Kenomagnathus: what you can do with only 2 bones

Spindler 2020
reports on a new basal pelycosaur, Kenomagnathus scottae (ROM 43608; Upper Pennsylvanian, Late Carboniferous, Garnett, KS, USA; Figs 1-3) known from a single lacrimal and maxilla (with teeth) exposed in medial view (Fig. 1).

Figure 1. Kenomagnathus in situ from Spindler 2020.

Figure 1. Kenomagnathus in situ from Spindler 2020. The halo of organic matter is interesting.

From the abstract:
“This is the oldest known diastema in synapsid evolution, and the first reported from a faunivorous member that lacks a precanine step, aside from Tetraceratops. This unique precanine morphology occurred independently from similar structures in Sphenacodontoidea.” 

See Spindler’s freehand drawing
of the ‘true diastema’ (Fig. 2). 

Figure 2. Kenomagnathus maxilla and lacrimal with the rest of the skull restored in lateral view. Note the deep jugal, as in Ophiacodon (Figs. 3, 4). Spindler's freehand drawing indicates a deeper orbit, smaller jugal.

Figure 2. Kenomagnathus maxilla and lacrimal with the rest of the skull restored in lateral view. Note the deeper jugal (cyan), though not as deep as in Ophiacodon (Figs. 3, 4). For that reason the mandible of Ophiacodon was used in this restoration. Spindler’s freehand drawing indicates a deeper orbit, shallower jugal and smaller naris along with a larger mandible.

It is worth noting
that maxillary teeth shrink toward the naris in Ophiacodon (Fig. 3). A diastema may be present in Pantelosaurus (formerly Haptodus saxonicus, Fig.3). These pertinent taxa were not illustrated in Spindler 2020.

Figure 3. Pertinent synapsid skulls to scale. The origin of the Pelycosauria + Therapsida is marked by phylogenetic miniaturization, as in so many other clade origins. Note the depth of the jugal in basal taxa here.

Figure 3. Pertinent synapsid skulls to scale. The origin of the Pelycosauria + Therapsida is marked by phylogenetic miniaturization, as in so many other clade origins. Note the depth of the jugal in basal taxa here.

Spindler’s freehand restoration
increased the size of the orbit and decreased the depth of the restored jugal. So this is yet another cautionary tale highlighting the danger in using freehand drawings in scientific studies.

The shallow jugal depth in the Spindler freehand restoration
is a key oversight. When repaired (Fig. 2) the semi-deep jugal of Kenomagnathus transitionally links deeper jugal Ophiacodon (Fig. 3) to shallower jugal Pantelosaurus and Haptodus (Fig. 3) at the base of Pelycosauria + Therapsida in the large reptile tree (LRT, 1642+ taxa). While running the risk of ‘Pulling a Larry Martin’, there are so few traits to consider here (Fig. 1) and none contradict the present hypothesis of interrelationships. All that puts Kenomagnathus in the lineage of synapsids leading to therapsids, mammals, primates and humans.


References
Spindler F 2020. A faunivorous early sphenacodontian synapsid with a diastema. Palaeontologia Electronic 23(1):a01. doi: https://doi.org/10.26879/1023
https://palaeo-electronica.org/content/2020/2905-early-sphenacodontian-diastema

A reexamination of Milosaurus: Brocklehurst and Fröbisch 2018

I just found out that not one but two Aerosaurus specimens were tested and are to be found in the SuppData for this paper. So, what happened here? I’ll dig deeper to look for a solution. 

Solution: The cladistic analysis in the Brocklehurst and Fröbisch 2018 Milosaurus study recovered nearly 2000 most parsimonious trees for 60 taxa. So the phylogeny is not well resolved. By contrast the LRT is well resolved. Relatively few of the characters could be scored for Milosaurus in the Brocklehurst and Fröbisch study. None overlapped with Ianthodon, the purported closest relative. By contrast the LRT found a suite of traits that were shared by Milosaurus and Aerosaurus to the exclusion of all other tested taxa. 

