Cambrian Shutter of Doom Becomes Sucker of Worms

Summary

This article talks about the discovery of fossils of Anomalocaris, a Cambrian segmented invertebrate. It primarily concentrates on the shape of their bodies based on fossil evidence, which would impact the assumptions on the creature. The article presents the different hypotheses that paleontologists had on the structure of Anomalocaris. It begins by introducing paleontologist Charles Walcott’s idea that it was an archaic form of a jellyfish. However, the discovery of their mouth structures gave evidence that Anomalocaris was a predator that fed on hard-shelled trilobite. This was, again, contradicted by evidence that claimed Anomalocaris didn’t feed on hard-shelled trilobites due to their mouth shapes. It is then presented that the Anomalocaris probably fed on small prey in the mud by sucking them up instead of chomping on them.

Fossils used to determine age of organism

The age of a fossil can determine the age of the organism and seeing which layer of rock it is found in can identify the fossil’s age. The older the fossil, the deeper it would be located in the layers. According to the article, scientists found out the age of the Anomalocaris fossil by identifying other fossils found nearby in the same site (Burgess Shale Site.) The article reveals that the fossils were 505 million years old.

Morphology providing evidence for common ancestry

Anomalocaris was such a strange creature to the point where scientists assumed the parts that made up the creature were from different creatures and it was only recently that scientists found out what it looked like. The creature is described to have stalked eyes, swim flaps, spiked grasping appendages, and plated mouths making it look alien-like. When scientists assumed it was an archaic jellyfish, this was primarily due to a flattened ring that was identified on the fossil, which were the squishy discs on the mouth of the Anomalocaris. This misconception, however, was replaced with the discovery of its different body parts that made it become categorized with arthropods in the fossils. This is due to Anomalocaris’s segmented body that its swim flaps extend out from. This means that current arthropods such as shrimps have a high chance that they evolved from Anomalocaris.

http://www.wired.com/images_blogs/wiredscience/2011/05/Image-1-Laggania-660x440.jpg

What it is

The four major evolutionary developments start with the development of three tissue layers, continuing with the development of, in order, bilateral symmetry, internal cavity, and type of zygote cleavage. The three tissue layers that are developed in triploblastic creatures such as arthropods are the ectoderm, endoderm, and mesoderm. However, tissues were never brought up in the article so there is no way to determine if Anomalocaris are animals through this development. Despite this, we can see from the picture above that Anomalocaris is an animal because it has a bilateral symmetry. Each half of Anomalocaris has an eye, a spikey appendage, and swim flaps. Thus, this gives support to the argument that Anomalocaris are animals. It is implied in the article that Anomalocaris have internal cavities simply because it discusses about the mouth of the creature and what it probably fed on (hard-shelled trilobites or scraps on the ocean floor such as worms.) This means that Anomalocaris is either a pseudocoelomate or a coelomate. Psuedocoelomates and coelomates have body cavities unlike acoelomates but psuedocoelomates develop their body cavities between the endoderm and mesoderm whereas coelomates develop their coeloms in the mesoderm. According to the article, it is not mentioned whether the Anomalocaris is a psuedocoelomate or a coelomate. It is known that the type of zygote cleavage developed primarily applies to coelomates in which they can be divided into two groups of protostomes and deuterostomes. In protostomes, the zygote divides into two cells, which keep dividing and end up forming the blastula. The blastopore, which is the region where initial opening occurs during the early stages of embryonic development, forms to become the mouth in protostomes. While in deuterostomes, the blastopore becomes the anus first. The article, however, does not discuss anything related to the type of zygote cleavage that Anomalocaris could have possibly developed. Based on this evidence alone, we can identify that the Anomalocaris was an animal that had bi-laterally symmetrical bodies and, in addition, was either a pseudocoelomate or coelomate.

What it isn’t

It is not radially symmetrical because if it was sliced in several different ways, there wouldn’t be any matching sections. It is also not an acoelomate because they although they are bilaterally symmetrical like acoelomates, they have body cavities. Therefore we can rule out all claims that it was either Ctenophoras, Cnidarias, Poriferas, or Choanoflagellates as these are all organisms that do not have bilateral symmetrical bodies. Anomalocaris also are not Platyhelminthes or Acoelomorphas because these are organisms that do not have body cavities.

What is not preserved in a fossil sample

Tissue is not preserved in fossil samples because it does not preserve well, especially compared to bones. Therefore, it is hard for scientists to determine a creature’s tissue sample based on fossil evidence. This is probably why the article did not discuss issues such as whether Anomalocaris were pseudocoelomate or coelomates, which in turn would make it hard to determine whether it was a protostome or deuterostome. These facts can only be hypothesized based on evidence from modern world descendants of the Anomalocaris such as shrimps.

Source: http://www.wired.com/wiredscience/2012/05/cambrian-shutter-of-doom-becomes-sucker-of-worms/