The search for sterna

This is step 2 in a search for sterna in the Tetrapoda.
Please consider this a work in progress because several hard-to-find appearances of the single sternum or dual sterna in tetrapods (and one fish, Fig. 9) may have been overlooked. Sterna also tend to come and go. They may be poorly ossified if present. We first and last looked at sternum distribution five years ago here with far fewer taxa. 

The sternum
This second anchor for pectoral muscles may be a single medial element (Fig. 1) or side-by-side elements slightly separated by the posterior process of the medial interclavicle, the first anchor for pectoral muscles. Almost universally the sternum is posterior to the interclavicle (Fig. 1). Sometimes the sternal rim anchors sternal ribs.

Since the sternum seems to be missing
more often than present, the various appearances of this bone in disparate clades appears to be largely convergent—with a genetic underpinning based on anterior gastralia in the most primitive reptiles.

Sometimes, as in birds,
the sternum (breast bone) can be the largest bone in the body.

The most primitive appearance of a sternum is in frogs,
like Rana (st in Fig. 1). Other basal tetrapods lack a sternum, but have a large interclavicle.

Figure 1. Rana the frog. Note the tiny sternum in the upper left inset, posterior to the coracoids.

Figure 1. Rana the frog. Note the tiny sternum in the upper left inset, posterior to the coracoids.

The basalmost reptiles in the LRT,
Gephyrostegus and Silvanerpeton (Fig. 2) have short gastralia in the shape and place of sterna. These are immediately posterior to the coracoids and are precursors that variously evolve into sterna, if they don’t disappear, which happens more often than not.

Immediately following these basalmost reptiles,
the first dichotomy splits the new Lepidosauromorpha and the new Archosauromorpha.

Figure 1. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Figure 2. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Sterna in basal Lepidosauromorpha:

  1. Thuringothyris has small paired post-coracoid elements not seen in sister taxa.
  2. Stephanospondylus (a pareiasaur ancestor) has an anterior ‘procoracoid’ and a  coracoid apparently not homologous to sterna. No gastralia are present.
  3. Both soft-shell and hard-shell turtles develop a plastron, a set of bones presently considered not homologous with sterna or gastralia.

Figure 3. Saurosternon, the first taxon in the lepidosauromorph lineage with sterna.

Sterna in basal Lepidosauriformes:

  1. Saurosternon (Fig. 3) has paired sterna posterior to the coracoids.
  2. Jesairosaurus has a single, posteriorly indented sternum.

Sterna in Rhynchocephalian Lepidosauria:

  1. Sphenodon has a diamond-shaped sternum, but sister taxa lack one.
Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.

Figure 4. Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.

Sterna in Tritosaurian Lepidosauria:

  1. Almost all tritosaurs (e.g. Huehuecuetzpalli through Cosesaurus, Fig. 4 have a sternum except the hyper-neck taxa, Tanystropheus and Dinocephalosaurus.
  2. All pterosaurs and Longisquama fuse the sternum to the clavicles and interclavicle to form a sternal complex (Fig. 4).

Sterna in Protosquamata and Squamata:

  1. Homoeosaurus has a single sternum.
  2. MFSN 19235 (= ‘Renestosaurus‘) has a single sternum.
  3. Lyriocephalus and Chlamydosaurus have a single sternum.
  4. Moloch and Trioceros have a single sternum.
  5. Eolacerta and Gekko have a single sternum.
  6. Varanus and Tylosaurus have a single sternum.

Sterna in basal Archosauromorpha:

  1. Eldeceeon and Diplovertebron have paired sterna.

Sternal elements in Synapsida
Following these two, the sternum is absent in basal proto-synapsids and basal synapsids.

Sternal elements in Mammalia
In monotremes a string of one to several articulated sternal elements appear (Fig. 5) where the clavicles are green, the interclavicle is red, the sternal manubrium is blue. The interclavicle disappears in the opossum Didelphis and its descendants, all higher therians. Only the manubrium and sternal elements remain. In many placentals the sternal elements fuse together (Fig. 5 image at right) as they anchor dorsal ribs that wrap all the way around from the back.

Figure 1. The pectoral girdle of basal mammals and their relatives. Note the presence of an interclavicle (red), clavicles (green) and a new bone, the manubrium (deep blue), which develops where the sternum develops in other tetrapods.

Figure 5. The pectoral girdle of basal mammals and their relatives. Note the presence of an interclavicle (red), clavicles (green) and a new bone, the manubrium (deep blue), which develops where the sternum develops in other tetrapods.

Sterna in basal Diapsida

  1. Only in Petrolacosaurus and Araeoscelida is a sternum present.
Figure 1. Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Figure 6. Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Sterna in marine Younginiformes and Enaliosauria

  1. Thadeosaurus, and Hovasaurus have paired sterna (Fig. 6).
  2. Tangasaurus has a single large sternum (Fig. 6). these are all basal taxa in this clade.

Sterna in terrestrial Youngininformes and Protorosauria are not present.

Figure 7. Champsosaurus sternum (yellow).

Figure 7. Champsosaurus sternum (yellow).

Sterna in Archosauriformes

  1. Champsosaurus has a small, narrow sternum (Fig. 6). Due to its size and shape a closer examination of related taxa is warranted, but currently a sternum has not been identified.
  2. Crocodylus appears to have a short, broad ‘sternum’ anchoring elongate coracoids, but this is the inter cruciform interclavicle. Basal archosaurs, including basal dinosaurs lack sterna.
Figure 3. Hummingbird skull for comparison to the stilt in figure 2. Image courtesy of Digimorph.org and used with permission.

Figure 8. Hummingbird skull and skeleton. Note the large sternum at bottom, anchoring flight muscles. Image courtesy of Digimorph.org and used with permission.

Sterna in Dinosauria

  1. Scipionyx. Compsognathus and Struthiomimus have paired sterna.
  2. Zhenyuanlong and Tianyuraptor have paired sterna.
  3. Velociraptor, Balaur, HaplocheirusShuuvia and Mononykus have paired sterna.
  4. Limusaurus and Khaan have paired sterna.
  5. Microraptor and Sinornithosaurus have paired sterna.
  6. Troodontids have paired sterna.
  7. Birds (Fig. 8) have large fused sterna, except the enantiornithine, Sulcavism which lacks sterna, replaced with gastralia, as in basalmost reptiles (Fig. 2). Talk about a reversal!!
  8. Camarasaurus, Brachiosaurus, Apatosaurus, and other sauropods have paired sterna.
  9. Psittacosaurus. and ceratopsians have paired sterna. Hard to find them elsewhere.
Figure 1. Rhombichthys, a tiny Late Cretaceous tarpon with deep scutes creating a sternum.

