Placoderm jaw de-volution

About an hour ago, 
the question of pelvic girdles before jaws in vertebrate (more specifically, placoderm) evolution was reviewed in light of the LRT.

Now let’s re-examine
another tiny placoderm whose interrelationships were originally misinterpreted due to taxon exclusion.

Hu, Lu and Young 2017
studied jaw structure in a really tiny unnamed Devonian placoderm, ANU V244 (Fig. 1, shown 3x larger here) preserved in 3D. Unfortunately, Hu, Lu and Young followed tradition when they thought placoderms represented the genesis of jaw evolution, preceding the appearance of jaws in sharks and bony fish.

By contrast,
the large reptile tree (LRT, 1697+ taxa, subset Fig. 2) recovers sturgeons and Chondrosteus at the genesis of jaws, immediately preceding sharks + bony fish. In the LRT placoderms nest deep within bony fish, after the great dichotomy. Placoderms like the ANU specimen represent a reduction of jaw elements, not the acquisition. Placoderm precursors, like Cheirodus and Eurynotus, lose or fuse the maxilla. The ANU specimen also loses the premaxilla and reduces the mandible and dentary, which retains teeth.

Figure 1. The tiny ANU V244 specimen in various views. Note the scale bars.

Figure 1. The tiny ANU V244 specimen in various views. Note the scale bars.

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Figure 2. Subset of the LRT focusing on the catfish + placoderm clade, starting with a spiny shark, Diplacanthus.

The ANU specimen nests with the much larger Romundina (Fig. 2), a bottom feeder with reduced jaw elements and large cheeks.

The ANU specimen
is only one of several placoderms with reduced jaws (Fig. 3).

The LRT has been adding fish taxa over the past year, when the first catfish was nested with the first few placoderms. As it stands now, catfish are still closely related to placoderms in the LRT (subset Fig. 2).

The origin of placoderms would make a great PhD thesis, seen from all angles.

Figure 2. A sample of taxa related to Autroptyctodus with homologous skull bones color identified

Figure 3. A sample of taxa related to Autroptyctodus with homologous skull bones color identified.

Once again,
a valid phylogenetic analysis that includes a sufficient number of pertinent taxa is key to understanding interrelationships. Don’t get turned around by using the traditional list of too few taxa. Don’t assume your predecessors and professors are correct. Test their hypotheses. Add taxa to provide, determine and validate the proper phylogenetic context in all cases. In can tell you from experience, it will be rewarding.


References
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y

The new ‘Titanichthys’ is not Titanichthys

Figure 1. Mandible of Titanichthys compared to scale with reconstruction of Bonnerichthys.

Figure 1. Mandible of purported Titanichthys from Coatham et al. 2020,  compared to scale with reconstruction of Bonnerichthys.

Coatham et al. 2020 bring us a new ‘Titanicthys
from Late Devonian Morocco based on a single bone: a gently curving toothless mandible (PIMUZ A/I 4716; Fig. 1). However, when that mandible is compared to a more complete Titanichthys (CMNH50319; Boyle and Ryan 2017; Figs. 2,3) similarities cannot be found. The closer match is to Bonnerichthys (Fig. 1), a late survivor from an Early Devonian radiation among tested taxa in the large reptile tree (LRT, 1694+ taxa; subset Fig. 5).

Figure 2. Titanichthys skull animated and colorized. I flipped the mandible upside down to make more sense as a bottom scraper. CMNH50319 (Titanichthys cf. clarki)

Figure 2. Titanichthys skull animated and colorized. I flipped the mandible upside down to make more sense as a bottom scraper and to match the 3D reconstruction in figure 3.

I emailed Dr. Coatham for more data
asking if more placoderm elements were found in association with the Morocco mandible.  Evidently the authors did not consider the possibility of a non-placoderm taxon, perhaps due to the great size of the specimen (Fig. 1) and its chronology.

BTW
the Bonnerichthys clade precedes the placoderm-catfish clade in the LRT.

Added moments after publication, Sam Coatham replied.
“Hi Dave, I take your point that the jaw used in the study has some disparity with the Cleveland specimens. However, there is a large degree of morphological variation even in just the jaws of the Cleveland species (detailed here: https://beforethebolide.wordpress.com/2017/06/21/how-many-species-of-titanichthys-are-there/), so I don’t know if this is enough to bring into question its status as Titanichthys. It’s interesting that you bring up Bonnerichthys, I wasn’t aware of any geographic or temporal crossover between the two. However, I believe that it has also been suggested as a suspension-feeder (Friedman et al, 2013) – it seems more likely to me that the morphological similarity is a result of convergence resulting from their shared feeding strategy. As we outline in the paper, similar jaw adaptations have been observed in numerous taxa containing giant suspension-feeders. Thanks, Sam.”

Figure 3. Titanichthys bones. Note the man bile, which is upturned anteriorly as in sister taxa.

Figure 3. Titanichthys bones. Note the man bile, which is upturned anteriorly as in sister taxa. Note how the jaw tips rise and bend hard toward the midline. Compare to figure 2.

