Palaeos Vertebrates 60.100 Placodermi: Arthrodira

*Palæos:*		Unit 60: Placodermi
*The* *Vertebrates*		100: Arthrodira

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Placodermi: Arthrodira

Taxa on This Page

The Arthrodires -- Introduction

The name Arthrodira means "jointed neck" These advanced preditory placoderms had a movable joint between the head (cephalic) and the thoracic (body) armor (you can see the way the armour is divided into two halves in the above photo). The mouth is interesting because as the lower jaw moved down the head shield moved up, thus giving a wide gape. Like all placoderms, they lacked teeth, instead using the sharpened edges of a bony plate as a biting surface.

There is a misconception that these creatures were sluggish bottom-dwellers. But as discussed below concerning this successful group, the Arthrodires were not only one of the most diverse and numerically sucessful vertebrate clans of the Devonian, but they occupied or even dominated virtually every ecological guild which would admit vertebrates, from top predator to scum-sucking bottom-dweller, for about fifty million years.

Arthrodires changed little during their long (50 million years is only slightly less than the time mammals have ruled the Earth), although during the end of their reign on lineage did increase enormously in size, perhaps exploiting the food source provided by diversifying sharks and bony fish. The largest genus of this group, Dunkleosteus, was a true superpreditor of the latest Devonian (Famennian epoch), reaching 3 to as much as 9 meters in length. The mighty head-shield alone (see above photo) was upto a meter in height in large forms. Note the sharpened edges of the jaw, which functioned as teeth. Like other sharp-sighted animals (birds, ichthyosaurs, some archosauriforms and dinosaurs) the eye sockets are protected by a bony ring.

The arthrodires were not out-competed by more advanced fish, as is usually but incorrectly believed. Rather they were eliminated by a terminal devonian extinction event, allowing the sharks to diversify into the vacated ecological niches during the Carboniferous period. MAK

More on Arthrodires

Placoderms make an interesting and useful study because there is little danger of being misled by living species. The closest living relatives of the Arthrodires are lampreys, in one direction, and holocephalians in the other - which suggests just how far in the taxonomic wilderness this group is. There is never a lack of unanswered questions in paleontology, but the placoderms do seem to have more than their fair share. Only a very few of these questions are addressed here, and certainly none are answered.

Some scholars seem to have consigned the placoderms to a slow, dim life as bottom-feeders, scraping by on detrital scraps fallen from the tables of the evolving sharks and actinopterygian ray-fin fish who whizzed above them like F-14s passing a garbage truck. This general concept of the placoderms cannot survive contact with the Arthrodira. The arthrodires were not only one of the most diverse and numerically successful vertebrate clans of the Devonian, but they occupied or even dominated virtually every ecological guild which would admit vertebrates, from top predator to scum-sucking bottom-dweller (both positions are, of course, now occupied by lawyers) for about fifty million years. There are altogether too many illustrations of the 3 meter Dunkleosteus chasing the 50 cm shark, but they do make a valid point.

Why was the dentition so conservative? After 50 My, the last arthrodire had two superognathal plates and one inferognathal, not terribly different from the first. Even the dentally unimaginative dinosaur clades often did better than this. No placoderm ever developed teeth from scales in the way of other vertebrate groups, and it is unclear how, if at all, placoderms replaced damaged dentition. The form of the gnathal plates seems to have been subject to considerable variation. Some had sharply se cutting surfaces, some had broad crushing surfaces, others had little or nothing recognizable. However, the basic gnathal formula was extraordinarily stable.

The rigid head and body armor is also unique among gnathostomes, but is perhaps more easily explained. The reader is cautioned that the following argument may be reasonable, but is entirely speculative at present. The vertebral support of the placoderms, in fact almost all Devonian gnathostomes of all groups, is fairly weak. Strong vertebral support is obviously less critical for aquatic organisms. However, strong longitudinal support allows stronger muscle movements in swimming than a notochord might permit. Furthermore, aguilliform (eel-like) movement excepted, efficiency of the notochord as a device for storing and directing the energy exerted by swimming muscles goes down strongly as a function of body length; nor is it possible to simply increase the diameter of the notochord as the diameter of the animal expands. [There, may, in addition, be a variant of the classic square-cube problem] There is only so much one can do with a rod of hydrated gristle.

The placoderms solved these problems in a very straightforward manner -- by evolving an exoskeleton, or at least by retaining an exoskeleton that their eugnathostome relatives had been so foolish as to squander. To be more exact, they evolved half an exoskeleton. The other half of the body remained free to make extremely strong swimming motions, anchored on the rigid craniothoracic exoskeleton without the body as a whole being forced to subscribe more than one half of a sinusoidal wave (this being, roughly, what makes a tuna fast and an eel slow but "slippery" and agile). The posterior of placoderms, where known, is thinner than expected for the equivalent osteichthyan or shark and generally scale-less, although there are exceptions. Quite likely this was because its principal function was to exert motive power -- more along the lines of a gigantic flagellum than an extension of the body cavity as such. Undoubtedly, it made better sense to attach the internal organs to the reinforced exoskeleton, near the center of gravity and of rotation, rather than dispersing them along the trunk in the manner of modern fish or tetrapods.