Figure 9. Rhombichthys, a tiny Late Cretaceous tarpon with deep scutes creating a sternum.

Sternum in fish

  1. Rhombichthys (Fig. 9), is a tiny Cretaceous tarpon that looks like an angelfish. Here the ‘sternum’ is created by fusion of several dozen elongate scales that are not pelvic or anal in origin. This is the only sternum present in a fish taxon in the LRT.

Summary
Paired and median sterna appear and disappear throughout the clade Tetrapoda. Since some sterna are small and/or poorly ossified, their distribution within the Tetrapoda may be greater than currently counted. Primitive gastralia proximal to the coracoids appear to be homologous to derived sternal plates proximal to the coracoids. The sternum fuses to the interclavicle and clavicles in pterosaurs and their allies.

Editor’s Note:
WordPress has recently revised their creative methods, now offering buttons for [EDIT], which delivers a blank page permitting no inputs whatsoever and [CLASSIC EDIT], which permits traditional editing. Unfortunately when you press on the [ADD NEW] button you no longer get a blank format ready to be filled, but another blank page permitting no inputs whatsoever. Let’s hope these ‘bugs’ get fixed soon. I have about a week of posts ready to go, but no more possible afterwards given the present ‘bugs’. 


References
Vickaryous MK and Hall BK 2006. Homology of the reptilian coracoid and a reappraisal of the evolution and development of the amniote pectoral apparatus. J Anat. 2006 Mar; 208(3): 263–285. doi: 10.1111/j.1469-7580.2006.00542.x

wiki/Sternum

 

More details on Parahesperornis

Bell and Chiappe 2020
provide additional insight and valuable photos of Parahesperornis alexi (Martin 1984; Fig. 1; Late Cretaceous ~90 mya) a smaller sister/ancestor to Hesperornis (Fig. 1) with more plesiomorphic traits.

Figure 1. Parahesperornis (from Bell and Chiappe 2020) compared to Hesperornis (Marsh 1890) to scale and not to scale. Here the glenoid to tail tip lengths are the same. Everything is exaggerated in Hesperornis.

Figure 1. Parahesperornis (from Bell and Chiappe 2020) compared to Hesperornis (Marsh 1890) to scale and not to scale. Everything is exaggerated in the derived taxon, Hesperornis.

Backstory
According to Bell and Chiappe, “The Hesperornithiformes constitute the first known avian lineage to secondarily lose flight in exchange for the evolution of a highly derived foot-propelled diving lifestyle, thus representing the first lineage of truly aquatic birds. First unearthed in the 19th century, and today known from numerous Late Cretaceous (Cenomanian-Maastrichtian) sites distributed across the northern hemisphere, these toothed birds have become icons of early avian evolution.”

Figure 2. Hesperornis cladogram from Bell and Chiappe 2020. Compare to LRT results in figure x.

Figure 2. Hesperornis cladogram from Bell and Chiappe 2020. Compare to LRT results in figure 3 where more taxa are tested and nested. Gansus should be closer to Hesperornis. Many taxa are omitted between Archaeopteryx and Asparavis here.

Figure 1. Click to enlarge. Toothed birds of the Cretaceous to scale.

Figure 3. Click to enlarge. Toothed birds of the Cretaceous to scale. Compare to figure 2. See the difference when more taxa are added.

Cladistics
Bell and Chiappe and the Large Reptile Tree (LRT, 1694+ taxa, illustrated in figure 3) are in broad agreement regarding the phylogenetic nesting of Parahesperornis (Fig. 2). Unfortunately, Bell and Chiappe don’t include enough taxa to understand the nesting of toothed birds within the clade of toothless birds, as recovered by the LRT (Fig. 3).

And what the heck 
are Gallus, the chicken, and Anas, the duck, doing in figure 2 nesting together? They are not related to one another in the LRT, but… (and here’s the key)… absent ANY pertinent transitional taxa, figure 2 is actually correct, a match with the LRT. Taxon exclusion delivers this oversimplified and misinforming cladogram (Fig. 2). More taxa, not more characters, makes a cladogram more and more accurate.


References
Bell A and Chiappe LM 2020. Anatomy of Parahesperornis: Evolutionary Mosaicism
in the Cretaceous Hesperornithiformes (Aves). Life 2020, 10, 62; doi:10.3390/life10050062
Marsh, OC 1880. Odontornithes, a Monograph on the Extinct Toothed Birds of North America. Government Printing Office, Washington DC.
Martin L 1984. A new Hesperornithid and the relationships of the Mesozoic birds. Transactions of the Kansas Academy of Science 87:141-150.

wiki/Hesperornis

A solution to the Acanthodes problem

The namesake for the clade of spiny sharks,
Acanthodes (Figs. 1, 2), is often touted as the last and best preserved member of the Acanthodii. The braincase and hyomandibular arch are well preserved in 3D, but the cheek and rostral bones are entirely absent. Here (Fig. 1) those elements are restored based on phylogenetic bracketing.

Figure 1. Acanthodes skull with elements restored.

Figure 1. Acanthodes skull with elements restored. The fragility of those elements is why we don’t have them.

Acanthodes bronni (Anonymous 1880; Early Permian 290 mya; 20cm) is the latest occurring acanthodian, the largest and has the best ossified braincase. Davis et al. mislabeled the hyomandibular as a giant quadrate and the preopercular as the mislabeled hyomandibular. Acanthodes is toothless and presumed to be a filter feeder. No extra spines are present. Other species can reach 41cm.

Figure 2. Acanthodes in situ.

Figure 2. Acanthodes in situ.

Reports that acanthodians are the last common ancestors
of sharks and bony fish (Friedman and Brazeau 2010, Davis, Finarelli and Coates 2012) are not supported by the LRT. However, acanthodians are basal to a wide variety of stem lobefin bony fish and placoderms in the LRT.