So how does that affect results from Coatham et al. 2020
who published stress tests on the ‘aberrant’ toothless mandible vs. that of the giant placoderm, Dunkleosteus? I can’t say, other than to note that none of the living relatives of Bonnerichthys, like Osteoglossumare suspension/ plankton feeders. Instead they are opportunistic surface feeders. On the other hand, the real Titanicthys has a huge gape (larger still because the fossil is missing the large nasal bones (Figs. 2,3) that form the rostrum in sister taxa, like Coccosteus (Fig. 4) and it seems able to scoop up large amounts of whatever it wanted to. Several placoderms are bottom-feeders. So are their living relatives, the catfish.

Figure 3. Coccosteus is a placoderm that shares more traits with Kenichthys than any tested sarcopterygian.

Figure 3. Coccosteus is a placoderm that shares more traits with Kenichthys than any tested sarcopterygian.

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Figure 5. Subset of the LRT focusing on catfish + placoderm clade.

Whenever the label ‘aberrant’ appears in a paper it usually means the authors have the clade misidentifiedBonnerichthys is not mentioned in the text.

From the abstract:
“The Late Devonian placoderm Titanichthys has tentatively been considered to have been a megaplanktivore, primarily due to its gigantic size and narrow, edentulous jaws while no suspension-feeding apparatus have ever been reported.  Our results, therefore, conform to the hypothesis that Titanichthys was a suspension feeder with jaws ill-suited for biting and crushing but well suited for gaping ram feeding.”

In conclusion,
the older and more complete CMNH specimen may indeed be what Coatham et al. say it is, but the new PIMUZ specimen from Morocco (Fig. 1) is not the same thing. I’ll change that assessment if more associated skull material indicates placoderm affinities.


References
Boyle J and Ryan MJ 2017. New information on Titanichthys (Placodermi, Arthrodira) from the Cleveland Shale (Upper Devonian) of Ohio, USA. Journal of Paleontology 91, 318–336. (doi:10.1017/jpa.2016.136)
Coatham SJ, Vinther J, Rayfield EJ and Klug C 2020. Was the Devonian placoderm Titanicthys a suspension feeder? Royal Society Open Science. 7:200272
Newberry JS 1885. Palaeozoic fishes of North America. Monogram US Geological Survery 16:132.

Titanichthys agassizi (Newberry 1885; )
Titanichthys termieri (from Morrocco)
Titanichthys cf. clarki CMNH50319

wiki/Titanichthys
http://dx.doi.org/10.1098/rsos.200272

 

Fish cladogram: Cambrian period to the present day

When one layers established time periods
over the fish portion of the large reptile tree (LRT, 1673+ taxa; Fig. 1) the surprising length of certain ghost lineages and the ability of several clades to survive several hundred million years becomes apparent.

Figure 1. Subset of the LRT focusing on basal vertebrates (= fish). Colors indicate time periods. This chart documents the lack of fossils for several clades and genera in the Silurian and Devonian.

Figure 1. Subset of the LRT focusing on basal vertebrates (= fish). Colors indicate time periods. This chart documents the lack of fossils for several clades and genera in the Silurian and Devonian.

The antiquity of Silurian members in the highly derived lungfish clade
(Guiyu and Psarolepis) helps one understand the coeval Silurian appearances of so-called primitive fish, like acanthdians and placoderms (Entelognathus). Traditional cladograms assumed early taxa must be more primitive, not realizing that phylogenetic analysis indicates a vast undiscovered radiation of taxa in the Silurian (Fig. 1). Most of these are still waiting to be discovered.

What do Silurian and Early Devonian fossil fish in the LRT have in common?
Many were flat bottom dwellers with small eyes.

By contrast, coeval spiny sharks had large eyes and were free-swimmers. Even so they lost their flexible fin rays, they lost large teeth, they kept a large mouth, and they had vestigial skeletons. Such traits are associated today with slow-moving deep sea fish.

So known Silurian fish were not open sea visual predators with great swimming skills. Their ecological absence must have a reason. I wonder if such taxa were gobbled up before they could drift to muddy or silty anoxic regions of the sea floor where they could wait undisturbed to be buried for fossilization? Even a few exceptions are lacking. Very puzzling…

According to Google:
“In North America geologic activity over the last 417 million years has removed or covered up most Silurian rocks. Well-preserved fossils from Silurian reefs can be found in the Great Lake States of Minnesota, Wisconsin, Michigan, and Illinois.” So Silurian exposures are comparatively rare.

How do left column fish differ from right column (Fig. 1) fish?
As a general rule (allowing for many exceptions) left column fish do not appear to be the fast, open water swimmers seen in the majority of primitive right column fish in the Silurian and Devonian. It is noteworthy that not one taxon in the right column has a Silurian through Permian representative. I will add them as they come to my attention. It is also noteworthy that the left column has very few living representatives. I count nine.

Traditional cladograms
put more emphasis on time and exclude extant taxa. That’s why traditional cladograms often nest spiny sharks and placoderms near the base of the basal vertebrates, prior to sharks and bony fish. And they attempt to add tube-feeding sturgeons somewhere in the middle of bony fish. In the LRT taxon exclusion is minimized and more natural evolutionary patterns are recovered based on phenomics (traits).

