Cephalaspidomorphi : Galeaspida

Taxa on This Page

1. Cephalaspidomorphi X
2. Eugaleaspidiformes X
3. Galeaspida X
4. Huananaspidiformes X
5. Polybranchiaspidida X
6. Polybranchiaspidiformes X

Cephalaspidomorphi

"Cephalaspidomorphi" is an old name of rather uncertain meaning. In some sources, it meant a group consisting -- roughly -- of the Heterostraci and Osteostraci, the Galeaspida and Pituriaspida being largely unknown at the time. As the name implies, it included any jawless fish with a head shield. During the last decades of the XXth Century, the term fell out of use because the two groups appeared to be unrelated, and the Osteostraci were suspected to be paraphyletic, or at least the sister group of the Gnathostomata. The way things are presently moving, it seems more likely that the heterostracans will be shown to be paraphyletic. To be specific, we suspect that the Thelodonti are Heterostracomorphi, or perhaps even Heterostraci, and that both the osteostracans and gnathostomes are thelodonts.

However that may be, we now have a perfectly good, and highly descriptive name that isn't being used for anything worthwhile. We have therefore retooled it to refer to osteostracans + galeaspids, a large but otherwise nameless group of jawless fishes with head shields. ATW030604.

Galeaspida

The Galeaspida are a strange group of armored fishes possessing a massive, flattened, one-piece bony shield. The mouth was ventral. The dorsal shield was pierced by a single, large, nostril-like opening situated in the middle of the head in front of the eyes. The head shield was usually an elongate oval or a simple half-moon, but there were also bizarre (possibly nektonic?) forms with long tubular rostra ("nose spikes") and widely flaring cornual processes ("wings"). The body was covered by numerous ornamented scales. The bone structure of galeaspids shows hollow cavities within ornamental tubercles or blisters. These animals may have buried themselves in the sand or mud, with only their "blow hole" showing above the surface.

Polybranchiaspis, buried in the sediment, showing the median dorsal inhalant opening at the front (left) of the head, above the eyes. Illustration from Janvier 1996: 301).

The Silurian Hanyangaspis is perhaps the most generalized galeaspid, having a broad and almost terminal median dorsal opening and forward placed eyes. The more derived galeaspids may be divided into the Eugaleaspidiformes and the Polybranchiaspidida. Most of the Polybranchiaspidida (i.e., the Huananaspidiformes) have a long rostral process and slender cornual processes, which parallels the condition in the boreaspid osteostracans.

The Galeaspids were geographically limited primarily to the South China terrane (southern China and Vietnam) from the Early Silurian to the Early Devonian. A few are also found in Northern China, and even in Tibet (all of these areas were part of a single geographic region). The last known type is from the Late Devonian of Ningxia, northern China.

Descriptions

Cephalaspidomorphi (= Osteostracomorphi)

Range: Early Silurian to Late Devonian

Phylogeny: Thelodonti::: Katoporida + *: Galeaspida + (Pituriaspida + (Osteostraci + Gnathostomata))

Characters: massive endo- and exoskeletal head shield. ATW030605. Sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone (Sansom, 2009, text via Wikipedia) MAK111026

Comment: Osteostraci, Pituriaspida, and Galeaspida are thought to be either the sister-group of (Benton 2004, Nelson2006), or the larger clade including (Janvier2008), gnathostomes. The term Osteostracomorphi has been suggested, but we have retained the older term Cephalaspidomorphi, albeit in a modified context. Both terms could be used in either a Linnaean / Evolutionary systematic sense of an ancestral group of armoured jawless fish, or cladistic sense of a larger clade that includes both various armoured forms and the gnathostomes. For an alternative hypothesis of gnathostome origins, see our Thelodont pages.

Galeaspida: Hanyangaspis.

Range: Early Silurian to Late Devonian of South China.

Phylogeny: Cephalaspidomorphi: (Pituriaspida + Osteostraci) + *: Polybranchiaspidida + Eugaleaspidiformes.

Links: Galeaspida; Class Galeaspida; Geol 437 jawless fishes; Galeaspida.

Image: Bannhuanaspis from the Early Devonian Bac Bun Formation of Vietnam, after Janvier (1996). ATW010421.

Polybranchiaspidida:

Characters: $ >10 (up to 45) pairs of branchial fossae.

References: Janvier (1996).

Polybranchiaspidiformes: Dongfangaspis, Duyunolepis, Polybranchiaspis

Characters: headshield oval or rounded; median dorsal opening oval or slightly bean-shaped; elaborate rostral and cornual processes absent; most have star-shaped pattern at lateral end of transverse sensory lines on headshield.

Note: [1] Likely paraphyletic stem group of Polybranchiaspidida. [2] includes largest galeaspids (~20 cm).

Links: Galeaspida;

References: Janvier (1996). ATW030404.