Evolutionary anatomy and phyletic implication of the extrinsic cardiac nervous system in the Philippine tarsier (Tarsius syrichta, Primates) in comparisons with Strepsirrhines and New World monkeys

To provide information on evolutionary morphological differences and similarities between Tarsiformes (tarsiers) and other primates (Strepsirrhini and Platyrrhini), we examined the previously undescribed extrinsic cardiac nervous system (ECNS) and surrounding structures in three Philippine tarsiers (Tarsius syrichta). We found that the complicated ECNS morphology shows little variation in contrast to significant anatomical diversity of the surrounding somatic structures in tarsiers as reported in our previous primate studies. Further, our comparative morphological data on the ECNS show the following aspects of evolutionary anatomical significance; (1) the traits of the general relationship between the superior cervical ganglion and the spinal nerves are common to the Tarsii and Strepsirrhini, (2) the traits of the general composition of the cervicothoracic ganglion, its communicating branches to spinal nerves, and its cardiac nerve are common to the Tarsii and Platyrrhini, and (3) the traits of the general relationship between the middle cervical ganglion and the spinal nerves and its cardiac nerve are common among Strepsirrhini, Tarsii, and Platyrrhini. Therefore, the general anatomical characteristics of the ECNS in tarsiers retain a primitive state for some traits and exhibit a derived state for others. These characteristics of the ECNS also reflect an evolutionary history without the modification and specialization as seen in the surrounding somatic structures. It may play a key role in unifying future studies on molecular phylogeny and updating evolutionary anatomical information.