Some previously unrecognized relationships recovered by the LRT include:

  1. The wide radiation of clades in the Silurian.
  2. Devonian taxa take us rapidly to tetrapods, documented by Middle Devonian tracks
  3. Note the proximity of Silurian lobefins to Viséan (Early Carboniferous) tetrapods, including reptiles.
  4. Note the unbalanced fossil record with regard to the major dichotomy splitting bony fish
  5. Proamia is known from the Devonian while a sister taxon, Amia, is known from extant taxa, separated by 360 million years. This is the closest we get to a right column fish fossil in the Silurian or Devonian.
  6. The time span between tiny Silurian Loganiella and giant extant sisters Rhincodon + Manta is about 430 million years.
  7. A similar time span splits Hemicyclaspis from living sturgeons.
  8. A longer time span (~500 my) splits Branchiostoma from its Cambrian precursors.
  9. When comparing the LRT to traditional cladograms, check to make sure they have similar outgroup taxa. Too often taxon exclusion is an unaddressed issue in those papers, which make them fitting subjects for the next few blogposts.

Cautionary note:
The choosing of fish taxa for the LRT has not been random, but was made on the basis of availability and possible importance. At present the fossil record is skewed toward left column fish prior to the Permian. As more taxa are discovered and added, the subjective second reason will hopefully pale to become less of a factor.

 

Gogosardina: the genesis of the squamosal

Choo, Long and Trinajstic 2009 brought us
a small, Late Devonian actinopterygian, Gogosardina coatesi (Figs. 1, 2; holotype WAM 07.12.2) known from four crushed and incomplete specimens. One contains conodont elements lodged among the branchial arches.

Figure 1. Gogosardina from Choo, Long and Trinajstic 2009 shown at full size if shown on a typical 72dpi computer monitor.

Figure 1. Gogosardina from Choo, Long and Trinajstic 2009 shown at full size if shown on a typical 72dpi computer monitor. Gray areas indicate missing bones on skull.

Here
(Fig. 2) a few skull bones are relabeled according to their tetrapod homologs, as in all taxa entered into the large reptile tree (LRT, 1663+ taxa). The skull is nearly identical to coeval and similarly-size Mimipiscis with slightly rotated premaxilla, a straighter anterior maxilla, a higher naris and only a partial ‘razor back’ ridge anterior to the dorsal fin. The skull is proportionally larger as well. Both have a large pineal opening between the frontals (yes, the frontals), distinct from almost all fish. The excurrent naris is confluent with the orbit. This entire clade lacks postparietals.

Figure 2. Gogosardina soul from Choo, Long and Trinajstic 2009. New labels in red. The intertemporal anchors the large hyomandibular in all fish.

Figure 2. Gogosardina soul from Choo, Long and Trinajstic 2009. New labels in red. The intertemporal anchors the large hyomandibular in all fish.

Choo, Long and Trinajstic considered Gogosardina to be
a stem actinopterygian. No cladogram of relationships was published then. Wikipedia lists Gogosardina among the Palaeonisciformes (Hay 1902). In the LRT Gogosardina nests between Cheirolepis and Mimipiscis, all basal to the extant anchovy, Engraulis., which is not traditionally considered to be a paleonisciform.

Figure 3. Pteronisculus shows how the jugal splits to form a jugal and squamosal, a bone that will ultimately take over for the preopercular.

Figure 3. Pteronisculus shows how the jugal splits to form a jugal and squamosal, a bone that will ultimately take over for the preopercular in this clade.

Clade member, Pteronisculus
(Fig. 3) splits the jugal into four parts. The posterior two become the single squamosal in Strunius (Fig. 4), Onychodus (Fig. 5) and all later lobefins and ultimately tetrapods.

Figure 5. Strunius shows the next step in the enlargement of the squmosal and the two bones making up the preopercular.

Figure 4. Strunius shows the next step in the enlargement of the squmosal and the two bones making up the preopercular.

In Onychodus
(Fig. 5) the squamosal is beginning to take over the preopercular (= postsquamosal). Thereafter the postsquamosal is a vestige until it disappears in most tetrapods, only to reappear in a few basal tetrapods undergoing reversals.

Figure 1. Onychodus is typical of most fish having dual external nares strictly for olfactory sensing. Gill covers are part of the respiratory apparatus.

Figure 5. Onychodus continues the enlargement of the squamosal and the reduction of the preopercular (post squamosal) in our tetrapod lineage.

Wikipedia reports,
“The Palaeonisciformes (Hay 1902) are an extinct order of early ray-finned fishes (Actinopterygii) which began in the Late Silurian and ended in the Late Cretaceous. It is not a natural group, but is instead a paraphyletic assemblage of the early members of several ray-finned fish lineages.”

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

With regard to anchovies, Wikipedia reports, 
Clupeiformes (Goodrich 1909) is the order of ray-finned fish that includes the herring family, Clupeidae, and the anchovy family, Engraulidae.” 

