Gill chambers in basal chordates and vertebrates, pt. 2

Yesterday we looked at several basal chordates
that use their large atria / gill chambers to capture planktonic prey. Today, we’ll peek at a few morphologies that modify the gill chamber in diverse ways and take planktonic feeding to new levels in prehistoric and extant taxa.

We’ll start with a taxon we looked at yesterday,
the Middle Cambrian first fish: Metaspriggina (Fig. 1). It had relatively few body parts. The torso was dominated by swimming muscles, followed by the much smaller gill chamber, liver, gut, heart and eyes, in that order.

Figure 1. An early jawless, finless, lancelet-like fish from the Cambrian, Metaspriggina. Compare the placement of the eyes here with Birkenia in figures 2 and 3.

Ordovician Arandaspis
took body armor to turtle-like heights with a dorsal carapace and ventral plastron along with armored gill openings (Fig. 2) surrounding this basal fish / advanced lancelet. Compared to Metaspriggina (Fig. 1, the gill chamber of the Panserfische, Arandaspis, was several times larger, still servicing a small oral cavity. The eyes were still tiny and faced only forwards. Evidently Arandaspis wasn’t concerned with prey sneaking up from behind. If swallowed, it might have been too tough to bite and digest. No predatory arthropods could dismantle it. Like ancestral lancelets, perhaps Arandaspis burrowed into sandy sea floors.

FIgure 2. Arandaspis lived inside its gill chamber shell and armored tail.

Late Silurian Poraspis
 (Fig. 4) had fewer and larger tail plates. All gill plates were fused into one with a single lateral atrial water exit on the sides. The skull plates remained large and solid, protecting the head, still dominated by the enormous atrium / gill chamber. With the first appearance of a rostrum since the lancelet, Branchiostoma, the eyes moved laterally, protected by knight-like armor. Lateral line sensory canals first appear in such taxa.

Figure 3. Poraspis fuses the lateral gill plates together for greater armor, leaving only a slender single common opening for the exiting water. This was probably a sedentary taxon due to its inability to respire at a great rate.

Early Devonian Drepanasipis
evolved a flattened armor to protect its huge gill chamber with posterior atrial openings. 

Figure 4. The large gill chamber (cyan) of Early Devonian Drepanaspis.

Distinctly different was Middle Silurian Birkenia
(Fig. 5). This late survivor from an early undocumented Ordovician radiation did not have stiff, plate-like armor like Andraspis, Poraspis and Drepanaspis (above). Instead Birkenia was surrounded, supported and protected by hundreds of splinter-like, interwoven cartilage / bone, permitting much greater flexibility for this more mobile taxon. The tail fin was new, improving the efficiency of the swimming muscles. Small, barely mobile pectoral fins first appeared here. The eyes were larger, still protruding dorsally like those of Metaspriggina (Fig. 1) or a mudskipper like Periophthalmus. The rostrum of Birkenia extended anteriorly beyond the eyes. The oral cavity remained lancelet-like and lancelet-sized, with cilia surrounding the ventral opening. Likewise, the atrium / gill chamber remained relatively small. Nine gill openings were retained, a few more than in Metaspriggina (Fig. 1).

Figure 5. Birkenia is basically an armored Metaspriggina with a tail and pectoral fin.

These newest members of
the large reptile tree (LRT, 1611+ taxa; Fig. 6) nest at the base, helping us understand relationships among major groups, which have shifted slightly since their addition.

Figure 6 Subset of the LRT with the addition of several jawless taxa.

Metaspriggina walcotti (Simonetta and Insom 1993; Morris and Caron 2014; Cambrian, 500 mya; up to 10cm) is an early chordate, naked, jawless and finless, but with two anterodorsal eyes. The orbits comprise the proto-skull. The swimming muscles are larger, so this was probably a mobile feeder, using its eyes to seek prey and avoid predators. Not sure if an atriopore is present here, or if gill slits opened directly.

Arandaspis prionotolepis (Ritchie and Gilbert-Tomlinson 1977; Ordovician, 465mya; 15 cmlong) is a member of the Arandaspididae that looked like a large, armored tadpole. This jawless, finless filter feeder was basically an armored lancelet, like Metaspriggina, with a much larger gill chamber and a tail to improved propulsion. Both eyes and nostrils faced forward above the jawless mouth. Several armored gill slits appeared between the dorsal carapace and ventral plastron.

Poraspis brevis (Kiaer 1930; Late Silurian to Early Devonian, 410mya) is traditionally considered a member of the Heterostraci.

Drepanapis gemuendenensis (Schlüter 1887; Gross 1963; Early Devonian 405mya) was a flattened arandaspid with a superficially ray-like armored body. The common gill opening exited posteriorly. This bottom feeder with widely-spaced eyes is traditionally considered a member of the Heterostraci. This skull differs from diagrams produced by Gross 1963.

Birkenia elegans (Traquair 1899; Middle Silurian; up to 10cm) is a jawless, finless chordate with scales, twin nostrils and a hypocercal tail. Bony hooks top the dorsal region. Cilia line the permanently open jaws. Gill bars and gill openings are present. Birkenia is ancestral to Hemicyclaspis and the sturgeon Pseudoscaphirhynchus, along with all other extant fish and tetrapods.