The posterior trunk of placoderms is also surprisingly finless. Again, our knowledge of this anatomical region is limited. However, there is no evidence of a substantial dorsal fin or of anal fins in any arthrodire, and little enough evidence of caudal fins. The primary control surface was necessarily the pectoral fin. The oddly jointed pectoral fin armor is a memorable feature of the placoderms, especially Antiarchs. It is sometimes assumed that this armor was the fin. However, it may perhaps be better understood as the exoskeleton of a very large and powerful control surface. Recall that the head of most arthrodires was capable only of limited movement in the vertical plane, and none at all horizontally due to the accessory attachments on the exoskeleton. The movement of plates relative to each other was also minimal (recall the lack of overlap). The only possible control surface is the pectoral fin.

Thus, paradoxically, the arthrodires and, more generally, the placoderms may have succeeded because they were specialized for speed and for an active, pelagic lifestyle. They may, in fact, have overspecialized. Later arthrodires show some signs of convergence with osteichthyans, before the entire group was eliminated for an unknown reason towards the end of the Devonian. However that may have been, it cannot be assumed that the arthrodires were merely the leading clan in a caste of benthic snails. ATW

Phyllolepida

"Phyllolepis is a member of a poorly-known group of placoderms whose identifying trait is greatly enlarged plates of armor on both head and body. It was probably a rather primitive bottom feeder, much like Arctolepis and Coccosteus."

Dismissive statements like this one are intensely irritating. Let us, just for a moment, look at the world from the point of view of Phyllolepis and consider just who, as between the author of that sentence and Phyllolepis, might more accurately be described as a "rather primitive bottom-feeder."

Pennsylvania. Early summer. Mid-morning. Three hundred seventy million years ago. With brisk, liquid authority the river cuts west through a break in the cracked and barren brown foothills of the Taconic Range, its progress a dark procession within a waving crowded corridor of skeletal green lycopods and giant ferns. They scatter the harsh sun, throwing bits of colored light and shadow skittering over the smooth, dark surface. The bank is steep ochre clay, water-scoured by spring floods and melt from the retreating glaciers to the east. Insects pick delicately through exposed shallow roots. Below the surface, the river bottom is sand and rock and silt, windrows of plant debris, a kaleidoscopic maze of tans and green water plants glimpsed dimly through turbid currents and shifting light.

Phyllolepis hunts here. Not half a meter long, its broad head plates are tan and lumpy as the rock around it, etched with strange concentric ridges housing a light growth of filamentous algae. The algal threads blow and flutter in the currents, further confusing color and form. Now still, flat against the river bottom, it mimics some half-overgrown, long dead mollusk shell. Its long thin tail is an inconspicuous wedge behind, draped with deceptive languor in the shadow of a rock. Its eyes are well forward on the upper surface of the great, flat head. Half-buried in sand, they are small, weak, unblinking.

But the play of light and color, the universe of vision is for others -- foolish, energy-wasting acanthodians and lungfish, easily confused and misled in the dim, murky bottomland. The placoderm’s hunt is a no matter of speed and violence. It is a patient game of skill. Quietly, Phyllolepis builds a map of the microcurrents in its shelter. It tastes the world around it, flavored by the million chemical signatures of life and death, spiced and crunchy with electric fields which resonate to the music of muscle and nerve.

Here in the sand is the taste of the burrowing annelid worm - not far, but too old, too deep. There, the mark of some small, plant-climbing gastropod - too small to be worth the expense of capture. Time passes. A swirl of current, an electrical tingle warns of something large approaching, but the shade passes above without incident. And as time passes, Phyllolepis paints, in more and more detail, the complex landscape of current and the shape and texture of the chemical and electrical countryside around it.

Finally, there, beyond the gravel skree below the bank, it feels a slightly mistimed back-current, a hint of deeper, older sand and waste, a sudden bulge in an electrical field. With a minute flick of the tail, Phyllolepis begins to glide, edging up to the bank, anticipating, riding, following, flowing on the complex turbulent flow of current, through the rich carpet of energy and taste. The sense is stronger now. Some incautious crustacean is burrowing too close to the surface. In one huge, lurching dash, Phyllolepis is on it, blasting sand away with the force of its approach, crushing the thin shell with bony tooth plates and swallowing.

The sand settles, the mud is swept away, and Phyllolepis patiently begins to rebuild its map.

Descriptions

Characters: Teeth dermal bone, not replaced; jaw muscles medial to palatoquadrate; nostrils anteroventral; nasal capsules not fused to braincase; stalked eyeballs (as chondrichthians); sclerotic plates of 1-4 pieces; spiracle absent; probably 5 branchial arches; single branchial opening between cephalic & thoracic armor; unique neck joint; head and (variably) body covered with dermal bone plates; pattern of plates characteristic of each placoderm order; neural arches present and, in the tail region, hemal arches (no ossified centra); heterocercal tail; single dorsal fin; anal fin unknown or absent; internal skeleton of cartilage and perichondral bone; well-developed and unique pectoral fins; primitively, presence of semi-dentine with tear-drop shaped cavities for encased odontocytes; body often flattened (benthic).

