Cambrian

Ogygopsis klotzi

Introduction:

Now for a group of fossils that may not be the biggest or most popular, but are the most recognizable of the world; the trilobites. With their flat segmented bodies with antennae and legs. These now extinct group of marine arthropods first appeared in the early Cambrian which would have been the predecessors for today’s arthropods such as insects and crustaceans. It was also during this time where morphological diversity really started to bloom and although most were benthic, some developed a more pelagic lifestyle.

Location:

The reason why trilobites are so easily recognizable is because they were found everywhere. The origin however suggests the first was found in Siberia, back when Siberia was not as cold as it is today (closer to the southern hemisphere) based on phylogenetic biogeography. The Ogygopsis klotzi was found in a Burgess Shale.

Features of the Fossil:

Trilobite means “three-lobed” which gives it a clear distinction of cephalon, thorax and pygidium. These three parts make up the exoskeleton which is composed of calcite. They also have soft bodied appendages and one of the new morphologies was the development of eyes. These eyes were specialized compound eyes similar to those of modern day insects but could see in the dark as well.

Something Interesting:

During the Ordovician period, there was a glacial period which affected the warm water algae and dropped sea levels as water was being stored in glaciers. This led to one of the big five extinctions known as the Ordovician extinction wiping nearly 90% of the dominant trilobite species. Following the next extinction, it was the Devonian extinction which brought down the trilobites to a total of four different species and ultimately by the end of the Triassic, all trilobites were extinct.

How did it live in the past?

With over 15 000 species, trilobites would have had many lifestyles; from benthic scavengers to pelagic swimmers. They were also predators to other organisms that have not developed a hard shell yet such as worms but many would remain filter feeders.

What did it predesses for?

Trilobites were basal to many of the evolutionary trends. Notably, the development of a calcite shell. This would be the building block for defensive strategies as the Cambrian period is when predation become more relevant.

Trilobites also grew by molting their skeletons which suggests advancement of evolution. It offered protection as well as support for new muscle groups.

Limulus

Introduction:

Another arthropod that appeared during the Cambrian times are the chelicerates. Like the trilobite, chelicerates have segmented bodies with jointed limbs and an exoskeleton of chitin. The Limulus is just one of the genus of chelicerates and today, there is one extant species left; the Atlantic horseshoe crab. Similar to trilobites, these creatures would have been found all around the world.

Location:

Again, chelicerates would have been found anywhere ranging from North America to Europe and from freshwater to marine environments. They also were better swimmers with the adaptation of the chelicerae. The chelirates were also found in the Burgess Shale.

Features of the Fossil:

One of the main features are specialized appendages called chelicerae which were for holding onto things such as their prey. The other appendages were modified to become paddle like for better swimming. Unlike the trilobites, their bodies were long and narrow.

Something Interesting:

There is a living fossil cherlicerate named the Atlantic horseshoe crab, which is not actually a crab. It is found along Atlantic coast and in the Gulf of Mexico. What did it predesses for Developed a telson; tail spine. Also a trend towards increasing body size while having a shorter abdomen. A pair of appendages would also become the chelicerae; the pinchers.