Living fossils are evolving

Charles Darwin coined the term living fossil in On the Origin of Species. He didn’t use it the same way that it has come to be used. He suggested that living fossils are modern species that can be used to link to groups in the same way that fossils can. One of the examples he gave was the platypus, which lactates and lays eggs, which is evidence that mammals and reptiles share a common ancestor. I don't think he meant it to mean an unchanged relict, as some people interpret his words.

Today, a living fossil is a species that retains many features of their fossil ancestor so that it is recognisably closely related. There are some stunning examples of this, such as orb-weaving spiders. In 2011 a 165 million year old spider fossil was described by Seldon et al., which shared so many features with modern Nephila spiders that it was placed within the same genus. Interestingly, I have never heard of web building spiders being referred to as living fossils despite there being amazing conservation of traits in many groups.

The orb-weaving spiders Nephila clavipes (left) and N. jurassica (right) are separated by 165 million years, but placed within the same genus (image of N. clavipes from Wikipedia and N. jurassica from Seldon et al. 2011).

Unfortunately, living fossil has become synonymous with a species, or group of species, displaying no evolutionary change or very slow change. This is completely wrong. Although the conservation of morphology in Nephila is remarkable, there are more than 150 known species in the genus. Clearly there has been evolutionary diversification within the group. Indeed, whenever living fossils are examined in more than superficial detail it becomes difficult to see them as organisms that evolution forgot.

Horseshoe crabs are one of the most iconic living fossils. There are four living species in three genera. They are placed within the subphylum chelicerata, which makes them more closely related to spiders and scorpions than they are to true crabs, which are placed within the subphylum crustacea. Although there are fewer species of horseshoe crabs than Nephila, that fact that there are four species that are all different from fossil species is a strong indication that evolution hasn't stopped for them.

The Atlantic horseshoe crab, Limulus polyphemus, mating (photo Wikipedia)

The general shape of modern horseshoe crabs is strikingly similar to the fossils that date from about 450 million years ago. Close examination, though, shows that parts of their shape, their legs in particular, have changed over time. Briggs et al. 2012 looked at a fossil horseshoe crab from 425 million years ago, which is relatively early in their evolution. They found that modern horseshoe crabs are missing an entire set of limbs that were present in their ancestors.

All modern chelicerates, including living horseshoe crabs, have unbranched limbs; each limb is a single series of segments. Most crustaceans have limbs that branch at the base into two series of segments. Branched limbs, like those in crustaceans, are the ancestral condition and unbranched limbs are thought to have evolved several times among the arthropods. Indeed, Briggs et al. found that the fossil horseshoe crab had branched limbs, which have been lost in their descendents. 

Like horseshoe crabs, tadpole shrimp have a broad semi-circular carapace protecting their heads and are considered living fossils. There are 11 recognised species in two genera, Lepidurus and Triops. The two genera probably diverged about 180 million years ago, but there are fossil tadpole shrimp dating from about 250 million years ago. That's not as long as the really iconic living fossils, like horseshoe crabs and the coelacanths, but it is still an impressive amount of time to retain enough features to be easily recognised as related.

The tadpole shrimp, Lepidurus apus (photo Wikipedia)

The problem with relying on features that preserve in the fossil record is that it underestimates the actual amount of evolutionary change because generally only hard parts are preserved. A recent study of tadpole shrimp highlights this point. Mathers et al. 2013 used genetic analyses to construct the evolutionary relationships among the 11 species of tadpole shrimp. They found that there are actually 38 species and that these species arose relatively recently. This shows that rather than evolutionary stasis, there is likely to be high species turnover in the group.

There are many reasons why some features may be conserved over long periods of time. None of these have to do with natural selection taking a break. In fact, if natural selection did cease we should expect to see features wander under random genetic drift, as has been hypothesised for eyes in cave dwelling animals. Conserved features are much more likely to be the result of developmental constraints or stabilising selection.

References:

Briggs, D., Siveter, D., Siveter, D., Sutton, M., Garwood, R., & Legg, D. (2012). Silurian horseshoe crab illuminates the evolution of arthropod limbs Proceedings of the National Academy of Sciences, 109 (39), 15702-15705 DOI: 10.1073/pnas.1205875109 

Mathers, T., Hammond, R., Jenner, R., Hänfling, B., & Gómez, A. (2013). Multiple global radiations in tadpole shrimps challenge the concept of ‘living fossils’ PeerJ, 1 DOI: 10.7717/peerj.62

Selden, P., Shih, C., & Ren, D. (2011). A golden orb-weaver spider (Araneae: Nephilidae: Nephila) from the Middle Jurassic of China Biology Letters, 7 (5), 775-778 DOI: 10.1098/rsbl.2011.0228