Tomorrow,
we’ll look at several more taxa dominated by their gill chambers.

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References
Kiaer J and Heintz A 1935. The Downtonian and Devonian vertebrates of Spitsbergen. V. Suborder Cyathaspida. Part I. Tribe Poraspidei, Family Poraspidae Kiaer. Skrifter om Svalbard og Ishavet 40:1-138.
Ritchie A and Gilbert-Tomlinson J 1977. First Ordovician vertebrates from the Southern Hemisphere. Alcheringa 1:351-368.
Simonetta AM and Insom E 1993. New animals from the Burgess Shale (Middle Cambrian)and the possible significance for the understanding of the Bilateria. Bolletino Di Zoologia 60:97–107.
Traquair RH 1899. Report on fossils fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897: 72-76.

wiki/Branchiostoma
wiki/Metaspriggina
wiki/Arandaspis
wiki/Poraspis
wiki/Birkenia
wiki/Drepanaspis

Gill chambers in basal chordates and vertebrates, pt. 1

Adding taxa without skulls
to the the large reptile tree (LRT, 1611+ taxa) would seem to bring with it a slew of problems. As a solution, the scores “skull absent”, “orbit absent” and “jaws absent” were added last weekend to several of the traits created eight years ago.

A primitive – hypothetical – chordate,
just developing a coelum (middle tissue between skin and intestine) is shown here (Fig. 1). It is not a taxon in the LRT, but serves as a zero point from which derived traits can be added in the present report. Think of it as a worm stiffened longitudinally with a notochord.

No one knows the size of its gill chamber. Early members of this clade likely lacked gills. Oxygen would have been absorbed both externally and internally on this tiny wriggling worm made up of not much more than skin over intestine with a notochord ventral to the dorsal central nerve chord. Tiny food particles would have been processed sometime during the trip from mouth to anus/cloaca. Few to no sensory organs appeared near the oral opening (not quite a mouth yet).

Figure 1. Hypothetical chordate ancestor to known chordates. This is the starting point for looking at gills and throats in basal chordates and vertebrates. The ‘actual size’ is also hypothetical.

A primitive extant chordate,
the lancelet (genus: Branchiostoma; Fig. 2) documents the next stage in chordate evolution and serves as the new outgroup taxon for the LRT. Still lacking a head or anterior sensory organs, Branchiostoma has ring sets of cilia both outside and inside the oral cavity. These are followed by a large atrium / gill chamber lined with slender gill bars for gas exchange. Ventrally a stiff rod, the endostyle, creates a mucous strand that captures food and, using microscopic cilia, carries the particles posteriorly to the simple, linear intestine which terminates in an anus / cloaca, no longer at the tip of the tail (see Fig. 1). Water from the atrium is expelled from a single ventral opening, the atriopore, anterior to the anus/cloaca. The swimming muscles that envelope Branchiostoma from tip to tip evolve to a chevron shape.

Figure 2. Extant lancelet (genus: Amphioxus) in cross section and lateral view. The gill basket nearly fills an atrium, which intakes oxygen, water + food, sends the food into the intestine and expels the rest of the water. It also lacks a head and anterior sensory organs.

A Middle Cambrian ‘lancelet with eyes,’ aka primitive fish,
Metaspriggina (Fig. 3), apparently loses the cilia and atriopore of the lancelet (Fig. 2) and develops seven gill openings lateral to the relatively smaller gill chamber. Those tiny eyes both direct this tiny predator to tinier prey and serve to locate predators, setting off alarms that spur the tail to wiggle seeking a hiding place or to put distance between it and any approaching marauder.

Figure 3. An early jawless, finless, lancelet-like fish from the Cambrian, Metaspriggina. Compare the placement of the eyes here with Birkenia in figures 2 and 3.

Evolving in a different direction
the tunicate (Fig. 4) is mobile only as a juvenile. It become sessile (attached to the seafloor) as an adult. The tail is resorbed and the gill chamber takes over the majority of the body. Traditional workers consider this stage primitive to the lancelet, but they don’t employ a simple worm-like chordate as an outgroup taxon. Tunicates are quite derived relative to lancelets.

FIgure 4. A tunicate diagram turned on its side to replicate the morphology of the lancelet (Fig. 2). Here the atriopore enlarges to become a exit siphon. The cilia are reduced. The gill chamber is greatly enlarged.

Sessile tunicates gave rise to
barrel-shaped, planktonic  tunicates, otherwise known as salps (Fig. 5). They seem simple, but that’s because they have gotten rid of nearly every body part but the atrium / gill chamber. Salps alternate between sexual and asexual generations, each distinct in morphology. The atrium comprises the entire animal. Gonads, an endostyle, simple brain and digestive organs all migrate nside the atrium. The atriopore (exit siphon) rotates opposite to the entrance siphon, creating a little jet engine. Several morphologies have evolved (Fig. 5).

FIgure 5. Salp variety. Here the atrium is the animal with other organs inside the atrium. Color helps link homologous elements.

