Rivalry or coincidence: what causes the extinction of marine top-predators?
Final Report Abstract
Two main driving forces of evolution cause the amazing diversity on Earth: speciation and extinction. Based on this assumption, those individuals best adapted to their environments would more likely survive and reproduce, and each population of organisms within an ecosystem occupies a specific niche with distinct relationships to other parts of the system. Therefore “survival of the fittest” is the main organism-controlled process underlying the reorganisation of ecosystems. However, it is very difficult to track such competitive replacements in the fossil record and all attempts have so far been unsuccessful. In this study the long-lasting hypothesis should be tested that hybodont sharks were superseded by neoselachians in the marine realm, expelled to freshwater environments and finally drove to extinction in both habitats at the K/Pg-boundary 65 million years ago. Hybodonts were the most successful group of sharks during the first half of the Mesozoic, and they became extinct at, or near to, the K-Pg boundary. Up to now it is not entirely clear what caused their disappearance and their fate has often been linked to the rise of modern sharks (Neoselachii), which supposedly forced the slower and less efficient hybodonts into marginal habitats. However, the thorough and comprehensive investigation of all Mesozoic hybodont records, assigned to sedimentary environments using independent evidence, shows no evidence for this common and long-lasting hypothesis. In fact, hybodonts were highly successful, remaining abundant and diverse throughout the Mesozoic. Further, they maintained a nearly constant presence in brackish and fresh waters throughout the Mesozoic especially in Asia with no sign of a long-term trend of increasing preference for any habitat. A decline at the end of the Mesozoic is apparent but is not substantially greater than any other drop; this suggests that the eventual extinction of the last hybodonts was rather abrupt. Therefore, it rather seems that hybodonts did not apparently diminish in importance in the face of a diversifying Neoselachii. However, hybodonts might have been under extreme pressure from the much faster reproducing Osteichthyes for dwindling resources in early development and growth eventually leading to their extinction.
Publications
- 2014. Dental patterns of the stem-group hexanchoid shark, Notidanoides muensteri (Elasmobranchii, Hexanchiformes). Journal of Vertebrate Paleontology 34: 1292-1306
Kriwet, J. & Klug, S.
(See online at https://doi.org/10.1080/02724634.2014.874353) - 2016. Crassodontidanidae, a replacement name for Crassonotidae Kriwet and Klug, 2011 (Chondrichthyes, Hexanchiformes). Journal of Vertebrate Paleontology 36: e1119698
Kriwet, J. & Klug, S.
(See online at https://doi.org/10.1080/02724634.2016.1119698)