Figure 1. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale.

Figure 2. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale.

According to Davis et al. 2012:
“Acanthodes bronni remains the only example preserved in substantial detail, central to which is an ostensibly osteichthyan braincase.”

That’s because it nests with bony fish in the LRT (subset Fig. x).

Figure x. Newly revised fish subset of the LRT

Figure x. Newly revised fish subset of the LRT

Davis et al. continue:
“These data contribute to a new reconstruction that, unexpectedly, resembles early chondrichthyan crania. Principal coordinates analysis of a character–taxon matrix including these new data confirms this impression.”

Every time principal component analysis is used instead of phylogenetic analysis, things go awry. For example, Bennett 19xx is infamous for giving gender identities to large and small Pteranodon specimens, not realizing that phylogenetic analysis nests small taxa with Germanodactylus outgroups and large taxa as highly derived.

Davis et al continue:
“However, phylogenetic analysis places Acanthodes on the osteichthyan stem, as part of a well-resolved tree that also recovers acanthodians as stem chondrichthyans and stem gnathostomes.”

Unfortunately, Davis et al. do not employ a longer list of taxa to understand the tree topology the LRT recovers. Here acanthodians are recovered as stem tetrapods. The basal split between Amia and spiny sharks is missing from the Davis et al. 2012 cladogram.


References
Anonymous 1880. Royal Physical Society of Edinburgh (1880). “Proceedings of the Royal Physical Society of Edinburgh”. V: 115.
Davis SP, Finarelli JA and Coates MI 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486:247–250.
Friedman M and Brazeau 2010. A reappraisal of the origin and basal radiation of the Osteichthyes. Journal of Vertebrate Paleontology 30(1):36–56.

wiki/Acanthodii
wiki/Acanthodes

 

Another lepidosaur with a big antorbital fenestra

Quick backstory and summary:
Pterosaurs and their Middle Triassic precursors with a big antorbital fenestra are lepidosaurs (LRT 2020, Peters 2007). Macrocnemus is one of those Middle Triassic precursors, but this one is the only one has a large antorbital fenestra…by convergence.

Skull details on this specimen have been overlooked since 2007.
Macrocnemus fuyuanensis (Li, Zhao and Wang 2007; < 1 m in length; GMPKU P3001, Fig. 1), was the first and only member of this genus not considered conspecific by its authors (actually, no two are alike, see Fig. 3). Earlier we looked at the GMPKU specimen. Today the GMPKU specimen enters the large reptile tree (LRT, 1694+ taxa) today nesting with the T2472 specimen from Europe (Fig. 2).

Figure 1. Macrocnemus fuyuanensis (GMPKU-P-3001) in situ and as traced by the original authors, (middle) flipped with colors applied to bones, and (above) bone colors moved about to form a reconstruction. Darker yellow and darker green are medial views of premaxilla and maxilla. Note the long ascending process of the premaxilla and the palatal elements seen through the various openings all overlooked by those with firsthand access to the fossil. Epipterygoids are lepidosaur synapomorphies not present in protorosaurs.

Figure 1. Macrocnemus fuyuanensis (GMPKU-P-3001) in situ and as traced by the original authors, (middle) flipped with colors applied to bones, and (above) bone colors moved about to form a reconstruction. Darker yellow and darker green are medial views of premaxilla and maxilla. Note the long ascending process of the premaxilla and the palatal elements seen through the various openings all overlooked by those with firsthand access to the fossil. Epipterygoids are lepidosaur synapomorphies not present in protorosaurs.

This referred GMPKU specimen was brought to mind
when Scheyer et al. 2020 discussed in detail the larger holotype M. fuyuarnensis with the skull preserved in ventral view (IVPP V15001, Fig. 4). Scheyer et al. 2020 mistakenly considered it an archosauromorph due to taxon exclusion. Jiang et al. mistakenly considered it a protorosaurian due to taxon exclusion.

All prior workers also overlooked the twin epipterygoids
in the referred specimen (Fig. 1). This is a trait not found outside the Lepidosauria and is lost in several subclades of the Lepidosauria (e.g. Fenestrasauria).

All prior workers overlooked the tiny supratemporals,
which are easy to overlook unless you are looking for them based on phylogenetic bracketing. Taxon exclusion is, once again, the chief problem here. A poor tracing (e.g. Li et al. 2007; Jiang et al. 2011) is the secondary problem.

Figure 2. M. fuyuanensis GMPKU-P-3001 overall. This specimen nests with T2472 in figure 3.

Figure 2. M. fuyuanensis GMPKU-P-3001 overall. This specimen nests with T2472 in figure 3.

The antorbital fenestra
was previously (Li et al. 2007; Jiang et al. 2011) and recently (Scheyer et al. 2020) overlooked because earlier workers considered palatal bones to be rostral bones. That is repaired here (Fig. 1) using DGS methods.

Figure 1. Several Macrocnemus specimens to scale alongside the ancestral taxon in the LRT, Huehuecuetzpalli, and descendant taxa in the LRT, including Cosesaurus and the fenestrasaurs Sharovipteryx, Longisquama and Bergamodactylus. The similarities in transitional taxa should be obvious.

Figure 3. Several Macrocnemus specimens to scale alongside the ancestral taxon in the LRT, Huehuecuetzpalli, and descendant taxa in the LRT, including Cosesaurus and the fenestrasaurs Sharovipteryx, Longisquama and Bergamodactylus. The similarities in transitional taxa should be obvious.

The larger holotype IVPP V15001 specimen
(Fig. 4) preserves the skull upside down (mandible in ventral view). Other elements clearly show the pectoral girdle, pelvic girdle, manus and pes and other elements, more or less in articulation. These are typically scattered in European fossils of Macrocnemus.

Figure 7. The IVPP V15001 specimen of Macrocnemus fuyuanensis in situ. Colors and reconstructions added. Some disagreement here with the pectoral elements.

Figure 4. The IVPP V15001 specimen of Macrocnemus fuyuanensis in situ. Colors and reconstructions added. Some disagreement here with the pectoral elements. Note how the coracoids slide along the interclavicle bound by the sternum reidentified here from the original coracoid. The skull and mandibles are in the center in ventral view.