Brocklehurst and Fröbisch 2018 reexamine
“a large, pelycosaurian-grade synapsid” not from the Early Permian, but from the Latest Carboniferous of Illinois Milosaurus (Fig. 1) was first described by DeMar 1970 as a member of the Varanopsidae (= Varanopidae). Brocklehurst and Fröbisch note, “Milosaurus itself has received little attention since its original description. The only attempt to update its taxonomic status was by Spindler et al. (2018). These authors included Milosaurus in a phylogenetic analysis that, although principally focused on varanopids, contained a small sample of pelycosaurs from other families. Milosaurus was found nested within Ophiacodontidae, as the sister to Varanosaurus.”

Ultimately
Brocklehurst and Fröbisch nested Milosaurus with Haptodus within the Eupelycosauria.

Figure 1. The pes of Milosaurus in situ, reconstructed and compared to Aerosaurus, its sister in the LRT.

Figure 1. The pes of Milosaurus (FMNH PR 701) in situ, reconstructed and compared to Aerosaurus, its smaller sister in the LRT. PILs added to restore distal phalanges.

By contrast
the large reptile tree nested Milosaurus with Aerosaurus (Fig. 1; Romer 1937, A. wellesi Langston and Reisz 1981), a taxon not listed by Brocklehurst and Fröbisch. Based on the pes alone, Milosaurus was twice the size of Aerosaurus. Aerosaurus is a basal synapsid more primitive than Haptodus and the Pelycosauria. Aerosaurus and Milosaurus nest between Elliotsmithia + Apsisaurus and Varanops.

Unfortunately
Brocklehurst and Fröbisch included the unrelated clade Caseasauria in their study of Synapsida, and did not include Aerosaurus. They also include Pyozia, not realizing it is a proto-diapsid derived from and distinct from varanopid synapsids. So, once again, taxon exclusion and inappropriate taxon inclusion are the reasons for this phylogenetic misfit.

Distinct from Haptodus, and similar to Aerosaurus
in Milosaurus metatarsals 2 and 3 align with p1.1, not mt1. The base of mt 5 is quite broad. Other traits also attract Milosaurus to Aerosaurus, including an unfused pubis + ilium. I was surprised that so few traits nested Milosaurus in the LRT as it continues to lump and split taxa with the current flawed list of multi-stage characters.

References
Brocklehurst N and Fröbisch J 2018. A reexamination of Milosaurus mccordi, and the evolution of large body size in Carboniferous synapsids. Journal of Vertebrate
Paleontology, DOI: 10.1080/02724634.2018.1508026
DeMar R. 1970. A primitive pelycosaur from the Pennsylvanian of Illinois. Journal of Paleontology 44:154–163.
Langston W Jr and Reisz RR 1981. Aerosaurus wellesi, new species, a varanopseid mammal-like reptile (Synapsida: Pelycosauria) from the Lower Permian of New Mexico. Journal of Vertebrate Paleontology 1:73–96.
Romer AS 1937. New genera and species of pelycosaurian reptiles. Proceedings of the New England Zoological Club 16:90-96.

wiki/Aerosaurus

SVP 2018: Hipposaurus unprepared verts finally described with µCT scans

In the large reptile tree (LRT, 1306 taxa) Hipposaurus (Fig. 1; Haughton 1929, skull length 21cm, length 1.2m) nests at the base of the carnivorous therapsids, not far from finless pelycosaurs, like Haptodus and basal amonodonts like Stenocybus. For all that time the vertebral column has remained buried and unstudied.

Figure 2. The skull of Hpposaurus was larger than that of its sisters and predecessors among the basal Therapsida.

Figure 1. The skull of Hpposaurus was larger than that of its sisters and predecessors among the basal Therapsida.

We looked at Hipposaurus
earlier as a more likely trackmaker for Dimetropus ichnites.

Peecock et al. 2018
describe µCT scans of the previously undescribed vertebral column of Hipposaurus. With this data  they propose new relationships with other hipposaurids known only from vertebrae.

