Microbrachius dicki
- Rodrigo
- Mar 19
- 2 min read
Updated: 1 day ago
Age
Devonian（Emsian-Givetian）
393.3-382.7 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Placodermi
Order: Antiarchi
Family: Microbrachiidae
Genus: Microbrachius
Species: Microbrachius dicki
Morphological description
Microbrachius dicki is a small antiarch placoderm from the Orkney Islands of Scotland, typically measuring about 2–4 cm in length. The entire body is covered with robust dermal armor plates. The head is relatively large, while the thoracic armor is short and densely ornamented with small, distinct tubercles.
The most striking feature in males is a pair of dermal claspers located on the posterior ventrolateral plates. These claspers form curved, plate-like bony structures that extend laterally, reaching a width comparable to the entire trunk shield. Their surface bears a pronounced groove, which likely functioned in sperm transfer or enclosed a duct for conveying sperm. Along the outer margin, a row of larger spines is present, while the ventral surface is covered with numerous small posteriorly directed spines. Some specimens show that the paired claspers are sutured together along the midline, indicating that they were not mobile.
In females, a corresponding pair of blade-like genital plates occupies the same position. The inner surfaces of these plates bear curved ridges and marginal tubercles, representing the only inward-facing ornamentation in the dermal skeleton. These structures likely functioned as anchoring surfaces during copulation, allowing the male claspers to grip securely. The clear morphological differentiation between these structures in males and females indicates pronounced sexual dimorphism.
Etymology
The genus name Microbrachius was established in 1888 by the British palaeoichthyologist Ramsay Heatley Traquair. It derives from the Greek roots mikros (small) and brachion (arm), meaning "small arm", referring to the reduced pectoral appendages.
The species name dicki honors the fossil collector Robert Dick (1811–1866), one of the earliest individuals to discover and collect specimens of this fish.
Biological description
Microbrachius dicki lived approximately 385 million years ago during the Givetian stage of the Middle Devonian. It inhabited freshwater lakes and deltaic environments as a small benthic placoderm. It belongs to the family Microbrachiidae within the Antiarchi, representing a relatively basal lineage characterized by small body size but intricate morphology.
A study published in 2015 demonstrated that this species reproduced via internal fertilization. Males used their curved, grooved claspers to engage with females during mating. The individuals likely aligned side by side, allowing the claspers to directly contact the female cloaca. The female genital plates, with their inward-facing ridges and tubercles, likely helped secure the claspers in position, facilitating sperm transfer.
This discovery overturned the previous assumption that antiarchs lacked internal fertilization and reproduced solely through external spawning. It further suggests that internal fertilization may represent one of the earliest reproductive modes among gnathostomes. Similar structures identified in other antiarchs, such as Pterichthyodes and Bothriolepis, imply that this reproductive strategy may have been widespread within the group.
(Author: Rodrigo)
Reference
Long, J. A., Mark-Kurik, E., Johanson, Z., Lee, M. S. Y., Young, G. C., Zhu, M., Ahlberg, P. E., Newman, M., Jones, R., den Blaauwen, J., Choo, B., & Trinajstic, K. (2015). Copulation in antiarch placoderms and the origin of gnathostome internal fertilization. Nature, 517(7533), 196–199. https://doi.org/10.1038/nature13825
59.06, -3.43
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