Fossil frogs / The fossil record is not as bad as you think it is

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Conventional wisdom holds that the fossil record is data-poor compared to the modern record. Introductory biology textbooks always start their explanation of the fossil record by stating that it is imperfect. They point out that there are lots of holes in the fossil dataset: soft parts aren’t preserved, we can’t get DNA out of them, we don’t have that many examples per species, most species don’t leave a fossil record at all, etc. Obviously, to some extent this is true – if you want to study heart valve structure in extinct rhinos or look at genetic variation in Archaeopteryx, your dissertation will probably be pretty short.

Berlin Specimen of Archaeopteryx

Unsuitable for popgen studies
Image (c) 2009 H. Raab

But ask any paleontologist directly, and she will tell you that “exceptional” preservation (soft and/or delicate body parts preserved in good detail) is actually pretty common; that some species have hundreds or even thousands of fossil representatives (especially invertebrates, animals without backbones); and that we actually have a pretty good idea of which types of organisms were on the scene and where for the last 3.4 billion years. Although we don’t have dinosaur DNA yet, we do have it from mastodons, Columbian and woolly mammoths, and neanderthals (to name just four extinct species).

The fossil record is not just “good, considering all the things that prevent an organism from being preserved”; it’s actually good. We can acknowledge its limitations, but we don’t need to apologize for them at the expense of highlighting the wealth of very good data the fossil record bring to the table, or the types of questions it is uniquely suited to answer. If my posts accomplish anything, I hope it is to show that the fossil record offers a lot more than a basic catalog of the weird things that once lived on our planet.

Among my favorite exceptionally-preserved fossils are the many examples of fossil amphibians (especially frogs and salamanders) at various stages of development. I was excited to see this paper (“Post-metamorphic development of Early Cretaceous frogs as a tool for taxonomic comparisons”) by Zbynĕk Roček and colleagues in the most recent issue of Journal of Vertebrate Paleontology. In it, the authors report new specimens of the frog genus Liaobatrachus from the Cretaceous of China.

In living amphibians, limb bones (and some skull bones) start out as cartilage templates that convert to bone as the animal develops. Some parts of the skeleton will convert to bone early, some will convert late in development, and some will remain as cartilage permanently. Evolutionary relationships determine (in part) which elements of the skeleton convert, which don’t, and the timing and order of these conversions. Sorting out the species, genus, and/or family can be difficult for fossil amphibians if you’re not sure what developmental stage you’re looking at. If a particular element isn’t fully converted to bone (a process called ossification), is it because they are a species that converts late or never, or because the animal is too young to have converted yet? Not knowing could mean you misidentify the fossil, or misinterpret its relationships with other species. This is especially a problem for amphibians, because environment influences their body size greatly – a small animal could be young or an adult with stunted growth.

Salamander (Eurycea sosorum) skeleton stained to show bone (red) and cartilage (blue). Image from: City of Austin, TX website

Roček and his colleagues studied several frogs of the same species (Liaobatrachus), found in close proximity to each other at the same field site. All of the specimens had metamorphosed from tadpoles to frogs, but they were of various sizes and showed different stages of ossification. By looking at which bones were partially or fully ossified in different specimens, the authors were able to determine the order of conversion (= the sequence of ossification) for some of the bones. This meant they could order the specimens by age, even though late juveniles and young adults were very similar in body length.

Liaobatrachus from the Early Cretaceous of China

Image modified from Roček et al (2012), Figures 1B (left) and 1G (right). The juvenile (left) has not ossified its wrists, hands, or the ends of its arm and leg bones, unlike the adult (right). Images not to scale.

It also meant that Roček and his colleagues could confirm which features of the skeleton (the bumps, holes, and grooves found on particular bones) didn’t vary at all through development. This means we can identify future Liaobatrachus specimens using those characteristics, without having to worry that we’re looking at a feature that will change as the animal gets older. Features that change as animals get older are particularly problematic for sorting out evolutionary relationships; some closely-related species only start to look different late in development. Also, species that delay their development may resemble the juveniles of other species. To determine relationships among species, it’s best to look at unique features that remain the same throughout development.

So why care about the developmental stages of fossil frogs, beyond better IDs for future fossils? Well, from studying living amphibians, we know that there is a lot of variation in development across the group. For example, some families of salamanders are characterized by the retention of juvenile features (such as small eyes or gills) into adulthood. The evolution of these changes in developmental timing strongly shaped the diversity of living amphibians. Tracking the patterns of development and the evolutionary relationships of amphibians through their fossil record allows us to form hypotheses about when and why those transitions start happening in the first place.

axolotl, Ambystoma mexicanum

Some salamander species (like these axolotls) retain juvenile features such as gills and small eyes into adulthood). Image (c) 2010 ZeWrestler

 

Reference:
Z Roček, Y Wang, and L Dong. 2012. Post-metamorphic development of Early Cretaceous frogs as a tool for taxonomic comparisons. Journal of Vertebrate Paleontology 32: 1285-1292. doi:10.1080/02724634.2012.700666

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