FIgure 3. Hipposaurus compared Dimetropus. The overall and leg length is right, as are many of the digits. Unfortunately the medial digits are too short in Hipposaurus. Hipposaurus has a narrower gauge and lifted its belly of the ground, as did the Dimetropus trackmaker.

FIgure 2. Hipposaurus compared Dimetropus. The overall and leg length is right, as are many of the digits. Unfortunately the medial digits are too short in Hipposaurus. Hipposaurus has a narrower gauge and lifted its belly of the ground, as did the Dimetropus trackmaker.

The Peecock team concludes with caution,
“Phylogenetic analysis underscores the startling homoplasy between biarmosuchians and
archosauromorphs: when biarmosuchian vertebrae are coded into an archosauromorph data matrix, they form a monophyletic clade within Avemetatarsalia. Extreme caution is needed when interpreting Permian vertebrae as archosauromorphs.”

As we’ve seen before,
convergence is rampant in the LRT.

References
Boonstra LD 1952. Die Gorgonospier-geslag Hipposaurus en die familie Ictidorhinidae: Tydskr. Wet. Kuns 12:142-149.
|Haughton SH 1929.
 On some new therapsid genera: Annals of the South African Museum 28(1):55-78.
Peecock et al. (5 co-authors) 2018. Vertebral osteology of Hipposaurus boonstrai (Therapsida, Biarmosuchia) from the Middle Permian of South Africa, with implications for the evolution of Archosauromorpha. SVP abstracts.

 

wiki/Hipposaurus

SVP 2018: Pelycosaurian phylogeny (should not include Caseasauria!)

Wilson, et al. 2018
discuss various issues with pelycosaurian phylogeny, but then make the mistake of including Caseasauria, an unrelated clade in the large reptile tree (subsets in Figs. 1, 2).

It’s that simple.
Adding taxa shows that Casesauria nest within the new Lepidosauromorpha derived from Milleretta, while pelycosaurs nest within the new Archosauromorrpha, derived from Varanops. The lateral temporal fenestra is convergent and appears in many caseasaur sisters and cousins (Fig. 2).

Given that, the authors report:
“We recover a monophyletic Caseasauria and Eupelycosauria.” Well, of course, they did. That happens with unrelated taxa.

Figure 1. A monophyletic Pelycosauria if we can accept the changes suggested in the text.

Figure 1. A monophyletic Pelycosauria without the Caseasauria, which nests in the basal Lepidosauromorpha (see figure 2) when more taxa are added.

Adding taxa
going back to the origin of the Amniota (= Reptilia) would clarify issues. That’s what the large reptile tree is for. No one has to use the characters or the scoring, but it is a mistake not to use the relevant taxa revealed by the LRT.

Figure 2. Subset of the LRT: basal lepidosauromorpha, featuring Caseasauria.

Figure 2. Subset of the LRT: basal lepidosauromorpha, featuring Caseasauria. Pelycosaurs nest in the basal archosauromorpha when more taxa are added.

References
Wilson WM, Angielczyk KD, Peecock B, Lloyd GT 2018. Pelycosaurian “lineages”: a meta-analysis of three decades of phylogenetic research. SVP abstracts.

Metaanalysis is the statistical procedure for combining data from multiple studies. When the treatment effect (or effect size) is consistent from one study to the next, metaanalysis can be used to identify this common effect.

From the first post on this subject back in 2011:

The case for taking Caseasauria out of the Synapsida.

Figure 3. Which of these skulls does NOT belong with the others. The case for taking Caseasauria out of the Synapsida.

 

The Early Permian Ascendonanus assemblage

There are five specimens from the same pit
that were assigned to the varanid taxon Ascendonanus. Spindler et al. 2018 thought they were all conspecific.

Given their distinct proportions
(Fig. 1) and the phylogenetic differences recovered in 2 of the 5 so far (earlier one nested as a basal iguanid), we’re going to need some new generic names for at least one of the referred specimens. The others have not yet been tested in the large reptile tree (LRT, 1179 taxa).