Traditional cladograms
of chordate relationships (Fig. 6) nest tunicates basal to fish. Here both fish and tunicates are derived from lancelets, each evolving in different directions (mobile vs. sessile + planktonic). Tunicates have simplified and degenerated with fewer parts, distinct from the general trend in vertebrate evolution.

Figure 3. Traditional cladogram from Lingham-Soliar 2014. Missing from this cladogram are the lancelets. The LRT does not agree with this tree topology.

The above boneless, headless and finless taxa
are all filter feeders with large gill chambers, as are several primitive fish (= armored and bony lancelets) in the LRT. We’ll look at those in future blogposts.

New tanystropheid paper promotes archosauromorph myth

Colleagues. Stop promoting this myth.
Formoso  et al. 2019 report that Tanystropheus (Fig. 1) is an archosauromorph. Dr. Sterling Nesbitt, known for his widely cited, but poorly populated and scored  2011 cladogram, is a co-author. Their nesting of Tanystropheus as an archosauromorph is only possible by way of taxon exclusion and the omission of pertinent published works. Peters 2007 and the large reptile tree (LRT, 1611+ taxa, subset Fig. 2) firmly and unequivocally nest tanystropheids within the Tritosauria, within the Lepidosauria, within the Lepidosauriformes and within the Lepidosauromorpha.

Figure 1. Tanystropheus in a vertical strike elevating the neck and raising its blood pressure in order to keep circulation around its brain and another system to keep blood from pooling in its hind limb and tail.

Formoso et al. report,
“Tanystropheids are a unique group of archosauromorph reptiles, which likely appeared in the Late Permian (based on inferred ghost lineages) and diversified within five million years after the end-Permian extinction.”

By contrast
the LRT nest tanystropheids as sister taxa to pterosaurs, some of which had similar elongated cervicals, all derived from basal tritosaurs like Huehuecuetzpalli, Bavarisaurus macrodactylus and Tjubina. The protorosaur, Ozmik, had elongate cervicals. None of these are mentioned in the text. Strangely there is also no citation for Fuyuansaurus and Pectodens, the basalmost taxa in the Tanystropheus clade in the LRT (subset Fig. 2). They are all Middle Triassic taxa, as are all the macrocnemids. The only known Late Permian taxa in the LRT lineage of Tanystropheus are the basal arboreal lepidosauriformes, Saurosternon and Palaegama.

So where does this
“likely appeared in the Late Permian” supposition come from?

The authors uncritically cite Sennikov 2011
who mistakenly placed his tanystropheid, Augustaburiania vatagini, in the Early Triassic, perhaps based on Sennikov’s earlier similar mistake with regard to the coeval sauropterygian Tanaisosaurus kalandadzei. No other sister taxa for either taxon predate the Middle Triassic. Sennikov describes, “The Triassic beds of the Lipovskaya Formation are eroded, overlie Carboniferous marine limestones, and are covered by Middle Jurassic continental sands and clays.” Based on phylogenetic bracketing, the Lipovskaya is a Middle Triassic formation.

Formosa et al. also cite, “Early Triassic tanystropheid elements from the Sanga do Cabral Formation of Brazil (Olsen, 1979; Casey et al., 2007; Sues and Fraser, 2010; Sues and Olsen, 2015; Pritchard et al., 2015; De Oliveira et al., 2018; Lessner et al., 2018).”

1. Olsen 1979 refers to Tanytrachelos and the Newark Supergroup is Late Triassic-Early Jurassic.
2. Casey et al. 2007 also refers to Tanytrachelos, Cow Branch Formation of the Dan River Basin, part of the Newark Supergroup… Late Triassic (Carnian) age.
3. Sues and Fraser, 2010 is a book on Triassic life.
4. Sues and Olsen, 2015 does not appear on their list of references/citations
5. Pritchard et al., 2015 discuss “Late Triassic tanystropheids…”
6. De Oliveira et al., 2018 discuss,”Tanystropheid archosauromorphs in the Lower Triassic of Gondwana, Sanga do Cabral Formation of Brazil (see below).
7. Lessner et al., 2018) report on “New insights into Late Triassic dinosauromorph-bearing assemblages”
8. Formosa et al. assign their finds to the Middle Triassic (Anisian; 247-242 Ma).

De Oliveira et al. 2018 report,
“The fossil assemblage of the Sanga do Cabral Formation so far includes procolophonids, temnospondyls, and archosauromorphs. Vertebrate fossils are often found isolated and disarticulated. This preservation mode suggests extensive exposure and post-mortem transport of bones during the biostratinomic phase, and subsequent reworking after diagenesis.”

A Middle to Late Triassic time window for tanystropheids
is best supported here, along with a call for better editing among the several academic authors.

So, some phylogenetic and chronological problems surfaced
in Formosa et al. 2019. Reason: all scientists accept without testing, sometimes, because it’s easier. It remains important not to perpetuate myths in science, starting here and now.

Figure 2. Subset of the LRT focusing on the Tritosauria. Note the separation of one specimen attributed to Macrocnemus.

