Extinct Arachnids: Plesiosiro, Phalangiotarbi, Trigonotarbida & Uraraneida

Instructor: Derrick Arrington

Derrick has a master's degree in science education and has taught at the high school and community college level for 13 years.

Thinking and talking about spiders sends chills through many people. Scientists have discovered fascinating information about many types of extinct arachnids. In this lesson, we will examine four widely studied extinct arachnids.

Extinct Arachnids that Once Roamed the Earth

Arachnids are arthropods belonging to the class Arachnida. This grouping includes organisms such as spiders, ticks, and scorpions. All arachnids have a body that is divided into a cephalothorax and abdomen as well as eight appendages. The term cephalothorax is used to describe the fused head and thorax found in arthropods and other organisms, as opposed to a separate head region. Appendages refers to arms and legs adapted for locomotion and feeding. Let's dive into the world of extinct arachnids to investigate what you could have encountered millions of years ago!


Plesiosiro is a genus of extinct arachnid that includes one species that has been found to inhabit the United Kingdom, although there is some dispute about the exact location where the species was found. The only species, Plesiosiro madeleyi, was described in a 1911 British monograph about carboniferous period arachnids. The carboniferous period was the period of time from about 299 to 359 million years ago. In case you are not familiar with the term monograph, it refers to a work of writing that details a single subject to give clarity about its characteristics. According to the monograph, the arachnid had two long frontal appendages that may have been used to feel its way around their environment.


Phalangiotarbi is an order of extinct arachnids that have been found fossilized in coal beds in northern Europe and North America. After examining the impressions of the organism, arachnid paleontologists believe this order was likely related to harvestmen or ticks and mites. If you have ever seen those small arachnids on trees with really long legs that are often called granddaddy long-legs, then you have seen a harvestman.

A harvestman
An image of a harvestman insect.


Extinct members of the order Trigonotarbida are believed to have inhabited parts of Europe and North America around 350 million years ago. Fossilized impressions of these organisms have also been found in coal deposits. These spider-like arachnids did not have spinnerets to produce webs, but they did have a body resembling that of a spider. Evidence suggests that they had three to five segmented plates across their abdomens varying in size and thickness. Some of the seventy species that have been identified have armored plates with spines on them as a means of protection. Could you imagine running into an armored spider?

A fossilized Trigonotarbid impression
An image of a fossilized Trigonotarbid impression.


There are two known genera and species of arachnids that have been classified in the order Uraraneida. These organisms are believed to have lived approximately 350 million years ago. Fossils of these species have been found as impressions preserved in New York. These two organisms have been loosely classified as spiders since they do seem to have had active silk producing spigots used for making webs, although they are not the classic spinnerets found in spiders. Uniquely, these species seem to have had a long tail after the anus that acted as a flagellum. This characteristic is often found in some arachnids but not in spiders. A flagellum is a long whip-like tail found in many organisms that is used as a source of locomotion.

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