The holotype
remains Ascendonanus, but here it’s no longer a varanopid synapsid. Here it nests as a derived prodiapsid and the basalmost tested diapsid (Fig. 2), a little younger than the oldest diapsid, Petrolacosaurus.

Figure 1. The five specimens from the Ascendonanus quarry, all to the same scale. Most images from Spindler et al. 2018. Some have skulls 3x the occiput/acetabulum length. Others as much as 5x, the first hint that these taxa are no conspecific.

Figure 1. The five specimens from the Ascendonanus quarry, all to the same scale, counter plate flipped in every specimen. Most images from Spindler et al. 2018. Some have skulls 3x the occiput/acetabulum length. Others as much as 5x, the first hint that these taxa are no conspecific.

Some of these specimens
(Fig. 1) have an occiput/acetabulum length distinct from the others, ranging from 3x to 5x the skull length, the first clue to their distinct morphologies.

Figure 2. The Prodiapsida now include the holotypes of Ascendonanus and Anningia.

Figure 2. The Prodiapsida now include the holotypes of Ascendonanus and Anningia. Remember, the Diapsida does not include any Lepidosauriforms, which nest elsewhere.

Spindler et al. 2018
did not include several taxa typically included in pelycosaur studies and should not have included any caseasaurs, despite their traditional inclusion. Spindler et al. did not include any diapsids nor did they understand the role of the former varanopids now nesting as ancestors to the Diapsida (sans Lepidosauriformes).

Figure 3. Cladogram from Spindler et al. 2018. Colors refer to clades in the LRT.

Figure 3. Cladogram from Spindler et al. 2018. Colors refer to clades in the LRT.

The holotype 0924 specimen has more of a varanopid skull
than the 1045 specimen we looked at earlier. Prodiapsid sisters include varanopids ancestral to synapsids. Prodiapsids, as their name suggests, are late-surviving ancestors to diapsids like the coeval Araeoscelis (Early Permian) and the earlier Spinoaequalis (Late Carboniferous).

Figure 3. The Ascendonanus holotype skull as originally traced and as traced here.

Figure 3. The Ascendonanus holotype skull as originally traced and as traced here. Whether an upper temporal fenestra was present (as shown in the color tracing), or not (as shown in the drawings, makes no difference as this taxon nests at the transition. 

Not sure yet
where the other three specimens assigned to Ascendonanus nest. Enough muck stirred for the moment.

References
Rößler R, Zierold T, Feng Z, Kretzschmar R, Merbitz M, Annacker V and Schneider JW 2012. A snapshot of an early Permian ecosystem preserved by explosive volcanism:
New results from the Chemnitz Petrified Forest, Germany. PALAIOS, 2012, v. 27, p. 814–834.
Spindler F, Werneburg R, Schneider JW, Luthardt L, Annacker V and Räler R 2018. First arboreal ‘pelycosaurs’ (Synapsida: Varanopidae) from the early Permian Chemnitz Fossil Lagerstätte, SE Germany, with a review of varanopid phylogeny. DOI: https://doi.org/10.1007/s12542-018-0405-9

SVP 18 – the pelycosaur Dimetrodon via Dr. Robert Bakker

Bakker et al (2015)
show evidence that Dimetrodon (Fig. 1) fed on aquatic prey as there were too few terrestrial reptilian herbivores to sustain their numbers.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

