Astraspis desiderata
- Rodrigo
- Mar 19
- 3 min read
Age
Ordovician（Darriwilian-Hirnantian）
467-443 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Pteraspidomorphi
Order: Astraspidiformes
Family: Astraspididae
Genus: Astraspis
Species: Astraspis desiderata
Morphological description
Astraspis desiderata is an Ordovician vertebrate whose head shield is composed of numerous small tesserae. These elements are tightly packed, forming a continuous protective covering over the head and anterior trunk. On the dorsal surface, paired lateral line canals are present, indicating a well-developed sensory system. The eyes are relatively small and partially embedded within the thick dermal armor, with only a limited external opening exposed. One of the most distinctive features is the presence of eight gill openings on each side, arranged in a regular row posterior to the head shield, which serves as a key diagnostic characteristic of the genus. The tail region is covered with scales, suggesting a degree of streamlining and protection.
The middle layer of the dermal skeleton consists of thick anosteocytic bone, known as aspidin, formed by the centripetal deposition of mineralized tissue around vascular spaces, producing oval structural units called aspidones. This layer lacks cellular spaces and corresponds to the dermal skeletal structure observed in heterostracans. The basal layer is composed of lamellar anosteocytic bone with clearly organized collagen fibers. In some specimens, columns of small ovoid spaces are present, interpreted as chondrocyte-like lacunae. These structures indicate the presence of chondroid bone, a tissue combining features of cartilage and bone, representing one of the earliest known occurrences of such tissue in the fossil record.
On the surface of the dermal skeleton, the odontodes are arranged in a star-shaped pattern and often exhibit ridges and folded ornamentation. Developmental evidence indicates that dentine deposition occurs synchronously, with the growth of an individual odontode taking approximately 60–70 days at a rate of 0.5–5 μm per day. The growth pattern shows both short- and long-period periodicities, comparable to Andresen's lines and von Ebner's lines observed in the teeth of modern amniotes.
Etymology
The genus name Astraspis was established by Charles Doolittle Walcott in 1892. It derives from the Greek astron (star) and aspis (shield), referring to the star-shaped ornamentation of the dermal surface.
The species name desiderata originates from the Latin desiderare, meaning "to long for" or "to desire", reflecting that this fossil represented a long-anticipated record of the earliest vertebrates in North America.
Biological description
Astraspis desiderata was an early jawless vertebrate approximately 20 cm in length. Its heavily armored head and anterior body likely functioned primarily as protection against predators. The small eyes suggest limited visual capability, indicating that it was unlikely to be an active predator. Instead, it was probably a slow-moving benthic organism, feeding by suspension or suction. The presence of eight gill openings implies a strong water-flow respiratory system, allowing efficient respiration in nearshore environments with suspended particles.
Fossils of Astraspis desiderata have been recovered from the Harding Sandstone of Colorado, a shallow marine deposit rich in invertebrates and early vertebrates. This environment indicates that it coexisted with other heterostracans, early chondrichthyans, and various jawless vertebrates. The presence of lateral line canals in the dermal skeleton suggests a well-developed ability to detect water movement, which would have been advantageous for orientation and feeding in turbid aquatic environments.
References
Houée, G., Bardin, J., Germain, D., Janvier, P., & Goudemand, N. (2023). Developmental models shed light on the earliest dental tissues, using Astraspis as an example. Palaeontology, 66(6), e12682. https://doi.org/10.1111/pala.12682
Lemierre, A., & Germain, D. (2019). A new mineralized tissue in the early vertebrate Astraspis. Journal of Anatomy, 235(6), 1105–1113. https://doi.org/10.1111/joa.13070
Sansom, I. J., Smith, M. P., Smith, M. M., & Turner, P. (1997). Astraspis – the anatomy and histology of an Ordovician fish. Palaeontology, 40(3), 625–643.
Walcott, C. D. (1892). Note on Astraspis desiderata, a new genus and species of Ordovician fishes. Proceedings of the United States National Museum, 14, 187–188.
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