Lemdadella
Oldest known trilobite genus, key to Cambrian stratigraphy.
Lemdadella is an extinct genus of redlichiid trilobite from the early Cambrian Period. It existed during the late Atdabanian stage, roughly between 521 and 514 million years ago. Currently, it holds the record as the oldest known trilobite genus, appearing around 521 million years ago.
**Distribution** Fossils of *Lemdadella* have been found in several locations. In Morocco, an unidentified species occurs in the Lower Cambrian Botomian stage, within the Ounein A bioherm of the Lemdad 1 section in the High Atlas. In Spain, *Lemdadella linaresae* was collected from the Atdabanian stage of the Pedroche Formation (Member 1, Puente de Hierro Section). In Antarctica, both an unidentified *Lemdadella* species and *Lemdadella antarcticae* have been recovered from the Lower Cambrian. The unidentified species comes from the Toyonian stage, while *L. antarcticae* is from the Atdabanian stage; both were found in the Shackleton Limestone.
**Description** The glabella of *Lemdadella* tapers gently toward the front. A ridge, known as a plectrum, may run along the midline between the glabella and the cephalon’s border. The eye-ridges are crescent-shaped. The thorax is not divided into a prothorax and opisthothorax, and it lacks a macropleural segment.
**Biostratigraphy** Because *Lemdadella* is the oldest trilobite genus, its first appearance is crucial for Cambrian stratigraphy. The International Commission on Stratigraphy has proposed using the first appearance of trilobites as the lower boundary for Cambrian Series 2 and Stage 3. If this boundary is formalized as a Global Stratotype Section and Point (GSSP), it will likely be defined by the first appearance of *Lemdadella*, a species within this genus, or a trilobite zone that includes it. *Lemdadella* appears in two recognized trilobite zones: the *Fallotaspis* zone (found in Spain and Laurentia) and the *Eoredlichia-Yunnanocephalus* zone (found in Antarctica).
Quick Facts
- Taxon
- Lemdadella
Facts from the source article.
Lore & Background
Lemdadella species have been found in the Lower Cambrian of Morocco (Botomian, Ounein A bioherm, Lemdad 1 section, High Atlas), Spain (Atdabanian, Pedroche Formation, Member 1, Puente de Hierro Section), and Antarctica (Toyonian and Atdabanian, Shackleton Limestone). The genus includes named species such as Lemdadella linaresae and Lemdadella antarcticae, as well as unidentified specimens referred to as Lemdadella sp.
Morphologically, the glabella of Lemdadella is gently tapered forward. A ridge between the glabella and the border of the cephalon, called a plectrum, may be present. It has crescent-shaped eye-ridges, and its thorax is not divided into prothorax and opisthothorax, with no macropleural segment.
Reader's Guide
Lemdadella holds exceptional significance in Cambrian stratigraphy because it is currently the oldest trilobite genus known, dating to about 521 million years ago. The International Commission on Stratigraphy has proposed that the first appearance datum of trilobites should be used as the lower boundary of Cambrian Series 2 and Cambrian Stage 3. If this proposal is defined as a Global Boundary Stratotype Section and Point (GSSP), it will most likely use the first appearance of Lemdadella, a species of Lemdadella, or a trilobite zone that includes Lemdadella. Its age and global distribution—spanning Morocco, Spain, and Antarctica—make it a key index fossil for correlating early Cambrian rock layers across different continents. The trilobite zones that contain Lemdadella include the Fallotaspis zone (Spain, Laurentia) and the Eoredlichia-Yunnanocephalus zone (Antarctica). Thus, Lemdadella is not only a marker for the earliest trilobites but also a critical tool for understanding the timing and spread of early animal life during the Cambrian explosion.
Did You Know?
- Lemdadella is currently the oldest trilobite genus known, dating back to about 521 million years ago.
- Its first appearance is proposed as a potential marker for the lower boundary of Cambrian Series 2 and Stage 3.
- Lemdadella species have been found in Morocco, Spain, and Antarctica.
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