For those who forget how important the pectoral girdle is
in Macrocnemus and its descendants, others of you might remember the migration of the sternum to the interclavicle, the erosion if the anterior coracoid rim, the elongation of the scapula, the wrapping of the clavicles and the development of the anterior process of the interclavicle that gradually evolves to become the sternal complex in pterosaurs and their flapping precursors, the fenestrasaurs (Fig. 5). This is why it is vitally important to include more taxa in your analyses in order to keep the specimen you are describing in a proper phylogenetic context. All prior workers who studied Macrocnemus lack this context.

Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.

Figure  5. Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.

The Tritosauria (“third lizards”)
is a new squamate clade, now all extinct. The Tritosauria flourished in the Triassic, was reduced to only the Pterosauria during the Jurassic and Cretaceous, and became extinct thereafter. Several members have an antorbital fenestra, most in the lineage of pterosaurs. The GMPKU specimen has an antorbital fenestra convergent with those taxa.

In 2020 pterosaur experts
still have not presented a better hypothesis for the origin of pterosaurs, but prefer to follow their professors who taught them pterosaurs belong with dinosaurs (e.g. Avemetatarsalia, Ornithodira). When will the first one of them break away from promoting this myth?


References
Jiang D-Y, Rieppel O, Fraser NC, Motani R, Hao W-C, Tintori A, Sun Y-L and Sun Z-Y 2011. New information on the protorosaurian reptile Macrocnemus fuyuanensis Li et al., 2007, from the Middle/Upper Triassic of Yunnan, China. Journal of Vertebrate Paleontology 31: 2011-1237, DOI:10.1080/02724634.2011.610853
Li C, Zhao L and Wang L 2007.
A new species of Macrocnemus (Reptilia: Protorosauria) from the Middle Triassic of southwestern China and its palaeogeographical implication. Sci China Ser D: Earth Sci, 50(11): 1601–1605.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Scheyer TM, Wang W, Li C, Miedema F and Spiekman SNF 2020. Osteological re-description of Macrocnemus fuyuanensis (Archosauromorpha, Tanystropheidae) from the Middle Triassic of China. Vertebrata PalAsiatica. DOI: 10.19615/j.cnki.1000-3118.200525

wiki/Macrocnemus

Here’s a project ripe for a PhD dissertation: Youngina and kin

Summary for those in a hurry:
Specimens nesting at the base of the marine and terrestrial younginiforms need a good review, as in a doctoral dissertation. Many of the specimens below have not been described and the collection has not been tested in a phylogenetic analysis, except here in the LRT. And let’s not forget headless Galesphyris (Fig. 4), the last common ancestor of this monophyletic clade of (at present) wastebasket “young-” younginids (Youngina, Youngolepis and Youngoides) needs to be part of the picture. The Late Carboniferous diapsid, Spinoaequalis (Fig. 2), is the outgroup taxon in the LRT.

A new ‘Youngina’ specimen came to my attention
(Fig. 1) published in Sues 2019. Unfortunately no museum number was provided. Pending acquisition of that number, the new specimen was added to the large reptile tree (LRT, 1694+ taxa) just to see where the new one would nest among the many Youngina, Youngoides and Youngolepis specimens (Figs. 2, 3) already in the LRT. Scale bars indicate it’s a big one.

Figure 1. Unidentified specimen attributed by Sues 2019 to Youngina capensis. Here it nests with the much smaller BPI 375 specimen basal to protosaurs.

Figure 1. Unidentified specimen attributed by Sues 2019 to Youngina capensis. Here it nests to scale with the much smaller BPI 375 specimen basal to protosaurs, like the AMNH 9520 specimen assigned to Prolacerta.

Relatively few workers
have published on the Youngina, Younginoides and Youngolepis specimens. That is unexpected considering the key position in the LRT of these largely ignored taxa at the bases of several major clades.

Figure 1. Terrestrial Yonginiformes + Galesphyrus representing the marine clade, all to scale except the toned area containing protorosaurs, which have their own scale.

Figure 2. Terrestrial Yonginiformes + Galesphyrus representing the marine clade, all to scale except the toned area containing protorosaurs, which have their own scale.

One traditional Youngina specimen, 
short-legged BPI 3859, does not nest with the terrestrial taxa in the LRT, despite the many similarities.

Figure 3. The odd one out, the BPI 3859 specimen assigned to Youngina does not nest with the others, but with marine taxa.

Figure 3. The odd one out, the BPI 3859 specimen assigned to Youngina does not nest with the others, but with marine taxa.

However,
if headless Galesphyris turns out to be a junior synonym of Youngina, then the genus would be monophyletic across tested taxa. Let’s leave open that possibility. Otherwise, let’s rename them all appropriately.

Figure 4. If Galesphyrus was Youngina, the genus would be monophyletic.

Figure 4. If Galesphyrus was Youngina, the genus would be monophyletic.

At nine cm in length, the skull of the new specimen
is the largest skull assigned to the genus Youngina. Like the smaller BPI 375 specimen, it nests basal to protorosaurs in the LRT. Other specimens nest basal to Archosauriformes. As noted above, the BPI 3859 specimen nests basal to Claudiosaurus in the LRT along with other marine younginiformes, including plesiosaurs, mesosaurs and ichthyosaurs.


References
Broom R 1914. A new thecodont reptile. Proceedings of the Zoological Society of London, 1914:1072-1077.
Broom R and Robinson JT 1948. Some new fossil reptiles from the Karroo beds of South Africa: Proceedings of the Zoological Society of London, series B, v. 118, p. 392-407.
Gardner NM, Holliday CM and O’Keefe FR 2010. The braincase of Youngina capensis (Reptilia, Diapsida): New insights from high-resolution CT scanning of the holotype. Paleonotologica Electronica 13(3).
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Olson EC 1936. Notes on the skull of Youngina capensis Broom. Journal of Geology, 44 (4): 523-533.
Olson EC and Broom R 1937. New genera and species of tetrapods from the Karroo Beds of South Africa. Journal of Paleontology 11(7):613-619.
Smith, RMH and Evans SE 1996. New material of Youngina: evidence of juvenile aggregation in Permian diapsid reptiles. Palaeontology, 39 (2):289–303.
Sues HD 2019. The Rise of Reptiles: 320 Million Years of Evolution.
Johns Hopkins University Press, Baltimore. xiii + 385 p.; ill.; index.
ISBN: 9781421428673 (hc); 9781421428680 (eb).

wiki/Youngina

Plesiosaur necks: not so flexible after all

With a neck WAAAYYY longer than half the total length
elasmosaurs, like Albertonectes (Figs. 1, 2), have been traditionally referred to as ‘a snake threaded through a sea turtle’ (going back to the Buckland lectures 1832, full story online here). Snakes have no trouble swimming, but so far, paleontologists have not considered the long, minimally flexible neck of elasmosaurs a propulsive organ, as in sea snakes. That might change a little today.

