Pathologies in the fish fossils from the Upper Devonian, Famennian deposits of Latvia

The Upper Devonian deposits of Latvia are rich in fossil vertebrate remains that have been studied for several decades. Attention to pathologies in these remains has been given only recently.

The paleontological record of parasitic behaviour is quite incomplete, particularly regarding the parasites of vertebrates. Although fossil remains of parasitic organisms are very rare, traces of their action and parasite-induced pathologies can be found more often even though these are hard to identify and it is even more complicated to determine what was the organism that left the traces. Various skeletal pathologies from Middle (Givetian) to Upper Devonian (Frasnian, Famennian) fishes from Latvia, Estonia and Western Russia were attributed to attacks of parasites (Lukševičs et al. 2009) and predators (Lebedev et al. 2009). The pathologies caused by parasite action described by Lukševičs et al. 2009 include round and oval fossulae, swellings, variously shaped attachment buttresses, openings on various skeletal elements of fishes, and porous spongy formations found on the non-overlapped surface of the scales of porolepiform sarcopterygians. These pathologies were explained as caused by cestode and trematode infestations, attacks of parasites similar to copepod crustaceans, by activity of pathogenic fungi, bacteria, protozoans or algae. Bite marks on skeletal parts of placoderm antiarchs and arthrodires, sarcopterygian porolepiforms and osteolepiforms, and agnathan pteraspidiforms and psammosteiforms were also reported (Lebedev et al. 2009).

Subsequent excavations from 2018-2020 have provided new paleontological material. The paleontological material from the field work and collection of the Museum of University of Latvia has been studied. Pathologies have been found in porolepiform holoptichiid Holoptychius sp. and Ventalepis ketleriensis scales, osteolepiform Cryptolepis grossi scales from the Upper Devonian Tērvete and Ketleri Formations, antiarch bothriolepid Bothriolepis ciecere from the Ketleri Formation and B. ornata armour plates from the Tērvete Formation. Most common pathologies are regular pits on the overlapped and non-overlapped sarcopterygian scale surface similar to those described earlier. Thin sections and CT scans  that have been carried out for the first time confirm that they were formed during animal life. Shallow pits have also been found on the surface of the placoderm armour plates usually close to plate boundaries. Small drill holes on the placoderm armour plates and sarcopterygian scales, in one case connected with a swelling within the scale, might indicate endoparasite activity. Predator bite marks have also been found on the bones both as lifetime damage traces, in some cases accompanied by pathological regrowth of the bone element, as well as post-mortem scratches and marks. Some of the scales and the armour plates also exhibit lesions of unspecific shape on the bone surface, exact cause of these is currently unclear. Difficulties in interpreting the exact cause of the pathologies still should not disencourage further research.