Shown is a picture of a Mesolimulus specimen. Mesolimulus is a kind of animal commonly called a horseshoe crab. It belongs to the arthropod group, Chelicerata. Chelicerata is the same group that the arachnids (spiders, scorpions, and ticks) belong to. Mesolimulus comes from Late Jurassic sediments, supposedly making it between 160 to 145 million years old.
Is there anything that sticks out about the photo or the text describing Mesolimulus?
Sharp creationists might recall that horseshoe crabs are still alive today. The most familiar living genus is Limulus, which is very similar to Mesolimulus. Matter of fact, Mesolimulus was initially classified as Limulus, highlighting just how similar the two species are to one another. Therefore, the fossil shown above is a “living fossil,” a creature that has remained nearly unchanged for supposedly hundreds of millions of years.
Very sharp creationists might have noticed something else. The Mesolimulus shown shown is a little less than six inches long. Why is the length of Mesolimulus important? Because the living Limulus can achieve lengths of up to two feet long. Mesolimulus is significantly shorter than its closest living relative. As a further note, the Mesolimulus specimen shown above is not small, it is average for its genus.
The discrepancy in size between Mesolimulus and Limulus is important because it is a significant contradiction to the idea that every species found in the fossil record is larger than their relatives today. Some things have very large relatives known only from the fossil record. There are dragonfly fossils that have a wingspan of nearly three feet. There are crocodiles known only from the fossil record that are fifty feet long. Many creationists claim that these fossils are organisms that lived in the pre-flood world and that it was the pre-flood world that allowed them to achieve such great sizes. One common idea is that there was a water canopy around the earth that shielded earth’s inhabitants from harmful radiation. This shielding allowed the animals to live longer and since some animals, such as reptiles, grow throughout their lives, the large fossil reptiles are probably very aged animals. Additionally, the water canopy applied a lot of pressure to earth’s atmosphere. The greater pressure allowed a more efficient transfer of oxygen into an organism’s body and this more ready supply of oxygen helped contribute to their large size.
But what are we to make of Mesolimulus? Surely, it too belonged to the pre-flood fauna. How come it is so small compared to its living relative? Did it escape the benefits of the pre-flood world, or is there some other reason?
One explanation is that the horseshoe crab kind has a wide variety of sizes coded for in its genes. So perhaps Mesolimulus simply received all of the small genes while the living Limulus received all of the large genes. That could very easily explain the discrepancy in size.
However, the genetic explanation of the size difference between living and extinct horseshoe crabs poses a problem: couldn’t the same explanation work in reverse? In other words, maybe the three foot dragonfly and the fifty foot crocodile are not large because they lived in a different environment, maybe they simply have genes for large size and their living relatives lack these genes.
How can we test these two ideas?
If the pre-flood environment was responsible for the large size of many creatures known from fossils, then we would expect that fossil specimens would be consistently larger than their living relatives. For instance, all fossil crocodile specimens would be large, all dragonfly specimens would be large. There might be a few exceptions, but there should be a general trend.
On the other hand, if the large size of fossil specimens is due to genetic variation, then we would expect some of the genetic variation that exists today to also exist in the fossil record. So, while there might be a very large crocodile and a very large dragonfly, there would also be average sized crocodiles and dragonflies.
As it turns out, the latter is the case. There is a fossil dragonfly named Petalura that averaged one and a quarter inches long. That is similar to modern dragonflies. There is also a fossil alligator named Diplocynodon. Granted, it is an alligator, not a crocodile, but both animals probably belong to the same kind. Diplocynodon was around ten feet long, making it longer than the living Chinese alligator but far shorter than the American alligator. Diplocynodon could be considered average for living alligators.
These are example of just a few animals, but there is a similar pattern in most other creatures where there is a relative in the fossil record that is larger than living species. Yes, there are fifty foot fossil sharks, but here are also three foot fossil sharks. Yes, there are elephants thirteen feet at the shoulder, but there are also elephants just a few feet at the shoulder. Yes, there are giant lizards, but there are also lizards that are just a few inches long. Yes, there are enormous giant tortoises, there are also tortoises of an average size.
In other words, there is no trend of gigantism in the fossil record. Almost all of the gigantic animals have a relative that are far smaller. There are also the exceptions like Mesolimulus, where the fossil species is actually smaller than the living species. A better explanation is that what we are seeing is a natural variation in sizes. Some of that variation may be lost so that we may never see a gigantic specimen of a dragonfly today. But it is not necessary to claim that a unique pre-flood environment is responsible for the gigantic size of species known from the fossil record.
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