Figure 1. A weak attempt at making sine waves in the neck of Albertonectes.

Figure 1. A weak attempt at making sea snake-like sine waves in the neck of Albertonectes. Note the minimum of bending through effort. Relaxation realigned the neck.

Earlier a vertical configuration was suggested
to explain the weird and extreme morphology of elasmosaurs, entering fish and squid schools from below, distinct from all other oceanic predators. While the flippers were powerful propulsive organs for long distance, when it came to fine tuning while hovering, perhaps the increasingly longer (Fig. 2), snake-like necks helped some. It also moved the bulky flapping torso further from the mouth, so the school of fish would be less and less  likely to notice the intruder in the middle.

Figure 3. Click to enlarge. Albertonectes reconstructed. This 11 m elasmosaur is the longest thusfar recorded. This may be the breathing pose, swallowing air, then submerging the neck. When horizontal the air could be passed back to the lungs, as hypothesized for Dinocephalosaurus.

Figure 2. Click to enlarge. Albertonectes reconstructed. This 11 m elasmosaur is the longest thusfar recorded. This may be the breathing pose, swallowing air, then submerging the neck. When horizontal the air could be passed back to the lungs, as hypothesized for Dinocephalosaurus.

By contrast, Noe, Taylor and Gomez-Perez 2017 reported,
Based on the anatomy of the articular faces of contiguous cervical vertebral centra, neural arches, and cervical ribs, the plesiosaur neck was mainly adapted for ventral bending, with dorsal, lateral and rotational movements all relatively restricted. A new model is proposed for the plesiosaur bauplan, comprising the head as a filter, straining, sieve feeding or sediment raking apparatus, mounted on a neck which acted as a stiff but ventrally flexible feeding tube, attached to the body which acted as a highly mobile feeding platform.”

“The neck increased drag due to its form and large surface area, but was also potentially part of an integrated locomotor system, for instance affecting steering (as it lies in front of the locomotor apparatus) and because the rear of the neck acted as anchorage for musculature from the anterior limb girdles. Hence, any explanation of neck function should consider both slow speed locomotion and more rapid movement during respiration, feeding and predator avoidance.”

Their study looked at
Muraenosaurus (Figs. 3, 4), Cryptoclidus and Tricleidus (none if these yet in the LRT) as exemplars of long-necked plesiosaurians. All are related to one another, not to elasmosaurs. Noe, Taylor and Gomez-Perez presented a history of plesiosaur neck interpretation and presented their own interpretation (ventral flexion, Fig. 5). Given that comprehensive review, apparently no prior workers envisioned a sea-snake analog for the long neck of elasmosaurs, nor have any envisioned a vertical feeding orientation.

Figure 2. Muraenosaurus in dorsal and lateral views. Compare to figure 1.

Figure 2. Muraenosaurus in dorsal and lateral views. Compare to figure 1.

Rather than a flexible ball-and-socket joint
between cervicals, each plesiosaur vertebra consisted of a spool-shaped centrum with flat or slightly concave articular surfaces (Fig. 4). Most cervical centra are wider than deep. according to Noe, Taylor and Gomez-Perez, but that is largely due to a dorsal indentation for the neural spine. Cervicals preserved in situ indicate no intervening cartilage between centra. So, think of plesiosaur centra as Incan wall stones. There are no spaces between either. This compaction between vertebrae greatly restricts movement between individual cervicals and restricts cervical movement overall. Even so, even half a degree per centrum magnified by 76 cervicals can add up (Fig. 1) permitting some movement. Short, L-shaped cervical ribs are fused to each centrum.Their distal processes do not articulate with one another, but hypothetical ligaments extending from anteroposteriorly-oriented distal tips may have done so.

Figure 5. Muraenosaurus cervical sections from Noe et al. 2017 alongside a ghosted diagram of a complete Muraenosaurus neck.

Figure 4. Muraenosaurus cervical sections from Noe et al. 2017 alongside a ghosted diagram of a complete Muraenosaurus neck. The space between centra can be compared to the space between Incan wall stones. In other words: none. That is not shown in the ghosted reconstruction.

Noe, Taylor and Gomez-Perez conclude,
The consistent presence of numerous cervical segments that lack bony stiffening adaptations, however, is also strong evidence that flexibility was an important functional element in plesiosaur necks (Evans 1993), and gives the potential for a considerable range of movement in the living animal (cf. Zarnik 1925–1926).” The authors compare plesiosaurs to stiff-necked tanystropheids (with only 12 cervicals) to emphasize their point. They overlooked the tight articulations of each centrum with its neighbors. 

From a historical perspective, Noe, Taylor and Gomez-Perez report, 
“Previous workers have considered the degree of neck flexibility in plesiosaurs to range from: extreme mobility (Hawkins 1840; Zarnik 1925–1926; Welles 1943; Welles and Bump 1949), including the ability to arch the neck like a swan (Conybeare 1824; Andrews 1910; Brown 1981b); through relative inflexibility (Hutchinson 1897; Williston 1914; North 1933; Shuler 1950; Storrs 1997); to almost complete rigidity (Buckland 1836; Watson 1924, 1951; Cruickshank and Fordyce 2002; Figs. 3, 9); although some of this variation in interpretation may be due to differences between the species studied (Watson 1924, 1951).”

Clearly some of these workers were right and others were wrong.
But which ones? Zoe, Taylor and Gomez-Perez conclude, to their credit, “Overall, the range of movement available to the plesiosaur neck was strictly limited.”

Figure 7. Illustration from Noe, Taylor and Perez-Gomez showing their view of plesiosaur feeding and escape configurations.