Interesting phylogenetic note:
The clade Tanystropheidae is defined as the most recent common ancestor of Macrocnemus, Tanystropheus, Langobardisaurus Renesto, 1994, and all of its descendants (Dilkes 1998). In the LRT, that definition includes pterosaurs and their outgroups (Fig. 2).

Figure 3. Tanystropheus and kin going back to Huehuecuetzpalli. Note the scale change from the white zone to the yellow zone with duplicated taxa.

Size
Formosa et al. report,“The Moenkopi tanystropheid cervical vertebrae belong to a considerably smaller tanystropheid than the largest Tanystropheus, but we determined that its body length was approximately three times larger than Tanytrachelos ahynis known primarily from the eastern United States.” Earlier Pritchard et al. 2015 described relatively giant Tanytrachelos specimens from the same formation. Those are not the same specimens described by Formosa et al.

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References
Formosa KK, Nesbitt SJ, Pritchard AC, Stocker MR and Parker WG 2019. A long-necked tanystropheid from the Middle Triassic Moenkopi Fromation (Anisian) provides insights into the ecology and biogeography of tansytropheids. Palaeontologia Electronca 22.3.73 online
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Pritchard AC, Turner AH, Nesbitt SJ, Irims RB & Smith ND 2015. Late Triassic tanystropheids (Reptilia, Archosauromorpha) from northern New Mexico (Petrified Forest Member, Chinle Formation) and the biogeography, functional morphology, and evolution of Tanystropheidae. Journal of Vertebrate Paleontology 35(2):e911186, 20pp
Sennikov AG 2001. Discovery of a Primitive Sauropterygian from the Lower Triassic of the Donskaya Luka (Don Basin) and the Range of Triassic Marine Reptiles in Russia. Paleontological Journal 35(3):301–309.
Sennikov AG 2011. New Tanystropheids (Reptilia: Archosauromorpha) from the Triassic of Europe. Paleontological Journal 45(1): 90–104.

https://pterosaurheresies.wordpress.com/2015/09/26/relatively-giant-tanytrachelos-specimens/

From jawless fish to toothless jaws: Hemicyclaspis to Chondrosteus

Figure 1. Top to bottom: Hemicyclaspis, an extensively armored ostracoderm. Thelodus a soft jawless fish with a ventral oral opening and gill slits, retaining diamond-shaped armor laterally. Acipenser brevirostrum, a short-listed sturgeon with a protrusible tube for a mouth and reduced armor. Chondrosteus, a fish with jaws, but no marginal teeth.

Adding jawless fish
to the large reptile (LRT, 1611+ taxa) sheds new light on the origin of jaws and the basic topology at the base of the LRT.

Osteostraci,
like Hemicyclaspis (Fig. 1), have a ventral opening at the front of ventral surface of the skull, similar to their ancestors, like Birkenia, which retain lancelet-like cilia surrounding the oral opening. Perhaps Hemicyclaspis did, too. The oral cavity is poorly preserved.

Thelodus
(Fig. 1) was crushed to a thin film with a ventral exposure. Here the round lacrimal and angular oral opening is highlighted. The lateral armor (green) is barely ossified.

Sturgeons,
like Acipenser (Figs. 1, 2), have a longer rostrum and a posterior tube mouth. The maxilla and dentary are not yet present. Those bones grow teeth. Teeth are not present. Neither are the bones that grow them. So the lacrimal and surangular create the protrusible rim of that tube mouth and neither connects to the quadrate. Nesting sturgeons at the base of fish with teeth is the opposite of traditional cladogram topologies, in which sturgeons are considered ‘aberrant’ or ‘regressive’ (see below).

Figure 2. Old woodcut illustration labeling the upper mouth tube bone the lacrimal. Mn = mandible. h = quadrate. g = hyobranchial. Weave of bones above the lacrimal are palatal bones (pterygoid, ectopterygoid, palatine and vomer, plus a remnant gill bar. This taxon really exaggerates the rostrum, similar to the related spoonbill.

As you can see (Fig. 2), I am not the first worker 
to determine that the traditional ‘maxilla’ on sturgeons is instead the lacrimal.

Sturgeons, continued.
Gill covers (operculum) appear. While feeding on the bottom with the mouth buried in sediment, water cannot enter the mouth. So instead water enters the top of the operculum and exits out the back for respiration.

Note the close correspondence
between the torso ossifications, fin placement, tail shape and skull shape on the sturgeon and its osteostracan ancestor, Hemicyclaspis (Fig. 1).

Figure 3. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

Are sturgeons jawless fish?
In the LRT sturgeons are transitional between jawless fish and traditional gnathostomes.

Jollie 1980 reported in his growth study on sturgeons,
“It is a conclusion that the endocranium has been drastically altered in form and in the reduction of its ossifications but that the dermal head skeleton is basically that of an actinopterygian fish which shows many regressive tendencies such as the variable multiplication of ossified units. The jaws in this group are unique both in terms of suspension and in lacking a premaxilla. The post-temporal of the pectoral girdle has a unique relationship with the endocranium which involves the exclusion of the lateral extrascapular. An interclavicle is present. In spite of such features, the developmental story and adult ossifications of the sturgeon support the idea of a common, and understandable, bone pattern in actinopterygians and osteichthians.”

Jollie did not place Acipenser and Hemicyclaspis
in a phylogenetic context. In the LRT (subset Fig. 4) Pseudoscaphorhynchus is a tested sturgeon.

Figure 5. Subset of the LRT focusing on basal vertebrates. Note the clade Holocephali nests apart from Chondrenchelys and kin, including moray eels.

Are sturgeons bony fish?
Not according to the LRT. Much of their skeleton is cartilaginous and they nest basal to cartilaginous taxa. So between cilia and jaws, the transitional trait is a tube. Marginal teeth seem to have appeared three times by convergence in this scenario and once gained, were quickly lost in placoderms + catfish. Add to those palatal tooth carpets found in catfish, mantas and whale sharks.

Apologies for earlier errors.
As I’ve often said, I’m teaching myself vertebrate paleontology one taxon at a time using the LRT as a terrific tool for figuring things out.

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References
Jollie M 1980. Development of head and pectoral girdle skeleton in Acipenser. Copeia 1980(2):226–249.

Actinopterygii = ray fin fish
Osteichthyes =  bony fish

wiki/Gnathostomata

Dinosaurs in the Wild video, plus a backstory video

There’s a new(?) dinosaur exhibit
in England and several visitors have uploaded YouTube videos of it. Most of these are at least one year old, so I may be the last one to learn about this.

Visitors go back in time
and every so often put on 3D glasses to see dinosaurs outside the ‘windows’ of the exhibit. Looks like a thrill a minute with up-to-date dinos.

Plus
Dr. Darren Naish provides a behind-the-scenes YouTube video.

Figure 1. Evidently, when they had to animate Quetzalcoatlus, they got rid of that membrane down to the ankles, distinct from all previous illustrations of Quetzalcoatlus, but only when standing. Baby steps…

References

For more YouTube listings click here.

 

A catfish with barbels from the Silurian

Finally
a catfish from the Silurian with preserved barbels (Figs. 1, 2).

Ironically
catfish are members of the order Siluriformes (from ‘silurus’ Latin = large river fish). Previous oldest member of this clade: Late Cretaceous, 100mya.  Sir Roderick Murchison (1792–1871), a wealthy Scottish aristocrat, named the Silurian Period after an ancient Welsh Celtic tribe, the Silures. Appears to be a coincidence. The slow genesis of plants and arthropods on land occurred in the Silurian, along with a rise in oxygen levels, a rise in temperature and a rise in sea levels after the massive glaciation of the Ordovician.

Figure 1. Originally considered another Silurian Thelodus, this specimen nests with catfish in the LRT. Here’s where DGS tracing helps pick out the details from a ‘fish silhouette’ fossil.

Not sure what the museum number is on this one.
In the large reptile tree (LRT, 1602 taxa; subset Fig. A) this taxon is labeled ‘unnamed Sil. catfish‘ (in the purple clade). In the LRT the new taxon is not as primitive as the armored catfish, Hoplosternum. Worthy of note, basal catfish in the LRT are air breathers employing the intestine or modified gill arches, not their air bladder, which they need to swim upright. Clarias is the famous walking catfish (Figs. 3–5) able to traverse land in search of other ponds. The spiny pectoral fins (Fig. 4) keep it upright and act as ground undulates as it wriggles from pond to pond.

Figure A. Subset of the LRT focusing on basal vertebrates (fish). The base of the LRT will change by the next time you see it with the addition of several jawless fish. 

The identity of this specimen
might have been overlooked because it appears like a silhouette, offering little detail. Digital Graphic Segregation (DGS) enables details to be colored, identified and later scored in the LRT.

Figure 2. Silurian catfish face. Note the left barbel is aligned with a crack.

Extant velvet catfish,
members of the clade Diplomystidae, are considered primitive.

Figure 3. Clarias head with barbels in vivo.

Clarias batrachus (Linneaus 1758, up to 50 cm in length) is the extant walking catfish. The skull bones are nearly identical to those in the placoderm, Entelognathus. The spiny pectoral fins keep the walking catfish upright as it wriggles from pond to pond. No scales or bones appear on the surface. The teeth are short bristles on pads. The maxilla is absent.

Figure 4. Clarias, the walking catfish skull bones identified. Note the ossified spines at the leading edge of the pectoral fin. 

Figure 5. Clarias batrachus, the walking catfish, in vivo. The pelvic fin is tiny. The single dorsal fin is elongate. The anal fin is also elongate. The skull is flat and provided with sensory barbels.

Generally recognized fossil catfish
include Qarmoutus hitanensis from the same Eocene North African beds as the early whale, Basilosaurus. Reported by NatGeo.com (citation below): “Even though the fossil is relatively old in the way we ordinarily think of ages in millions of years, it is still essentially anatomically modern and directly comparable to living catfishes,” says John Lundberg of Drexel University’s Academy of Natural Sciences. “It’s one of the best preserved and oldest of its family.”

