lecture 5 and 6 V

What does Agnatha mean?

Jawless

What does Agnatha consist of?

Extinct: Cambrian vertebrates, conodonts, ostracoderms

Extant: Cyclostomata (Hagfish and lampreys)

What does the cambrian vertebrates consist of”

Earlist “fishes”. Haikouella, Myllokunmingia, metaspriggina, nuucichthys, and Haikouichthys.

Class Myxini common name

Hagfishes

What are some synapomorphies of Cyclostomata compared to other living fish?

• Possess rudimentary vertebral elements

• A single median nostril rather than two

• A branchial basket that supports the gills

• A velum that pumps water over the gills

• A tongue with keratinous teeth

• Their notochord persists through life

What are some traits of the class Myxini (Hagfishes?)

No scales, no traces of mineralized bone (only keratin teeth) and no paired fins

• One semicircular canal, no lens in eye, electroreceptors or neuromasts

• Semi-open circulation with multiple non-innvervated hearts

What are the synapomorphies of the different hagfish species that separates them from lampreys?

• Body fluids like seawater (isotonic)

• Slime glands

What is slime used for by hagfish

To deter predators and allow them to slip out of grasps

What order contains the lampreys?

Petromyzontiformes

What are lmapreys?

Similar size and shape fish to hagfish that are well known parasites. They undergo two life stages where as larvae, they are small filter feeders at the bottom of rivers called Ammocoetes

What are the synapomorphies of the Lampreys that separates them from Hagfish

• 7 pairs of gill pouches

• Round mouth located at bottom of buccal funnel

• Nostril does not connect to oral cavity or pharynx

• Pineal eye

• Piston cartilage that supports tongue

Why are lampreys sometimes referred to as primitive vertebrates?

Because they lack some features seen in more derived vertebrates like jaws

How do mouthparts relate to eachother in cyclostomata?

Both hagfish and lampreys are jawless but they each have effective specialized mouth parts. They function differently as they were not homologous.

What do modern studies suggest about lampreys?

They have retained complex vertebrate organization like certain gene networks involved in neural crest development. This suggests that the common ancestor of all vertebrates already had a fairly complex nervous system

What is another feature that groups cyclostomata together?

Hagfish and lamprey each share 4 microRNAs that are not seen in Gnathostomes or in earlier common ancestors.

What group of fish includes cambrian vertebrates all the way up to osteostracans?

Agnatha—meaning jawless

Describe the conodonts

sister group to lampreys means cone tooth—find conodont elements in microfossils which are teeth that line the pharynx. Also have V-shaped myomeres and a notochord

What are conodont elements

Teeth like structure that line the pharynx of conodonts. Made from calcium phosphate and hydroxyapatite

W-shaped myomeres

Typically found in vertebrates

V-shaped myomeres

Typically found in earlier chordates

Vertebrate versus chordate features of conodonts

They have V shaped myomeres which places them closer to early chordates but have large eyes similar to vertebrates

How were conodont “teeth” arranged

Two upper lips armed with fang-like teeth and a tongue with comb-likek teeth. Food was crushed by 2 pairs of molar like pharyngeal teeth

What are conodonts found in phylogeny

Between lineage of cyclostomes and ostracoderms

How do early conodonts relate to later conodonts

Early conodonts did not share the same tooth structure as later ones. only later ones have a crown like structure on the tip of the teeth

How do we know conodont “teeth” are different than true vertebrate teeth?

The makeup of dermal teeth (vertebrate odontodes) is different meaning the crown structure of conodonts is not homologous to vertebrate odontodes. They evolved in parallel

When did mineralized dermal skeleton originate?

Right after conodonts branched off right before the ostracoderms

What are Odontodes?

Skin denticles. Projections of dentine from a living bony base covered in enamel. Similar to placoid scales of sharks.

What covers bony fishes?

Dermal scales derived from ancestral odontodes

How did Anatolepis fossil confuse scientists?

Once thought to be first vertebrate with odontodes lived over 500 mya in the Cambrian, looks like it has a pulp cavity and dentine tubules but there was no cellular bone for the bony base. Found that it wasn’t even a vertebrate but an arthropod with similar structures.

What is the outside in hypothesis

Says sensitive structures (e.g., teeth) developed first on exoskeletons.

How did Haridy support outside in hypothesis

Found that dentine which is the inner layer of teeth that transmits sensory information to nerves inside the pulp first evolved as sensory tissue in armored exoskeletons of ancient fish.

When did earlist evidence for vertebrate dental tissue actually arise

Middle-Ordovician—Astrapis fossil

What are the Ostracoderms

Armored fishes with no jaws. Had bony armor made of dermal plates.

How did jawed and unjawed ancestors relate

Jawless forms diversified as Ostracoderms for 150 my and coexisted with Gnathostomes for 100 my

Describe Anaspids

Split after heterostracans, have no shield but a dermal bone as a large triradiated spine. Have paired lateral fin folds and an anal fin. Known not to be cyclostomata as dermal bone evolved after cyclostomata

What are all the Ostracoderms we talk about

1. Heterostracans

2. Anaspids

3. Osteostracans

Name for an early armored fish (heterostracans)

Pteraspids

Name for a later armoured fish (anaspids and osteostracans)

Myopterygians

Describe heterostracans

Diplorhina (a type of heterostracan) Had 2 nostrils which was evidence they are later lineage than cyclostomata. Also had wing shields made of dermal bone. Not true fins but extensions of bony armor used for hydroplaning

Describe osteostracans

Had head shields which protect internal organs and are more active swimmers than winged shields (heterostracans). Paired pectoral fins, different from true fins as they don’t come from an internal skeleton and positioned differently, point from back of head.

What is cellular bone

Bone that comes from osteocytes

When did cellular bone originate (what we thought)

In the osteostracans. Heterostracans had narrow canals but no evidence of osteocytes. Osteostracans had osteocytes

Where do we place the origin of cellular bone before discovery of Astraspis

At the base of the common ancestor of Osteostracans

When did cellular bone (bone remodeling) actually orinate

Astraspis was found with osteons used in bone remodeling more than 100 million years earlier than originally thought. Makes sense as even jawless fishes would probably need a mechanism to repair their armor. No osteocytes suggesting may not have been neccessary.

So where do we place the real origin of cellular bone

At least at the base of the heterostracans according to the Astraspis fossil

Clade including both Ostracoderms as well as gnathostomes (fishes with bony skeleton)

Osteognathostomata

When did ostracoderms exist

From the Ordovician to the Devonian