Figure 5. Illustration from Noe, Taylor and Perez-Gomez showing their view of plesiosaur feeding and escape configurations. Usually paleo illustrations are more anatomically accurate than this.

Elasmosaurs were morphologically different than anything else in the sea. 
And they became more and more different as time went by (Fig. 2). So, something was working better and better as evolution selected for more extreme neck lengths.

Once again, let’s broaden our scope and look at the environs,
including coeval predators. All of these were robust, fast, streamlined, short-neck predators that swam horizontally preceding an attack from outside in. All of this is the opposite of elasmosaurs who hypothetically loitered below schools of fish unobtrusively rising to slip only their head in from below with minimum turbulence in order to remove fish or squid at leisure from the inside out.

Plesiosaur respiration at the surface
had to take place horizontally due to air pressure constraints. Alternatively, elasmosaurs could have gulped air, then assumed a horizontal or diving orientation to let the air bubble travel back through their long neck back or up to their lungs. With such tiny nostrils, gulping air seems more reasonable than narial inhalation.

Exhalation could have been more leisurely
and might have involved producing a ‘bubble net’ from stale air stored in the long trachea and released through the tiny nares. Extant baleen whales sometimes produce a bubble net to herd fish and plankton as they rise to feed on them. Perhaps elasmosaurs did the same, again based on their vertical orientation.

Fins at all four corners
Noe, Taylor and Gomez-Perez report, “With limbs at the four corners of the body, plesiosaurs could potentially produce vectored thrust from different limbs, to provide fine control of movement in all directions, and around all axes. This is more useful in slow swimming or hovering animals than simple shark-like control fins, which require movement in order to generate a current over the control surfaces.” Exactly. Unfortunately, these authors did not consider plesiosaurs to have a vertical orientation. Instead they focused on the ability of the neck to flex ventrally from a horizontal orientation.

Stomach stones
Noe, Taylor and Gomez-Perez report, “Swimming efficiency was further impaired by the mass of the neck, and the stomach stones commonly preserved in plesiosaurs. This stone ballast was probably needed to establish trim control and longitudinal stability to enable the animal to swim slowly horizontally and to hover, especially when diving in shallow water when the animal was positively buoyant.” The other explanation is that stomach stones helped weight the body below the more buoyant neck (filled with stagnant air), again supporting a vertical orientation when not swimming to other locations.


References
Noe LF, Taylor MA and Gomez-Perez M 2017. An integrated approach to understanding the role of the long neck in plesiosaurs. Acta Palaeontologica Polonica 62 (1): 137–162.

Sclerosaurus and the evolution of turtle respiration

Lyson et al. 2014 brought us their view
on the origin of ventilation (= respiration) in turtles using fossils and extant taxa. Similarly, and in the same year, Hirasawa et al. 2014 did the same from a different perspective: turtle embryos.

Unfortunately
neither put their finger on the correct phylogenetic origins of turtles (Fig. 1) due to taxon exclusion. You can’t get a valid phylogenetic solution without a valid phylogeny.

Figure 1. Carbonodraco enters the LRT alongside another recent addition, Kudnu, at the base of the pareiasaurs + turtles.

Figure 2. Subset of the LRT focusing on the dual turtle clades (pink) and their ancestors.

Both sets of authors
overlooked/omitted the ancestor taxa of turtles recovered by the large reptile tree (LRT, 1694+ taxa; subset Fig. 1), which tested all current candidates for turtle ancestry. That means both sets of authors stepped into the morass that is convergence.

Figure 8. Sclerosaurus insitu. This turtle ancestor still bas a flexible spine, but the pectoral girdle has migrated anterior to the dorsal ribs. A hypoischiuum is present.

Figure 2. Sclerosaurus insitu. This turtle ancestor still bas a flexible spine, but the pectoral girdle has migrated anterior to the dorsal ribs. A hypoischiuum is present.

Here,
the LRT (subset Fig. 1) minimizes taxon exclusion due to its wide gamut of included taxa. Here turtles had dual origins from small horned pareiasaurs. Basal to hard-shell turtles, Elginia documents the genesis of cranial traits. Post-crania is poorly known. Basal to soft-shell turtles, Sclerosaurus (Figs. 2–4) documents the genesis of soft-shell turtle traits. These remain (at present) the best clues we have to the genesis of stem hard-shell turtle post-cranial traits. Those are lacking until we go back to the large pareisaur Bunostegos.

Figure 1. Softshell turtle ancestor, Sclerosaurus animated walking in dorsal view. Dorsal armor initially does nothing to prevents lateral undulation here, as shown by the in situ fossil.

Figure 1. Softshell turtle ancestor, Sclerosaurus animated walking in dorsal view. Dorsal armor initially does nothing to prevents lateral undulation here, as shown by the in situ fossil.

Key to the present discussion,
Sclerosaurus had a wide set of dorsal ribs that were not immobilized by the sprinkling of armor over the dorsal vertebrae. The specimen (Fig. 2) is preserved bending far to the left. So it undulated when it walked (Fig. 3). Sclerosaurus lacked a plastron and/or gastralia.

Figure 4. Sclerosaurus walking with an imagined ventral cross-brace, a plastron. Now this more closely resembles turtle locomotion.

Figure 4. Sclerosaurus walking with an imagined ventral cross-brace, like a turtle plastron. Now Scleromochlus locomotion more closely resembles turtle locomotion. Compare to figure 1.

Immobilzation of the thorax in soft shell turtles
occurs with the genesis of the plastron in Odontochelys (Fig. 5). If we give Sclerosaurus a hypothetical ventral cross brace to stiffen its thorax in the above animated graphic (Fig. 4), it suddenly walks like a turtle (Fig. 4). At first that permits breathing while walking by overcoming Carrier’s constraint. Extant turtles have such a low metabolism that breathing is the last thing they think to do. Sea turtles hold their breath for long periods underwater.

Immobilization of the thorax in Odontochelys
prevented costal ventilation (expanding the ribcage). This is reflected in turtle embryos, which lose intercostal muscles as they develop a rigid shell, according to Hirasawa et al. 2014. Three sets of internal thoracic (hypaxial) muscles take over respiration, expanding to press on the lungs between them or relaxing to initiate inspiration, according to Lyson et al. 2014.

Figure 3. Sister taxa according to Bever et al. Eunotosaurus purportedly nests between Ascerosodontosaurus and the turtles. The large reptile tree, on the other hand, finds that only the turtles are related to each other.

Figure 5. Sister taxa according to Bever et al. Eunotosaurus purportedly nests between Ascerosodontosaurus and the turtles. The large reptile tree, on the other hand, finds that only the turtles are related to each other.

Lyson et al. 2014 
suggested, “the ventilation mechanism of turtles evolved through a division of labour between the ribs and muscles of the trunk in which the abdominal muscles took on the primary ventilatory function, whereas the broadened ribs became the primary means of stabilizing the trunk.” Unfortuantely their ‘early member of the turtle stem lineage’ was the unrelated turtle mimic, Eunotosaurus (Figs. 5, 6). We discussed taxon exclusion errors several times earlier here, here and here.

Figure 3. Subset of the LRT with Martensius added to the base of the Caseasauria + another clade of similar lepidosaurs, all derived from Milleretta.

Figure 6. Subset of the LRT with Martensius added to the base of the Caseasauria + another clade of similar lepidosaurs, all derived from Milleretta. Note the placement of Eunotosaurus with sisters, none of which is close to turtles in the LRT.

Lyson et al. hypothesized,
“an easing of structural constraints through division of function (divergent specialization) between the dorsal ribs and the musculature of the body wall facilitated the evolution of both the novel turtle lung ventilation mechanism and the turtle shell.”
This is likely correct, but they used the wrong outgroup taxon, a turtle mimic, rather than a valid stem turtle. Lyson et al. thought the initial thoracic stiffening occurred in the carpace, as it does in Eunotosaurus, which lacks a plastron or more than 5 pairs of slender gastralia not in the radiating pattern of a plastron. Some Eunotosaurus specimens have overlapping ribs. Turtles don’t do this. Mutual side-by-side suturing is the turtle rib pattern and that’s just the beginning of a long list of non-turtle traits found n Eunotosaurus, which nests with Acleistorhinus and other near caseids in the LRT (Fig. 6), all with lateral temporal fenestrae, making them all synapsid mimics.

As you’ll note above,
Sclerosaurus does not have expanded ribs. They begin to expand with Odontochelys (Fig. 5). By contrast, the turtle-mimic, Eunotosaurus, has much more expanded dorsal ribs than those in Odontochelys. That’s the reverse of the order one would expect. The LRT indicates that Lyson et al. should have expanded their taxon list. Sins of omission are also considered sins in paleontology.

Lyson et al. fell prey to a classic error in paleontology
when they ‘Pulled a Larry Martin,‘ listing traits the turtle mimic, Eunotosaurus, shares with turtles. That’s why a good taxonomist saves listing traits until AFTER a comprehensive phylogenetic analysis determines what is related to what and what converges with what.

Hirasawa et al. 2014
attempted to provide ‘answers to the question of the evolutionary origin of the carapace… Along the line of this folding develops a ridge called the carapacial ridge (CR), a turtle‐specific embryonic structure.’ More important to the present discussion is the genesis of the plastron.

A little backstory on Sclerosaurus
Sclerosaurus armatus (Meyer 1859) Middle Triassic ~50 cm in length, was originally considered a procolophonid, then a pareiasaurid, then back and forth again and again, with a complete account in Sues and Reisz (2008) who considered it a procolophonid.

Here, based on data from Sues and Reisz (2008), Sclerosaurus nests between pareiasaurs and basal softshell turtles like ArganacerasOdontochelys and Trionyx. Their analysis also suffered from taxon exclusion. Sclerosaurus is also a sister to another small horned pareiasaur, Elginia and thus is only slightly more distantly related to Meiolania, the hard-shelled horned basalmost turtle in the LRT.

Overall smaller than other pareiasaurs, Sclerosaurus had a wide, flat body, like the horned lizard, Phrynosoma. The backbone remained quite flexible, as shown by the in situ fossil. Only a sparse sprinking of dermal bones lined the dorsal vertebrae. Note the hypoischium posterior to the ischium and the position of the pectoral girdle anterior to the dorsal ribs, as in Odontochelys.


References
Hirasawa T, Pascual‐Anaya J, Kamezaki N, Taniguchi M, Mine K and Kuratani S. 2015. The evolutionary origin of the turtle shell and its dependence on the axial arrest of the embryonic rib cage. J. Exp. Zool. (Mol. Dev. Evol.) 324B:194–207.
Lyson TR et al. (7 co-authors) 2014. Origin of the unique ventilatory apparatus of turtles. Nature Communications 5:5211.
Meyer H von 1859. Sclerosaurus armatus aus dem bunten Sandestein von Rheinfelsen. Palaeontographica 7:35-40.
Sues H-D and Reisz RR 2008. Anatomy and Phylogenetic Relationships of Sclerosaurus armatus (Amniota: Parareptilia) from the Buntsandstein (Triassic) of Europe. Journal of Vertebrate Paleontology 28(4):1031-1042. doi: 10.1671/0272-4634-28.4.1031 online

wiki/Sclerosaurus

The genesis of the manta ray

Figure 3. The gill chamber and digestive track of Manta shown in ventral view.

Figure 3. The gill chamber and digestive track of Manta shown in ventral view.

Manta is different than other rays.
That difference is reflected in the large reptile tree (LRT, 1694+ taxa). Manta does not nest with other rays, despite a long list of convergent traits. In the LRT Manta nests with the extant whale shark, Rhincodon, but closer to the Early Devonian taxon, Turinia (Fig. 2). Traditionally this boneless enigma is considered a thelodont. But, hey, we’re all thelodonts in a phylogenetic sense, like birds are dinosaurs and mammals are synapsids. Manta and Rhincodon have just changed less than the rest of the vertebrates. Flat and low Loganellia is an Early Silurian sister without the ray-like morphology.

Figure x. Turinia in ventral view. Colors indicate body (lavender), pectoral fins (magenta), digestive tract (yellow).

Figure x. Turinia in ventral view. Colors indicate body (lavender), pectoral fins (magenta), digestive tract (yellow).

Manta has one of the largest
brain-body ratios in the animal kingdom.

Figure 2. Turinia is a basal ray, possibly ancestral to mantas or cow nose rays.

