XXII.3.1 The accuracy of dating the emergence and extinction of a species depends on the abundance of its populations and age of the event
The age of fossils can be determined on the basis of leading fossils present in the particular layer.Redeposition, i.e. release of fossils from the original rocks during weathering and secondary transport to layers of a different geological age, can, of course, lead to certain complications.However, the main problem associated with dating of evolutionary events is related to the problem of how to determine when the particular species evolved and became extinct on the basis of the discovered fossil.Basically, the discovery of a fossil of a certain age only allows us to state that the particular species was already and simultaneously still present in nature at the particular time.However, it is not possible to determine when the species appeared in nature or when it disappeared.If a certain species disappeared during a well-dated event, for example during a mass extinction event, its fossil documents will most probably disappear from the paleontological record at an earlier date (Signor & Lipps 1982).For abundant species, it is possible that the youngest fossils will be present close to the layer whose age corresponds to the time of extinction of the particular species.However, if a relatively rare species was involved, only a very few fossils will be available.In this case, it is probable that even the youngest fossils will come from layers that are substantially older than the time when the species still actually existed.This phenomenon is denoted as the Signor-Lipps effect and is valid both for dating the extinction of a species and also for dating its evolution; here, this is sometimes (humorously) called the Lipps-Signor effect.These effects do not constitute such a great problem for marine invertebrates, because their fossils were mostly preserved in large numbers.However, it constitutes a major problem in dating the evolution of terrestrial vertebrates, only a few of whose fossils are found.
The apparent time shift in the fossil record as a consequence of the Signor-Lipps effect introduces substantial uncertaintyinto determination of the causes of extinction of entire taxa.For example, for dinosaurs, there is considerable uncertainty as to whether their extinction was caused by a catastrophe at the boundary between the Cretaceous and the Tertiary, at the KT-boundary, or whether the diversity of this long-successful taxon began to decrease long before this event.In this case, terrestrial vertebrates were involved to a major degree; while their fossils are striking, not many of them have been preserved.Approximately half of approximately 350 species of dinosaurs have been described on the basis of a single preserved specimen (Raup 1994).Under these conditions, it is very difficult to determine the moment of emergence and disappearance of the individual species.Even if an entire taxon were to become extinct at a single moment, most of the rarer species would disappear from the paleontological record long before the Cretaceous-Tertiary boundary.
The pull of the present and the consequent “telescopic character” of the fossil record constitute a further confusing factor affecting the information that can be obtained from the fossil record and the accuracy of dating evolutionary events.Fossils in the younger layers are better preserved and have mostly been preserved in greater numbers than older fossils.Consequently, when, for example, we study the development of biodiversity over time and find that the biodiversity gradually increased, this could possibly actually only correspond to the pull of the present.The same is true of comparison of the rate of extinction on the basis of the number of species or number of higher taxa that became extinct at the given time; this would necessarily yield a greater number of recent extinctions and a lower number of ancient extinctions.
Further substantial distortion can occur if recent species, i.e. contemporary species, are also included in the study.Recent species are far more accessible to study than extinct species and far more traits can be distinguished in them.As a consequence, we are capable of distinguishing a far greater number of species within most taxa than we would be able to distinguish if we were to study the same set on the basis of paleontological material alone.This problem is mostly resolved by not including contemporary species in paleontological comparative studies.