Logical Errors in Determining Size Sexual Dimorphism in Extinct Animals

Scientists, even professionals, can be fallible in their conclusions. In the study of paleontology, making assumptions is an inevitable part of the discipline. Assumptions, or hypotheses, can help set a course for investigation. However, there are good and logical assumptions to make, as well as illogical and bad assumptions. This article is partly related to my previous article "Size Sexual Dimorphism in Dinosaurs - A Hypothesis".

Imagine a world in the future where humans are long extinct, and a member of a species with advanced intelligence 50 million years from now is examining the fossilized remains of Homo sapiens. They come across a few average sized male specimens from the period of 500 B.C. Then they find a few specimens of average sized females from around the year 2000 A.D. They might make the mistake of concluding that there was very little if any size sexual dimorphism in humans. Because the average sized male from 500 B.C. was very close to the height of the average female from 2000 A.D. However, if they had found an average sized female from 500 B.C. they would find that she is much smaller than the average female of 2000 A.D. Conversely, if they found average males from 2000 A.D., they would find that they are significantly larger than the females. Due to humans developing the ability to mass produce food more efficiently, our complex knowledge of nutrition, and technological advances in health care, we were able to grow considerably larger in a relatively short period of time. Dinosaurs did not have advanced civilization like humans, so if they are able to confirm different sexes of fossils from 1,500 years apart, that might give an indication of sexual size dimorphism. However, if the ages of the fossils varied by a few million years, that would be a different case. Changes in the ecosystem can affect food type and quantity, which can affect the size of the population.

So you are a paleontologist. You have traveled the world and found a new species with four or so representative and relatively complete specimens. You are somehow able to confirm that there are two males and two females. The two females are slightly larger than the two males. You are also able to prove that they come from the same time period. So the females must definitely be larger, right? Maybe. But...not so fast. You found these specimens in different parts of the world, or different parts of a large continent? Here's another fact of nature...

The average size of a species can vary with different sets of populations. There can be several causes for this, including variations of diet, minor variations in genetics, geographic location, lifestyle, and availability of food.

Diet and/or lifestyle can be taken into account when examining the differences in sizes in the current world population of the killer whale. They are the same species, but have minor genetic variations. The types tend to avoid each other and are believed to have not bred together for at least the last 10,000 years.

The first type are called residents, who have extremely tight family bonds, travel in large groups, and feed almost exclusively on fish, occasionally eating squid. They travel in medium sized to large groups of between 8-25 members. Living close to shore, they usually stay in one area for an extended period of time before travelling.
The second type is referred to as transients. These orcas travel in smaller groups of usually 5-6 members. Their diet is vastly different than the residents, they prey on marine mammals and sea birds. They also live close to shore, but generally do not stay in the same area for as long.

The third type is known as offshore. They travel in large groups and have a diet similar to the residents, though they tend to feed on different types of fish than the residents. They live in large groups, usually 50 or more. They travel almost constantly and unlike the residents and transients, they do not live close to shore, but in the pelagic regions of the ocean.

The residents on average are the largest sized population of the killer whale, the transients are slightly smaller, and the offshore orcas consist of the smallest individuals. There are also subtle morphological differences such as the shape of the fin, coloring, and pattern of the “tuxedo”.

Failure to examine fossils accurately has led to embarrassing results. Size estimates were given for species such as Brachiosaurus altithorax and Giraffatitan brancai based on the most complete specimens found. Years later, further examination of the bones revealed that both of them were sub adults, and had not finished growing.

Baseless assumptions have also led to recanting of claims. “Sue”, the largest Tyrannosaurus specimen ever discovered, was long claimed by scientists to be female since the bones had wide hips. However the rest of the body was also robust, and they later admitted that they did not know the sex of “Sue”. The only Tyrannosaurus specimen whose sex was confirmed was one named “B-Rex”. She was found to be female due to the presence of medullary tissue on the fossilized bone. Medullary tissue is formed by female birds during ovulation. No other specimen of Tyrannosaurus has a confirmed sex.