From the abstract
“In restorations, Dimetrodon often appear feeding upon large land herbivores, e.g., Diadectes and Edaphosaurus. 􀁄􀁑􀀃􀁄􀁏􀁗􀁈􀁕􀁑􀁄􀁗􀁌􀁙􀁈􀀃􀁙􀁌􀁈􀁚􀀏􀀃􀀲􀁏􀁖􀁒􀁑􀂶􀁖􀀃􀀤􀁔􀁘􀁄􀁗􀁌􀁆􀀃􀀩􀁒􀁒􀁇􀀃 Base Theory (AFBT) recognizes non-terrestrial prey as key for dimetrodont food webs. Over 45% of the bones are severely tooth-marked; ubiquitous shed Dimetrodon teeth are mingled with tooth-marked bones in every depositional unit. The CBB lacks any structures that indicate high current energy, so the hydraulic forces probably did not wash in bones from beyond the trough, though bloated whole carcasses could have floated in. There are 39 Dimetrodon, one each of the large herbivores Edaphosaurus and Diadectes, three of the large non-herbivore, non-apex carnivore Secodontosaurus, and three of the semi-terrestrial amphibian Eryops calculated form postcrania. Did benthic amphibians and fish fill the gap in prey? The benthic amphibian Diplocaulus is abundant in every bone-rich unit. Xenacanth sharks are very common in several layers; each shark carried a large, well ossified head spine. AFBT is corroborated: dimetrodonts fed intensively on aquatic prey at the CBB.”

Combine this with what we know of Spinosaurus, and finback reptiles appear to have been largely aquatic in habitat. That’s heresy joining the mainstream.

There is also a good Dimetrodon video (52 min.)
on YouTube featuring Dr. Bakker as he describes how the vast majority of Dimetrodon tails are missing, neatly cut and probably carried away for their meat (because that’s where the most of it is!) by other Dimetrodons.

References
Bakker RT et al. 2015. Dimetrodon and the earliest apex predators: The Craddock bone bed and George Ranch Facies show that aquatic prey, not herbivores, were key food sources. Journal of Vertebrate Paleontology abstracts.

Ianthodon: a basal edaphosaur without tall neural spines

A new paper
by Spindler, Scott and Reisz (2015) brings us new data on the basal pelycosaur Ianthodon schultzei (Fig. 1; Garnet locality, Missourian Age; 305-306 mya, Middle Pennsylvanian, Late Carboniferous). The authors reported that Ianthodon represented a more basal sphenacodontid than Haptodus. In the large reptile tree Ianthodon was derived from a sister to Haptodus and nested at the base of Edaphosaurus + Ianthasaurus + Glaucosaurus, all edaphosaurids.

Figure 1. Ianthodon schultzei was considered a basal pelycosaur, and it is, but here nests as a basal edaphosaur. And it has no tall neural spines. So pelycosaur sails were convergent, not homologous.

Figure 1. Ianthodon schultzei (image modified from Spindler, Scott and Reisz 2015) was considered a basal pelycosaur, and it is, but here nests as a basal edaphosaur. And it has no tall neural spines. So pelycosaur sails were convergent, not homologous. Spindler, Scott and Reisz considered this specimen a juvenile due to its incomplete ossification.

Notably Ianthodon does not have tall neural spines. Earlier we wondered whether the tall neural spines of Edaphosaurus and Dimetrodon were convergent or homologous. Now it is clear, via Ianthodon, and Sphenacodon (sorry I did not notice this yesterday) that the tall neural spines of Edaphosaurus and Dimetrodon were convergent.

Most well-known pelycosaurs
were Early Permian in age. Ianthodon demonstrates an earlier origin for their carnivore/ herbivore split. And it retains carnivore teeth! Therapsids likewise originated in the Late Carboniferous according to this new data.

Phylogenetic history
Spindler, Scott and Reisz (2015) report, “In the original description and phylogenetic analysis of Kissel and Reisz (2004), Ianthodon was found to nest surprisingly high within Sphenacodontia, as a sister taxon to the clade that included Pantelosaurus, Cutleria and sphenacodontids. In a subsequent, large-scale analysis, Ianthodon was found to be more basal, near the edaphosaurid–sphenacodont node (Benson, 2012), but its exact position remained poorly resolved. In the latter analysis, Benson (2012) extensively revised the character list and included all known “pelycosaur” grade synapsids, while Kissel and Reisz (2004) used data and taxa derived from Laurin (1993), which mainly followed Reisz et al. (1992). Another recent analysis of sphenacodont synapsids by Fröbisch et al. (2011), as part of a description of a new taxon, recovered Ianthodon, Palaeohatteria and Pantelosaurus in an unresolved polygamy.”