The LRT nests few traditional clupeiformes,
but the wolf herring, Chirocentrus, nests elsewhere (at a more basal node, along with toothy Trachinocephalus) apart from the anchovy, Engraulis. So this seems to be a paraphyletic clade based on these two disparate taxa.

Putting related taxa in phylogenetic order
helps us visualize the less dramatic processes of evolution that no one ever talks about, like the origin of the squamosal. which ultimately creates the dentary-squamosal jaw joint in mammals and humans.


References
Choo B, Long JA and Trinajstic K 2009. A new genus and species of basal actinopterygian fish from the Upper Devonian Gogo formation of Western Australia. Acta Zoologica (Stockholm) 90 (Supp 1):194–210.

wiki/Gogosardina (not online yet)
wiki/Palaeonisciformes
wiki/Clupeiformes

In memoriam: Professor Jennifer Clack

If you never met her,
here’s your second chance, via YouTube videos.

This week marks the passing of Professor Jennifer Clack (1947-2020),
a renown specialist in Devonian tetrapods, especially Acanthostega (Fig. 1). In the above 4-minute YouTube video from 2017, Clack introduces her concept that the first tetrapods, like her discovery of Acanthostega, had more than five manual digits. This is confirmed by Middle Devonian tetrapod tracks (Fig. 3) with more than five digits.

Figure 4. Acanthostega does not have much of a neck.

Figure 1. Acanthostega does not have much of a neck. Note the narrow torso, taller than wide, distinct from lobefin fish that phylogenetically led to basal tetrapods, like Trypanognathus in figure 4.

But not
according to the large reptile tree (LRT) which recovers Acanthostega as a terminal taxon, not a transitional one, far from the main line of tetrapod origins. Four digits are found in Panderichthys, Greererpeton and many other basal tetrapods, as we learned earlier here, here and here. More than five digits are found in only a few derived taxa, including the stem reptile, Tulerpeton, far from the origin of digits.

A more complete and technical account
of basal tetrapod traits is provided by Clack in this 20-minute YouTube lecture video from 2016 (above).

It may be that Clack only saw evolutionary progress
without considering the possibility of evolutionary reversal, as happens when taxa return to a more aquatic niche from a less aquatic niche, reducing the importance of their digits and limbs. In the above video, Clack does not provide a phylogenetic analysis, like the LRT (subset Fig. 2) that includes more primitive, but late-surviving basal tetrapods, all of which follow the pattern of a wider than deep torso, as in ancestral fish with embedded arm bones in their lobefins. Rather, she concentrates on individual traits, which while valuable, set her up for ‘Pulling a Larry Martin‘, rather than concentrating efforts on determining a phylogeny that minimizes taxon exclusion and lets the software determine (= mirror) evolutionary events, as the LRT does while minimizing taxon inclusion bias.

Figure 4a. Subset of the LRT focusing on basal tetrapods. Note the displaced positions of Acanthostega and Ichthyostega.

Figure 2. Subset of the LRT focusing on basal tetrapods. Note the displaced positions of Acanthostega and Ichthyostega.

Only after a phylogeny is documented and validated
can one then discuss the various traits and their uses by the creature that possessed them.

Lest we forget
the first tetrapod tracks (Fig. 1, Niedźwiedzki et al. 2010) predate fossil tetrapods, including Acanthostega, by 20 to 30 million years, as we looked at here. And even they had more than five toes. Thus the phylogenetic origin of tetrapods goes back even further. The early Devonian must have provided quite a few niches for such rapid evolution to take place.

Figure 3. Best Devonian Valentia track with various overlays.

Figure 3. Best Devonian Valentia track with various overlays.

We need to look more closely at
Trypanognathus (Fig. 4; latest Carboniferous), which is the most primitive, but by far not the earliest, taxon in the LRT to document fingers and limbs, rather than lobe fins. Note the anterior eyes, wide flat skull and body, and primitive sprawling limbs. Can someone count the fingers and toes on this specimen? I find no more than four digits. Some may be hiding here.

Figure 1. Trypanognathus in situ, colorized to bring out ribs and limbs.

Figure 4. Trypanognathus in situ, colorized to bring out ribs and limbs is the most primitive, but not the earliest taxon with limbs and toes, not lobe fins.

We’ve seen the chronology of several fossil finds
at odds with their phylogeny in the LRT (e.g. multituberculates, bats, Gregorius). That keeps it interesting, but only a wide gamut phylogenetic analysis based on traits will deliver a valid tree topology. As time goes by and more discoveries are made the competing hypotheses will someday converge.

Figure 2. Silvanerpeton from the Upper Viséan (331 mya) is the outgroup taxon for Gephyrostegus and the Amniota.

Figure 5. Silvanerpeton from the Upper Viséan (331 mya) is the outgroup taxon for Gephyrostegus and the Amniota.

And one more thing,
Clack 1994 described Silvanerpeton (Fig. 5, Viséan, 335 mya) first as an anthrcosauroid and later (Ruta and Clack 2006) as a stem tetrapod, all without recovering it as the basalmost reptile, as shown in the LRT. Adding taxa, creating a wider gamut phylogenetic analysis, would have brought even more fame to this well-respected paleontologist.


References
Clack JA 1994. Silvanerpeton miripedes, a new anthracosauroid from the Visean of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (for 1993), 369–76.
Niedźwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M and Ahlberg PE 2010. Tetrapod trackways from the early Middle Devonian period of Poland Nature 463, 43-48. doi:10.1038/nature08623
Ruta M and Clack, JA 2006 A review of Silvanerpeton miripedes, a stem amniote from the Lower Carboniferous of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 97, 31-63.

https://www.zoo.cam.ac.uk/news/professor-jenny-clack-frs-1947-2020

http://www.theclacks.org.uk/jac/Biography.html

https://www.pbs.org/wgbh/nova/link/clack.html
(make sure to click on the parts 2-4 links therein)

 

Remora adhesion disc: origin, evolution and ontogenetic development

Figure 1. The head of a remora showing in three views of the adhesion disc that make this fish the one and only 'shark-sucker.'

Figure 1. The head of a remora showing in three views of the adhesion disc that make this fish the one and only ‘shark-sucker.’ Suction is created by raising the Venetian blind-like strips shown here once the seal is made with the rim of the disc.

As reported at NationalGeographic.com
The remora is so ridiculous that no one would try to make it up. The top of its head is a giant, flat suction cup. It uses the cup to lock onto the bodies of bigger animals, such as sharks, sea turtles, and whales. As the big animal swims for miles in search of a meal, the remora hangs on for the ride. When its host finds a victim, the remora detaches and feasts on the remains.”

“Remains”? According to Williams et al. 2003, 
remora diets are composed primarily of host feces.

FIgure 1. The origin of the remora starts here with the swift, open water predator, the barracuda (Sphyraena). Note the long body, jutting lower jaw and flat top skull.

FIgure 2. The origin of the remora starts here with the swift, open water predator, the barracuda (Sphyraena). Note the long body, jutting lower jaw and flat top skull.

As reported at NationalGeographic.com
“Their closest relatives include Mahi-Mahi and amberjacks, neither of which has anything on their head that even faintly resembles the remora’s sucker.”

According to the Friedman et al. 2013,
and the large reptile tree (LRT, 1556 taxa) the closest relatives of remoras include the barracuda (Sphyraena, Fig. 2), the cobia (Rachycentron, Fig. 3) and Opisthomyzon (Figs. 4, 5). Basal to these nests the open seas predator, mahi-mahi (Coryphaena) and its sister, the cave-dwelling wolffish (Anarhichas) and prior to these, the shorter-bodied carp and perch, derived from the more primitive piranha. Amberjacks, like Seriola revoliana, are more derived, basal another list of slower-moving taxa.

As reported at NationalGeographic.com
“Britz and Johnson’s research indicates that the remora suction disk started out, improbably enough, as a dorsal fin.” (Fig. 9)

That statement requires a bit of explanation
because outgroup taxa, like the barracuda (Fig. 2) and the cobia (Fig. 3) don’t have a traditional dorsal fin near the skull. The latter taxon does have a series of tiny hooks with spine bases (Fig. 9) and it is this structure that spreads laterally, develops a surrounding lip and moves forward over the flat skull during the ontogeny of remoras (Fig. 9), one step of which is tentatively shown in the early Oligocene pre-remora, Opisthomyzon (Fig. 4, 5). Those tiny little hooks could barely attach themselves to a larger host, a little like Velcro, not well and not often, but those fish that were better at it due to various morphological modifications (mutation), survived and reproduced better in their chosen niche, ultimately evolving to become full-fledged remora.

FIgure 2. The remora transition starts here: with the cobia (Rachycentron). Note the overall resemblance, lacking an adhesion disc. Instead six to nine tiny spine-hooks appear where an anterior dorsal fin appears on other fish.

FIgure 3. The remora transition starts here: with the cobia (Rachycentron). Note the overall resemblance, lacking an adhesion disc. Instead six to nine tiny spine-hooks appear where an anterior dorsal fin appears on other fish. Those little hooks could barely attach themselves to a larger host, but not well. Improvements led to more hooks, wider plates, then suction as the adhesion disc evolved.

Rachycentron canadum (Kaup 1826; 2m; Fig. 3) is the extant cobia. Like the remora but without the adhesion disc, this fish also follows larger hosts (Fig. 8) seeking bits and pieces of the detritus and excrement. The first ‘dorsal fin’ has 6 to 9 short sharp spines. Females spawn 30 times a season, producing thousands of planktonic eggs 1.2mm in diameter.

Figure 3. The early Oligocene pre-remora, Opisthomyzon, with a small adhesion disc at the back of the flat skull.

Figure 4. The tiny  early Oligocene pre-remora, Opisthomyzon, with a small adhesion disc at the back of the flat skull. Note the smaller dorsal fin and elevated pectoral fins.

Opisthomyzon glaronensis (Friedman et al. 2013; early Oligocene) is a small prehistoric remora with only a small posterior sucker. This specimen indicates that the adhesion disc originated in a postcranial position, and that other specializations (including the origin of pectination, subdivision of median fin spines into paired lamellae, increase in segment count and migration to a supracranial position) took place later in the evolutionary history of remoras.

Figure 4. The skull of Opisthomyzon in situ and reconstructed. Note the small adhesion disk at the back of the skull, essentially replacing the postparietals.

Figure 5. The skull of Opisthomyzon in situ and reconstructed. Note the small adhesion disk at the back of the skull, essentially replacing the postparietals.

This phylogenetic sequence of transformation
finds some parallels in the order of ontogenetic changes to the disc documented for living remoras (Britz and Johnson 2012).

Figure 2. A remora attached to a much larger shark with an adhesion disc atop its head. Gone are the 6 to 9 dorsal spines.

Figure 6. A remora attached to a much larger shark with an adhesion disc atop its head. Gone are the 6 to 9 dorsal spines.

Remora remora (Rafinesque 1810; 75cm) is the extant remora or shark-sucker. A flexible Venetian blind-like membrane rises due to blood flow atop the skull to produce suction (Flammang BE and Kenaley 2017). Hatchlings are less than a centimeter in length. At 3cm juvenile Remora has a fully formed 2mm sucking disc. Like its phylogenetic sister, the barracuda, the skull roof is otherwise flat and the lower jaw juts out beyond the upper one. Remoras eat the ectoparasites and feces of their host.

Figure 5. Skull of Remora with a large adhesion disc extending forward to the premaxilla.

Figure 7. Skull of Remora with a large adhesion disc extending forward to the premaxilla.

Notably
remoras lack a swim bladder. And they are more likely than not to attach themselves in an inverted or angled position on their host (Fig. 6). Most fish (Fig. 8) swim upright.

FIgure 7. Cobia and remora surrounding a whale shark. Cobia have to work harder to keep up. Remora rather easily hitches a ride instead.

FIgure 8. Cobia and remora surrounding a whale shark. Free-swimming cobia have to work harder to keep up. Shark-sucking remora rather easily hitch a ride instead.

Recent studies on disc origin
Britz and Johnson 2012 report: “We compared the initial stages of development of the disc with early developmental stages of the spinous dorsal fin in a representative of the morphologically basal percomorph Morone.” (Fig. 9)

Morone is a sea bass, not related to Remora. It is not long, like a barracuda and it has two dorsal fins, unlike a barracuda.

“We demonstrate that the “interneural rays” of echeneids are homologous with the proximal‐middle radials of Morone and other teleosts and that the “intercalary bones” of sharksuckers are homologous with the distal radials of Morone and other teleosts.”

Wish they had compared Remora to Rachycentron?

“The “intercalary bones” or distal radials develop a pair of large wing‐like lateral extensions in echeneids, not present in this form in any other teleost. Finally the “pectinated lamellae” are homologous with the fin spines of Morone and other acanthomorphs. The main part of each pectinated lamella is formed by bilateral extensions of the base of the fin spine just above its proximal tip, each of which develops a row of spinous projections, or spinules, along its posterior margin. The number of rows and the number of spinules increase with size, and they become autogenous from the body of the lamellae.”

And that’s the story, told both in ontogeny and phylogeny.

Figure 8. From Britz and Johnson 2012 showing a hatchling remora, focusing on the tiny spines in the cervical region that ultimately become the adhesion disc.

Figure 9. From Britz and Johnson 2012 showing a hatchling remora, focusing on the tiny spines in the cervical region that ultimately become the adhesion disc. Compare to Rachycentron (Fig. 3). Like a very primitive form of Velcro, such backward pointing spines dig in deeper whenever the host is accelerating relative to the hitchhiker and dislodged whenever the reverse is initiated.

Finally let’s return to the Devonian – Carboniferous
and the chimaera-like clade Iniopterygidae (Fig. 10), which share with remoras the trait of large elevated pectoral fins. 

Figure 2.I The Iniopterygidae include Iniopteryx, Promexyele, Iniopera and Sibyrhynchus. These reconstructions are from Zangerl and Case 1973 and the captions label them "tentative."

Figure 10. Imagine these Iniopterygidae (Iniopteryx, Promexyele, Iniopera and Sibyrhynchus) attaching their prickly fins to larger hosts. Remoras also have elevated pectoral fins, but without the tiny hooks.

Distinct from remoras,
members of the Iniopterygidae (= Iniopterygiformes, 15-46cm in length) have pectoral fins with tiny hooks. Now we can wonder if these fins and hooks enabled iniopterygids to hitch a ride on larger hosts. Something about that hypothesis makes sense in light of what we’ve learned about the evolution of remoras. Let me know if anyone has promoted this idea before and I will publish the citation.

Figure 1. The skull of the barracuda (genus: Sphyraena) with bones identified with colors.

Figure 11. The skull of the barracuda (genus: Sphyraena) with bones identified with colors.

PS
Before reading Britz and Johnson 2012, and after nesting Remora with the barracuda, Sphyraena (Fig. 11), I wondered if the subdivided adhesion disc of Remora might be evolved from the similar area in Sphyraena. Most fish postparietals are flat to gently convex. By contrast the barracuda postparietal (Fig. 11) is absent… AND the similar adhesion disc of Opistomyzon (Fig. 5) essentially replaces the missing postparietal in shape, size and position. Sphyraena deserves a closer look.


References
Britz R and Johnson GD 2012. Ontogeny and homology of the skeletal elements that form the sucking disc of remoras (Teleostei, Echeneoidei, Echeneidae). Journal of Morphology https://doi.org/10.1002/jmor.20105 online here.
Flammang BE and Kenaley 2017. Remora cranial vein morphology and its functional implications for attachment. Scientific Reports 7(5914). https://www.nature.com/articles/s41598-017-06429-z
Friedman M, et al. 2013. An early fossil remora (Echeneoidea) reveals the evolutionary assembly of the adhesion disc. Proc. R. Soc. B 280.1766 (2013): 20131200.
Williams EH, et al. (6 co-authors) 2003. Echeneid-sirenian associations, with information on sharksucker diet. Journal of Fish Biology. 63 (5): 1176.

nationalgeographic.com/what-good-is-half-a-sucker/

scientificamerican.com/how-the-sharksucker-got-its-suction-disc/

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

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

Mimipiscis: how Amphicentrum came to be

Quick one today
as today’s two fish (Fig. 1) look dissimilar, yet, given all the present candidates in the large reptile tree (LRT, 1556 taxa) they nest together. Likely Mimipiscis gave rise to Amphicentrum, given the former’s Devonian presence relative to the latter’s Carboniferous appearance. Both are in the lineage of extant tuna.

Figure 1. Mimipiscis the Devonian palaeoniscid, is a sister to Amphicentrum, a Carboniferous triggerfish mimic in the LRT.

Figure 1. Mimipiscis the Devonian palaeoniscid, is a sister to Amphicentrum, a Carboniferous triggerfish mimic in the LRT. Those dorsal and anal fins were more scaly than finny. 

If you think THAT’S untenable,
remember human, bat and bird ancestors all looked like salamanders (amphibian-like reptiles, like Silvanerpeton) were just starting to climb out on land at that time.

Mimipiscis bartrami (Choo 2011, formerly Mimia toombsi Gardiner and Bartram 1977; Late Devonian) is a paleoniscid sister, chronologically basal to Amphicentrum in the LRT. The nasal is enlarged, pushing the prefrontal to the back of the orbit and the postfrontal above it. The premaxilla rises to mid orbit.

Amphicentrum granulosus = Cheirodus granulosus (McCoy 1848, 1855; 20cm; Carboniferous) is a disc-shaped fish lacking pelvic fins, covered in large rectangular ganoid scales. Several skull bones are fused. The squamosal is tall. The preopercular and pelvic fins are missing. Teeth are absent from the upper jaws. Note the heterocercal tail.


References
Choo B 2012. “Revision of the actinopterygian genus Mimipiscis (=Mimia) from the Upper Devonian Gogo Formation of Western Australia and the interrelationships of the early Actinopterygii”. Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 102: 77–104. doi:10.1017/s1755691011011029
Gardiner BG and Bartram AWH 1977. The homologies of ventral cranial fissures in ostheichthyans. Pp. 227-245, 8 figs. in: Andrews, S. M., Miles, R, S, & Walker, A. D. (eds.): Problems in Vertebrate Evolution, London.
McCoy F 1855. A synopsis of the classification of the British Palaeozoic rocks, with a systematic description of the British Palaeozoic fossils. Fasciculus 3, Mollusca and Palaeozoic fishes. British Palaeozoic Fossils, Part II. Palaeontology 407-666.

SVP 2018: new Whatcheeria data from nearly 100 specimens

Last one for this year.
This post finishes up an inundation of about 40 2018 SVP abstract reviews. We’ll get back to a regular one-a-day look at paleo news later today.

Otoo, Bolt and Lombard 2018
bring us new information on Whatcheeria (Fig. 1), a basal tetrapod (Fig. 2) now known from nearly 100 specimens. In the large reptile tree (subset Fig. 2, LRT) the Whatcheeria clade nests between the primitive Ossinodus clade and the Ichthyostega clade plus all higher tetrapods. The authors report, “Whatcheeria is key for establishing character polarities on the tetrapod stem, particularly in the context of recent controversies about age of the tetrapod crown group and the timing and pattern of the lissamphibian/amniote split.” In the LRT Tulerpeton and Eucritta have taken over that role.

Figure 3. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

Figure 1. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

After a short description
of key Whatcheeria traits, and without describing a phylogenetic analysis, Otoo, Lombard and Bolt conclude: “This combination of features in the femur emphasizes the moasic of characters present in Whatcheeria, and, in conjunction with recent Tournaisian discoveries, emphasizes the complexity of post-Devonian tetrapod evolution.”

A subset of the LRT
(Fig. 2) portrays post-Devonian tetrapod evolution rather differently and rather simply

Figure 1. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered.

Figure 2. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered.

References
Otoo BK, Bolt JR, Lombard E 2018. A leg up: Whatcheeria and its new contributions to tetrapod anatomy. SVP abstracts.