Figure 2. Turinia is a basal ray ancestral to Manta. This is a ventral view of this Devonian taxon.

Manta birostris (formerly Cephalopterus manta, Bancroft 1829; up to 5.5m in length) is the extant manta ray. Traditinally a derived member of the guitarfish, skates and rays clade, Manta nests here with Rhincodon, including an anteriorly facing mouth, nares inside the mouth, tiny blankets of teeth and a diet of planktonic prey. The cephalic fins are like the lobes of cownose rays, but detached anteriorly. So, what to make of this ray with a whale shark mouth, nose, eyes and teeth?

Turinia pagei (originally Thelodus pagei Traquair 1896, 1898, Powrie 1870, Donoghue and Smith 2001; Early Devonian, 410mya) nests with the extant manta ray (Manta) in the LRT. This ventral view preserves soft tissue and only faint impressions of cartilaginous skeletal material with an open stomach cavity and a simple gut extending to the cloaca. The gill chamber was enormous. The pectoral fins had already reached the orbit and the weak tail is transitioning to a whip. The gill openings were ventral to the large pectoral fins. Tiny pelvic fins remain, as in Manta.


References
Bancroft EN 1829. On the Fish known in Jamaica as the Sea-Devil. The Zoological Journal. 4: 444–457.
Donoghue PCJ and Smith MP 2001. The anatomy of Turinia pagei (Powrie), and the phylogenetic status of the Thelodonti. Transactions of the Royal Society of Edinburgh: Earth Sciences 92:15–37.
Powrie J 1870. On the earliest known vestiges of vertebrate life; being a description of the fish remains of the Old Red Sandstone rocks of Forfarshire. Edinburgh Geological Society Transactions 1: 284–301.
Traquair RH 1896. The extinct vertebrate animals of the Moray Firth area. Pp. 235–285 in Harvie-Brown J.A and Buckley TE (eds.): A Vertebrate Fauna of the Moray Firth Basin, Vol. II. Harvie Brown and Buckley, Edinburgh.
Traquair RH 1898. Report on fossils fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897: 72-76.
Swenson JD et al. 2018. How the Devil Ray Got Its Horns: The Evolution and Development of Cephalic Lobes in Myliobatid Stingrays (Batoidea: Myliobatidae). Front. Ecol. Evol, published online November 13, 2018; doi: 10.3389/fevo.2018.00181
White WT et al. 2018. Phylogeny of the manta and devilrays (Chondrichthyes: mobulidae), with an updated taxonomic arrangement for the family. Zoological Journal of the Linnean Society, 2018, 182, 50–75.

wiki/Loganellia
wiki/Turinia
wiki/Manta

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

 

The basalmost primate in the LRT is alive and living in Madagascar!

Now you have a choice.
Either go out looking for crumbling bits and pieces of basal primate jaws and teeth over vast stretches of badlands… Or go to Madagascar to study basal primates in the wild, and have them feeding from your hand, according to the latest addition to the LRT.

The gray mouse lemur,
(Microcebus murinus; Figs. 1, 2) nests at the base of the all the tested primates in the large reptile tree (LRT, 1692+ taxa; subset Fig. 3), basal to both larger adapid lemurs, Notharctus and Smilodectes.

Figure 1. The gray mouse lemur (Microcebus murinus) nests basal to primates in the LRT.

Figure 1. The gray mouse lemur (Microcebus murinus) nests basal to primates in the LRT.

This largest species in this smallest genus of primates
also nests between two tree shrew taxa, Tupaia (basal to Glires) and Ptilocercus (Fig. 4; basal to Volitantia).

Though living today in Madagascar forests,
Microcebus likely radiated during the Cretaceous, prior to the splitting of Madagascar from Africa 88 mya. Later it gave rise to all extinct and extant adapids and lemurs on that island.

Millions of years ago lemurs were
worldwide in distribution. Now only a few lemurs find refuge in Madagacar. and only in Madagascar.

Figure 2. The skull of Microcebus murinus from Digimorph.org and used with permission. Here colors mark bones.

Figure 2. The skull of Microcebus murinus from Digimorph.org and used with permission. Here colors mark bones.

Microcebus murinus (Miller 1777) is the extant gray mouse lemur an omnivore found only in Madagascar. This nocturnal arboreal basalmost primate in the LRT forages alone, but sleeps in groups, sharing tree holes during the day. Twin babies are typical. Offspring can reproduce after one year. Lifespan extends to ten years. The eyes are large, typical of nocturnal mammals. Relatives include Hapalodectes and Ptilocercus. Descendants include Notharctus and Smilodectes.

The newly expanded clade Scandentia (tree shrews) now unites
Volitantia (bats + pangolins + colugos), Primates and Glires (rodents, rabbits, multituberculates and kin) in the LRT, subset Fig. 3). The addition of Microcebus as the smallest lemur held the possibility that it was the most basal form or one leading to smaller galagos and tarsiers. This time Microcebus turned out to be more primitive.

Figure 3. Subset of the LRT focusing on the clade Scandentia (tree shrews) and the three arboreal clades that arise from it.

Figure 3. Subset of the LRT focusing on the clade Scandentia (tree shrews) and the three arboreal clades that arise from it.

With the addition of Microcebus to the LRT,
the extant pen-tailed tree shrew, Ptilocercus (Fig. 4) nests basal to colugos, which also lack upper incisors. That means an older, more plesiomorphic fossil taxon with a complete set of upper incisors is out there waiting to be discovered somewhere in Early Jurassic fossil beds.

Figure 4. Ptilocercus is a sister to Microcebus nesting with colugos.

Figure 4. Ptilocercus is a sister to Microcebus nesting with colugos.

Paleontologists have been looking for the ancestor of primates,
colugos and bats for ages. They find fewer and smaller bony scraps the deeper they look.

Here’s a solution:
Add extant taxa. Phylogenetic analyses that includes extant taxa can sometimes help by nesting late survivors at basal nodes. Sure the fossil taxa are the real ancestors. Sure, living lemurs are late survivors, radiating into new morphologies and niches, but the soft, cuddly, active chatterboxes (Fig. 1) are still worth studying and scoring.


References
Miller JF 1777. Cimelia Physica p.25

wiki/Microcebus