Afterthought about fish with spines in their fins
Spiny sharks (Acanthodii), like Brachyacanthus (Fig. 6), also briefly appeared in the Silurian and Devonian. Since the walking catfish uses its spiny fins to ‘walk’ on land, I wonder if spiny sharks, especially those with longer, thinner pectoral and pelvic spines, did the same, perhaps on the sea floor, not on land?

Figure 6. Brachyacanthus has short, thick spiny fins, distinct from the long spines found in the walking catfish.

That might explain
why those extra spines appeared between the pectoral and pelvic fins, as extra hooks in the substrate?

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References

https://en.wikipedia.org/wiki/Catfish

https://www.nationalgeographic.com/news/2017/03/ancient-egypt-catfish-fossil-palaeontology-science/

https://pubs.geoscienceworld.org/gsa/geology/article-abstract/5/4/196/195354/Fossil-catfish-and-the-depositional-environment-of?redirectedFrom=fulltext

Tiny Late Cretaceous Najash: basal to burrowing snakes

Garberoglio et al. 2019
bring us long awaited skull data and several new partial skeletons of a Late Cretaceous snake with legs, Najash rionegrina (Figs. 1, 2). It must be said, the only evidence of legs supplied by the current authors was a caption labeled tibia on a tiny straight bone near the edge of the matrix. Nevertheless, legs and hips were described earlier in other headless specimens of Najash (Apesteguía and Zaher 2006; Fig. 1).

Figure 2. The pelvis of the protosnake with legs, Najash, compared to Heloderma (burrower) and Adriosaurus (swimmer). Heloderma appears to share more traits with Najash.

Most of the new specimens
were found in layered sandstone related to migrating aeolian dunes, along with abundant rhizoliths (root systems encased in desiccated mineral matter) and burrows.

Figure 2. Najash compared to Tetrapodophis (the last snake with legs) and Loxocemus, an extant burrowing snake without legs.

From the authors’ abstract:
“the evolutionary versatility of the vertebrate body plan, including body elongation, limb loss, and skull kinesis. However, understanding the earliest steps toward the acquisition of these remarkable adaptations is hampered by the very limited fossil record of early snakes.”

That’s not true.
In the large reptile tree (LRT, 1602+ taxa, subset Fig. 3) snakes have a well documented ancestry back to Cambrian lancelets. The cladogram presented by the nine co-authors was steeped in tradition and lacking in appropriate outgroup taxa. Contra Garberoglio et al. 2019, Varanus and its monitor lizard kin are not part of snake ancestry in the LRT.

Figure 3. Subset of the LRT focusing on geckos and their sister snake ancestors.

Garberoglio et al. continue:
“These new Najash specimens reveal a mosaic of primitive lizard-like features such as a large triradiate jugal and absence of the crista circumfenestralis, derived snake features such as the absence of the postorbital, as well as intermediate conditions such as a vertically oriented quadrate. The new cranial data also robustly resolve the phylogenetic position of this crucial snake taxon, along with other limbed snakes.”

1. The authors’ cladogram did not nest Najash with burrowing snakes, as in the LRT, but at a much more primitive node.
2. Perhaps this is so because Tetrapodophis and Barlochersaurus were not mentioned in the text.
3. The quadrate was sharply bent posteriorly at a right angle, a trait only found in burrowing snakes.
4. I found no primitive lizard-like features here, other than legs and hips, traits found in Tetraphodophis and Barlochersaurus, the last common ancestors of all living snakes.
5. Najash is a crown-group snake in the LRT until additional untested taxa move it out.

Najash rionegrina (Apesteguía and Zaher 2006; Garberoglio FF et al. 2019; Late Cretaceous) is a tiny burrowing snake that retained a pelvis and hind limbs, transitional between Tetrapodophis and Loxocemus. The premaxilla was tiny, as in terrestrial snakes. The mandible rose anteriorly, as in burrowing snakes. The jugal and vomers were retained.

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References
Apesteguía S and Zaher H 2006.A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature. 440 (7087): 1037–1040.
Garberoglio FF et al. (eight co-authors) 2019. New skulls and skeletons of the Cretaceous legged snake Najash, and the evolution of the modern snake plan. Science Advances 2019(5):eaax5833, 8pp.

wiki/Najash

Is ‘Vjushkovia triplocosta’ a jr synonym for Garjainia prima?

In other words,
are the two erythrosuchid holotypes (Fig. 1) sufficiently alike to be congeneric or conspecific? Garjainia was published first.

Butler et al. 2019 reported
“Two species of Garjainia have been reported from Russia: the type species, Garjainia prima Ochev, 1958, and ‘Vjushkovia triplicostata’ von Huene, 1960, which has been referred to Garjainia as either congeneric (Garjainia triplicostata) or conspecific (G. prima).”

“…little work has been conducted on type or referred material attributed to ‘V. triplicostata’. However, this material includes well-preserved fossils representing all parts of the skeleton and comprises seven individuals. Here, we provide a comprehensive description and review of the cranial anatomy of material attributed to ‘V. triplicostata’, and draw comparisons with G. prima. We conclude that the two Russian taxa are indeed conspecific, and that minor differences between them result from a combination of preservation or intraspecific variation.”