Chronology and phylogeny of basal tetrapods

Bottom Line:
The place to make future basal tetrapod discoveries is in Late Devonian/Earliest Carboniferous strata (Fig. 1, light blue). That’s where an undiscovered radiation appears to have taken place based on the widespread dispersal of basal tetrapods in the Visean (Early Carboniferous, light green).

Figure 1. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered.

Figure 1. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered.

Sometimes what you don’t see right away
is the important story. We should see lots of Devonian tetrapods, but currently we do not.

Earlier we considered the possibility that Acanthostega and Ichthyostega were secondarily a little more aquatic, based on ancestral taxa that were a little more terrestrial. That hypothesis is based on the current cladogram (subset in Fig. 1).

Tiktaalik was discovered by searching in the desired strata. So this process does work. Maybe we’ll find more basal tetrapods in slightly higher strata.

Ichthyostega and Acanthostega: secondarily more aquatic

More heresy here
as the large reptile tree (LRT, 1036 taxa) flips the traditional order of fins-to-feet upside down. Traditionally the late Devonian Ichthyostega and Acanthostega, bridge the gap between lobe-fin sarcopterygians, like Osteolepis.

In the LRT
Acanthostega, ‘the fish with limbs’, nests at a more derived node than its precursor, the more fully limbed, Ossinodus (Fig. 1). Evidently neotony, the retention of juvenile traits into adulthood, was the driving force behind the derived appearance of Acanthostega, with its smaller size, stunted limbs, smaller skull, longer more flexible torso and longer fin tail.

Figure 1. Ossinodus is the more primitive taxon in the LRT compared to the smaller Acanthostega, the tadpole of the two.

Figure 1. Ossinodus is the more primitive taxon in the LRT compared to the smaller Acanthostega, essentially the neotenous ‘tadpole’ of the two.

Likewise
Ichthyostega is more derived than both fully-limbed Ossinodus and Pederpes, which had five toes. As in Acanthostega, the return to water added digits to the pes of Ichthyostega. In both taxa the interosseus space between the tibia and fibula filled in to produce a less flexible crus.

Figure 2. Ossinodus, Pederpes were more primitive than the more aquatic Icthyostega.

Figure 2. Long-limbed Ossinodus and Pederpes were more primitive than the more aquatic Icthyostega.

So, Acanthostega and Ichthyostega were not STEM tetrapods.
Instead, they were both firmly nested within the clade Tetrapoda. Ossinodus lies at the base of the Tetrapoda. The proximal outgroups are similarly flattened Panderichthys and Tiktaalik. The extra digits displayed by Acanthostega and Ichthyostega may or may not tell us what happened in the transition from fins to feet. We need to find a derived Tiktaalik with fingers and toes.

Figure 3. Tiktaalik specimens compared to Ossinodus.

Figure 3. Tiktaalik specimens compared to Ossinodus.

In cases like these
it’s good to remember that ontogeny recapitulates phylogeny. Today and generally young amphibians are more fish-like (with gills and fins) than older amphibians.

It’s also good to remember
that the return to the water happened many times in the evolution of tetrapods. There’s nothing that strange about it. Also the first Devonian footprints precede the Late Devonian by tens of millions of years.

Figure 4. From the NY Times, the traditional view of tetrapod origins.  Red comment was added by me.

Figure 4. From the NY Times, the traditional view of tetrapod origins. 

Phylogenetic analysis teaches us things
you can’t see just by looking at the bones of an individual specimen. A cladogram is a powerful tool. The LRT is the basis for many of the heretical claims made here. You don’t have to trust these results. Anyone can duplicate this experiment to find out for themselves. Taxon exclusion is still the number one problem that is largely solved by the LRT.

You might remember
earlier the cylindrical and very fish-like Colosteus and Pholidogaster convergently produced limbs independently of flattened Ossinodus, here the most primitive taxon with limbs that are retained by every living tetrapod. By contrast, the Colosteus/Pholidogaster experiment did not survive into the Permian.

References
Ahlberg PE, Clack JA and Blom H 2005. The axial skeleton of the Devonian trtrapod Ichthyostega. Nature 437(1): 137-140.
Clack JA 2002.
 Gaining Ground: The origin and evolution of tetrapods. Indiana University Press.
Clack JA 2002. An early tetrapod from ‘Romer’s Gap’. Nature. 418 (6893): 72–76. doi:10.1038/nature00824
Clack JA 2006. The emergence of early tetrapods. Palaeogeography Palaeoclimatology Palaeoecology. 232: 167–189.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.
Jarvik E 1996. The Devonian tetrapod Ichthyostega. Fossils and Strata. 40:1-213.
Säve-Söderbergh G 1932. Preliminary notes on Devonian stegocephalians from East Greenland. Meddelelser øm Grönland 94: 1-211.
Warren A and Turner S 2004. The first stem tetrapod from the Lower Carboniferous of Gondwana. Palaeontology 47(1):151-184.
Warren A 2007. New data on Ossinodus pueri, a stem tetrapod from the Early Carboniferous of Australia. Journal of Vertebrate Paleontology 27(4):850-862.

wiki/Ichthyostega
wiki/Acanthostega
wiki/Ossinodus
wiki/Pederpes