Note: The body and head armor are primitively not articulated, with several independent lines developing articulated armor. 010420

Characters: Highly diverse and successful group in various roles, including dominant marine predator. $ Two sets of upper tooth plates (superognathals); gnathal plates may be tuberculated; large endocranial postorbital process; $ skull roof from 3 unpaired medial plates + 7 paired plates; cranial plates do not overlap; 3 cheek plates; hyomandibula fused w. cheek, not part of jaw support; inferognathal typically lies on Meckelian bone, with adductor muscles apparently running directly between Meckelian and palatoquadrate; anterior braincase with nasal capsules separated by perichondrally ossified "wall", the cranio-ethmoid fissure, which in some species completely separates nasal capsules from brain; olfactory capsules large and open ventrally (just ant to superognathals); cartilage eye-stalk links ball to braincase through (sometimes ossified) cup-shaped sclera; orbit hollowed with myodomes, especially posterior; palatoquadrate articulates with ethmoid and with dermal armor; braincase also articulates with gill arches; sensory line system in pits or grooves on dermal armor; $ endolymphatic opening in single paranuchal plate; main joint between braincase and synarcual; extra joint between head and body armor in most (but not Actinolepidoids) with condyles on anterior dorsolateral plate of thoracic armor & fossa on paranuchal plate of head armor (note that this stabilizes the head but prevents any significant lateral motion); primitively, elongated thoracic armor, with dermal plate closure posterior to pectoral; thoracic plates have small overlap; axial skeleton otherwise unknown; pectoral fin endoskeleton unknown, but scapulocoracoid shows narrow base for fin (stenobasal), with muscle attachments and vascular canals attaching external to base; some degree of pectoral armor common; dermal armor thick, cellular bone with external tubercles of semidentine (absent in some species which may have been skin-covered). (MAK 991218)

Image: from Science -- Burrow 300 (5626)- 1661b. Cross section of an

vertical section of arthrodire gnathal plate

arthrodire gnathal plate. See comments below. To us, these look about halfway between the conjoined denticles sometimes found internally in thelodont gills and gnathostome scales.

Characters: Peak(?) of nuchal plate near geometric center; ornamented with concentric ridges.

Actinolepida (= Actinolepina): Actinolepis, Sigaspis, Simblaspis, Wuttagoonaspis

Characters: radiation center of paranuchal plate near topographic center; elongated thoracic armor; spinal plates long & frequently ornamented with tubercles in concentric pattern.

Phyllolepida: Stensiö (1934). Austrophyllolepis Long (1984), Cobandrahlepis Young (2005), Phyllolepis Agassiz (1844), Placolepis Ritchie (1984), Yurammia Young (2005).

Range: Middle Devonian (Givetian) to Late Devonian (Famennian) of Antarctica, Australia, Europe, Greenland, North America, Scotland, & Russia. All Famennian except Australia & Antarctica.

Characters: 30-40 cm; eyes reduced, possibly absent; sclerotic ring absent; extensive armor; greatly flattened; nuchal plate enlarged, with width ≥ length [Y05]; nuchal plate surrounded by 4 paired plates plus paranuchal plate bearing postnuchal process [Y05]; trunk armor broad [Y05]; anterior dorsolateral plates with long, narrow exposed area [Y05]; anterior ventral and posterior lateral plates absent [Y05]; anterior ventrolateral plate short & broad [Y05]; ventrolateral plates flat, without lateral laminae & meeting ventromedially with non-overlapping sutures [Y05]; dermal ornament usually (except Yurammia) of conspicuous concentric pattern of raised ridges on plates [Y05]; highly developed lateral line system and probably electroreception.

Characters: Like primitive Arthrodira. Dorsoventrally compressed; orbits face dorsally; widely splayed pectoral fins, with cornua common; no post-pectoral plates; plates ornamented with linear rows of tubercles; strictly benthonic?

Characters: usually marine placoderm; <20cm. Look similar to Chimeroids. Armor back of head only. Mostly scaleless. Caudal fin much reduced. Very hard hypermineral "secondary dentine" on tooth plates. Claspers.

Note: I once described ptyctodonts as looking like an upper class Englishman dressed for dinner. I stand by that description. ATW020329.

Characters: Overall appearance very ray-like. Greatly flattened, jaw quite ray-like; gnathal plates with denticles; denticles on palatoquadrate (??); single skull roof plate and suborbitals; dorsal orbits and nostrils; single gill slit; small medial dorsal thoracic plate present; pectoral fin like rays, supported by wide basal and jointed radials; pectoral fins, body and most of head covered in small platelets. Probably marine.

Placodermi: Arthrodira

Abbreviated Cladogram

Contents

Taxa on This Page

The Arthrodires -- Introduction

More on Arthrodires

Phyllolepida

Descriptions