Figure 1. Vjushkova holotype compared to Garjainia. These two nest together in the LRT, but not by much. Several areas, including the antorbital and lateral temporal regions differ greatly. The dorsal view of both are quite distinct, overlooked by Butler et al. 

Combining elements from seven specimens
bears some risk of creating a chimaera. Since Butler et al. felt confident in doing so, and there is no alternative, then I do, too. Given the data presented by Butler et al. I reconstructed the skull from separate elements (Fig. 1), something Butler et al. did not do.

Although the two skulls are extremely similar
and the two taxa nest together in the large reptile tree (LRT, 1602 taxa) a few traits seem to distinguish these two taxa apart from one another, at least at the species level and perhaps at the generic level. Note the larger antorbital fenestra in Vjushkovia. Note the pinched upper portion of the lateral temporal fenestra. Note the concave posterior maxilla. Note the taller, narrower orbit. Note the much more robust quadratojugal and quadrate. Note the greater arch of the posterior postorbital. Note the posterior process of the squamosal. These differences appear to support the separation of these taxa at the generic level, IMHO. The lack of a reconstruction in Butler et al. 2019 may have hampered their decision in this case. The lack of graphic comparison in the paper (no images of the Garjainia holotype are shown side-by-side with those of Vjushkoiva) is also regrettable.

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References
Butler RJ, Sennikov AG, Dunne EM, Ezcurra MD, Hedrick BP, Maidment SCR, Meade LE, Raven TJ and Gower DJ 2019. Cranial anatomy and taxonomy of the erythrosuchid archosauriform ‘Vjushkovia triplicostata’ Huene, 1960, from the Early Triassic of European Russia. Royal Society Open Science 6: 191289. http://dx.doi.org/10.1098/rsos.191289

Criticisms of other papers by Butler as co-author:

https://pterosaurheresies.wordpress.com/2018/06/25/the-rise-of-the-ruling-reptiles-ezcurra-and-butler-2018-fiasco/

https://pterosaurheresies.wordpress.com/2019/10/21/teyujagua-paradoxa-still-no-paradox-in-the-lrt/

https://pterosaurheresies.wordpress.com/2019/04/05/mythbusting-prorotodactylus/

https://pterosaurheresies.wordpress.com/2019/02/06/what-is-gracilisuchus-add-more-taxa-to-find-out/

https://pterosaurheresies.wordpress.com/2018/12/12/ezcurra-et-al-2018-review-garjainia/

Birkenia and the origin of facial bones

The most important taxa
in the large reptile tree (LRT, 1601 taxa; Fig. A) are the basal forms at each clade and the basalmost forms at the base of the LRT. Trying to understand where we all came from, I’ve been adding fish to the LRT using generalized reptile traits with some success. Novel topologies have appeared due to testing taxa that have not been tested together before.

Figure 1. Not much of a face here. An early jawless, finless, lancelet-like fish from the Cambrian, Metaspriggina. Compare the protruding placement of the eyes here with Birkenia in figures 2 and 3.

Now the LRT is adding
a mid-Cambrian lancelet/fish without bones, Metaspriggina, (Simonetta and Insom 1993; Fig. 1) and Silurian jawless fish, like Thelodus and Birkenia elegans (Figs. 2, 3; Traquair 1898; Middle Silurian, 1.5-10cm long) in an effort to make sure the base of the LRT is not biased due to taxon exclusion. Turns out it was the right thing to do (Fig. A).

FIgure 2. Birkenia in situ and diagrams. Diagram from Blom et al. 2001. Closeup in Fig. 3.

With that short intro, on to today’s topic
About half of the traits in the LRT come from the skull. It is the one part of the vertebrate body that changes the most in evolution.  Metaspriggina is a Cambrian lancelet with eyes, muscles, guts and gill slits, but no bones. The LRT tests bones, principally. So let’s figure out where and how facial bones first appeared in the vertebrate fossil record. Up to this point, no one, it seems, has put in the effort to do so.

Certain clades of jawless fish enclose themselves
in turtle-like shells (e.g. Arandaspis) with only a scaly or bone armored tail sticking out the back. Those genera are not useful to the present purposes and may never be added to the LRT.

On the other hand, one of the earliest taxa with facial bones
is jawless Birkenia (Figs. 2, 3), a relative to extant lampreys. Here (Fig. 3) the bones are not sutured plates, the sort we expect to see. Rather, in Birkenia areas of parallel, interwoven and concentric splints, whether cartilaginous or bone, form the primordia on which bones are phylogenetically later produced.

Distinctly different,
the squamosal, quadrate and dentary are tiny splints, too (Fig. 3), but they are internal and little different from the other concentric gill bars behind them that remain gill bars before they eventually turn into jaw and throat elements in tetrapods.

Figure 3. Birkenia in situ with facial bones labeled. Frontals and quadratojugals evolve later.

Frontal and quadratojugal
bones are not present in finless Birkenia. Those appear in more derived taxa like Gogonasus. In Birkenia lamprey-like gill openings are retained along with the developing, transitional gill bars.