The Spindler, Scott and Reisz (2015) analysis
used 122 characters (vs. 228 in the large reptile tree). Their tree shows 12 taxa, 4 of which are suprageneric. In their tree Ianthodon nested between Edaphosauridae and Haptodus. (So close, but no cigar.) Their tree also nested two therapsid taxa (Biarmosuchus and Dinocephalia) with Cutleria, Sphenacodon, Ctenospondylus and Dimetrodon. Thus Spindler, Scott and Reisz appear to be excluding several key taxa and their tree topology differs significantly from the large reptile tree at the base of the Therapsida, with or without Ianthodon.

References
Spindler F, Scott D. and Reisz RR 2015. New information on the cranial and postcranial anatomy of the early synapsid Ianthodon schultzei (Sphenacomorpha: Sphenacodontia), and its evolutionary significance. Fossil Record 18:17–30.

Early Permian Sail-back Synapsids

Everyone knows
about Dimetrodon and Edaphosaurus, the two Early Permian sail back synapsid reptiles (Figs. 1, 2). Ianthasaurus was a more primitive sister to Edaphosaurus. Secodontosaurus was a sister to Dimetrodon. A taxon without a sail, Haptodus, was basal to both clades.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

Dimetrodon 
was a meat-eater. Edaphosaurus was a plant-eater. Every grade-schooler knows this. Skull size and sail design readily distinguish these two iconic taxa. Other traits, from teeth to toes also distinguish them.

Figure 2. Edaphosaurus, a sailback pelycosaur synapsid reptile of the Early Permian.

Figure 2. Edaphosaurus, a sailback pelycosaur synapsid reptile of the Early Permian. Note the tall caudal neural spines, distinct from Dimetrodon (figure 1).

Several specimens
of Dimetrodon are known (Fig. 3). Several attempts at reconstructing the skull of Edaphosaurus have been made (Fig. 2). I have the impression that there is not yet a single complete skull known for this taxon.

Figure 2. Click to enlarge. Sphenacodont skulls to scale. Figure 2. Click to enlarge. Sphenacodont skulls to scale.

Figure 3. Click to enlarge. Sphenacodont skulls to scale. See Figure 2 for Edaphosaurus skulls. Not sure why Sphenacodon is not considered a species of Dimetrodon. The skulls are nearly identical.

The two sails
are either convergent or homologous. At this point, we don’t know. They both have individual designs with Edaphosaurus having curved neural spines with short spars on each “mast”. If they are homologous, Ianthsaurus (Fig. 4) is close to that common ancestor. At present, sail-less Haptodus is the last common ancestor.

Figure 4. Ianthasaurus, a basal edaphosaur.

Figure 4. Ianthasaurus, a basal edaphosaur not far from the common ancestor to all tailback pelycosaurs.

Interestingly,
at the same time that sails were developing in one synapsid clade, another clade, the Therapsida, led by Cutleria and Stenocybus was developing in different ways. At present only skulls are known, but more derived therapsids had longer legs and apparently a more active lifestyle, again dividing at their origin into meat-eaters, like Biarmosuchus, and plant-eaters, like Niaftasuchus and the Dromasauria.

The Early Permian
reminds me of the Early Triassic with regard to the great amount of evolutionary novelty appearing then, likely in response to new environs, weather patterns, predators and experiments in raising the metabolism in several clades. At this time basal diapsids and basal lepidosaurs were diversifying as well.

References
Case ED 1878. Descriptions of extinct Batrachia and Reptilia from the Permian formation of Texas. Proceedings of the American Philosophical Society xvii pp. 505-530.
Cope ED 1882. Third contribution to the history of the Vertebrata of the Permian formation of Texas. