Heterostraci

From Palaeos

o Vertebrata
`--o Pteraspidomorphi
   |--Arandaspida
   `--o Heterostracomorphi
      |--Astraspida
      `--Heterostraci
         |--o Cyathaspidiformes
         |  |--Cyathaspidida
         |  `--Amphiaspidida
         `--o Pteraspidiformes
            |--Pteraspididae
            `--Psammosteida
               |--Drepanaspis
               `--Psammosteidae

Introduction

Heterostraci ("Different scales") is an extinct clade of jawless vertebrate that lived primarily in marine and estuary environments. They arose during the Silurian, and all, save for the Psammosteids, became extinct during the late Devonian. This last group of heterostracans, the Psammosteids, died out in the extinction event at the end of the Devonian.

Information

Heterostraci: Athenaegis.

Range: Early Silurian to Late Devonian.

Phylogeny: Heterostracomorphi: Astraspidae + Heterostraci: Cyathaspidiformes + Pteraspidiformes.

Description and Anatomy

The Heterostracans differed from other Paleozoic agnathan classes both in the arrangement of their scales, as well as the histology of their scales.

Most heterostracans had two plates which form a large dorsal shield and a large ventral shield, and had series of scales arranged in various patterns on the sides of their bodies, the exact pattern differing from one group to another. In a few primitive forms, such as Lepidaspis, the dorsal and ventral shields are composed of a mosaic of tiny scales. In most other known forms, though, these tiny scales have fused together to form the shield-plates.

The scales of heterostracans are histologically distinct from other vertebrates, having three layers composed of dentine and aspidine, an acellular bony tissue unique to this class. The middle layer was honeycombed with tiny spaces called "cancella."

As with many agnathan groups, heterostracans had no fins besides the tail or caudal fin. In some pteraspids, especially in the psammosteids, the ends of the branchial plates (the plates that covered the gills) is drawn out to form wing-like extentions.

Taxonomy

Heterostraca is divided into two main groups, the orders Cyathaspidiformes ("Cup Shields"), and Pteraspidiformes ("Wing Shields"). The Silurian heterostracan Athenaegis is regarded as being a sister group of both orders, while the Early Devonian Lepidaspis is regarded as being incertae sedis, possibly close to the original basal forms.

Cyathaspidiformes is divided into two main groups: the Amphiaspida of Early Devonian Siberia (grouped together with their relative Ctenaspis of Canada), and the Cyathaspidida (grouped together with their relative Nahanbiaspis.

The Pteraspidiformes is divided up into five families, Anchipteraspididae, Protopteraspididae, Pteraspididae, Protaspididae, and Psammosteidae.

Characters

Characters: Generally oblong, fusiform shape, with some species dorsoventrally flattened; fan-like lower oral plates; upper oral plate normally rigid; head normally covered with single large dorsal and ventral plates, typically elongated lateral branchial plates; orbits small; $ single pair of gill openings; possible prenasal sinus or naso pharyngeal tract partially separated from oral cavity; no indication of endoskeleton except head shield impressions of possible arcualia and possible gill arches; head shield has impressions of 2 SCCs, 8-10 transverse gills and possibly gill arch bones radial to gills; ventral plate shows median impression (lingual apparatus?); brain shows 3 segments (pineal, metencephalon or cerebellum, and medula oblongata); where known, posterior covered with large scales; large median dorsal and ventral ridge scales; no fins except caudal; caudal fin primitively di- (poly?-)phycercal with lines of scales rather than fin rays; tail may also be pad-like; no paired appendages; interior of plates smooth & shiny - no evidence of attachment for endoskeleton; subaponevrotic circulation known only in one species (Torpedaspis); trilaminar dermal skeleton with basal laminar layer, cancelous aspidine central layer, and tuburculate dentine outer layer showing characteristic grebeshkis.

Links

• UALVP Fossil Models & Replicas
• Class Pterapisdomorphi
• Geol 437 jawless fishes (NOT \AGNATHA\), Fall, 1995
• Heterostraci
• Gagnier
• The four other mass extinctions (French)
• xrefer - Heterostraci
• Lecture Notes, Nat. Hist. Vert., Emporia State Univ.
• vista verde news - Frühe Wirbeltiere: Ohne Biss (German)
• handout9_14
• 209 lecs02/20903s02.pdf
• geol.lsu.edu

Credits

• APW040316 Palaeos com (Information, Characters, Links)
• Wikipedia (Introduction, Description and Anatomy, Classification)