (i.e., layers or layer-like organization) and the double pyramidal cell morphology [cells of the medial pallium are uniformly pear-shaped; Roth and Westhoff, 1999] that characterize amniotes [Butler, 2017]. In addition, the amphibian medial pallium receives sensory input directly from the thalamus and olfactory bulb whereas, in amniotes, most sensory input is relayed through cortical areas.…”

“…The hippocampus is one of the best studied brain regions in vertebrates. However, we still struggle to understand how the evolution of its structure relates to the evolution of its function [Day, 2003;Salas et al, 2003;Striedter, 2016;Butler, 2017]. Ultimately, though, if we are interested in the evolution of the vertebrate hippocampus, work on amphibians is not just needed, it is imperative.…”