Birkenia is reported to have
a terminal, rather than ventral, sucking mouth. This specimen (Fig. 3) does not have a terminal mouth. Rather it has a ventral permanent opening, like a lancelet does. “Sucking” was not possible. Perhaps other specimens attributed to Birkenia have different mouth morphologies or some oddly crushed specimens (Fig. 2) have been misinterpreted.

Figure A. Subset of the LRT focusing on basal vertebrates (fish).

Today’s post may represent a novel observation
with regard to the origin of facial bones. If there is a similar earlier paper, let me know and I will cite it for proper credit.

Figure 4. Ventral view of the GLAHM V830 specimen of Thelodus. This appears to have fang-like teeth and a typical mouth. On the other hand, the mandible in this taxon appears to be a dead end experiment convergent with the mandible of all other vertebrates.

Final note:
The thelodont Thelodus (Fig. 4) appears to have a mandible and teeth similar to, but phylogenetically distinct from all other vertebrates. Those ‘teeth’ are probably derived from cilia, since basal vertebrate teeth are not like these. Crushing may have given this fossil the illusion of a mandible. Or this may represent a convergent appearance of a mandible that is not phylogenetically related to the jaws of other tested vertebrates. Or it may represent an early appearance of the mandible, since pectoral fins are also present here, distinct from Birkenia.

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References
Blom H, Märss T and Miller CG 2001. Silurian and earliest Devonian birkeniid anaspids from the Northern Hemisphere. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 92.03 (2001): 263-323.
Simonetta AM and Insom E 1993. New animals from the Burgess Shale (Middle Cambrian)and the possible significance for the understanding of the Bilateria. Bolletino Di Zoologia 60:97–107.
Traquair RH 1898. Report on fossils fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897: 72-76.

wiki/Birkenia 
wiki/Metaspriggina

 

Origin and evolution of gnathostome dentitions

Updated January 2, 2021 and June 3, 2022
with a new reconstruction of Gemuendina (Fig. x), which now appears to nest basal to Manta, close to Jagorina, but will not be entered into the LRT due to the large amount of skin and scale covering bone.

Figure x. Undistorted Gemuendina face.

Figure y. Gemuendina distorted face.

Johanson and Smith 2005
looked at the questions of teeth and pharyngeal denticles in placoderms.

Unfortunately
the large reptile tree (LRT, 1597+ taxa; subset Fig. 1) does not confirm the first sentence of the authors’ abstract: “The fossil group Placodermi is the most phylogenetically basal of the clade of jawed vertebrates but lacks a marginal dentition comparable to that of the dentate Chondrichthyes, Acanthodii and Osteichthyes (crown group Gnathostomata).”

The LRT nests placoderms along with catfish
between Hybodus and spiny sharks, deep into the Gnathostomata. Catfish are not mentioned in the Johanson and Smith text. They do mention, “the rounded or pointed denticles described for the Arthrodira may only be present in a limited number of taxa (Gemuendina (Fig. 2) Traquair, 1903).” Regrettably the authors did not know that some members attributed to this generic wastebasket of Gemuendina are catfish (Fig. 1), a clade closely related to traditional placoderms. So, taxon exclusion, once again, becomes a major issue.

Johanson and Smith also err
when they state, “The Arthrodira is a derived taxon within the Placodermi, hence origin of teeth in placoderms occurs late in the phylogeny and teeth are convergently derived, relative to those of other jawed vertebrates.” The LRT notes that Coccosteus is a basal placoderm, one that is closer to the outgroup taxon, Gregorius than are other less predatory taxa. This exemplifies a problem with this, and many other papers in that without a proper and validated cladogram, it is nearly impossible to determine whether the absence of teeth, or any other trait, represents a vestigial loss or a vestigial genesis situation.

Johanson and Smith report, 
“Tooth sets and tooth whorls in crown-group gnathostomes are suggested to derive from the pharyngeal denticle whorls, at least in sharks, with the patterning mechanisms co-opted to the oral cavity. A comparable co-option is suggested for the Placodermi.”

Figure 2. Whale shark (Rhincodon) tooth pads, not that much different from catfish tooth pads (Fig. 2).

The authors do not mention
the tooth carpets of Rhincodon (Fig. 2) and Manta. The LRT indicates that these taxa represent the origin of teeth within the jaws, not on the margins, which remain toothless, but on the palate, reusembling shark skin.

The authors likewise do not mention
the angel shark Squatina. The LRT indicates this taxon represents the origin of teeth along the margins of the jaws.

The LRT indicates
placoderms lose teeth and sometimes develop sharp, turtle-like gnathal plates, some of which retain vestigial tooth-like bumps. Their sister clade, the Siluriformes (catfish) lose the maxilla and retain tooth carpets only in the mandible (Fig. 1). This begins with the basalmost catfish, traditionally considered a basal placoderm, Entelognathus.

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References
Johanson Z and Smith MM 2005. Origin and evolution of gnathostome dentitions: a question of teeth and pharyngeal denticles in placoderms. Biology Review 80:1–43.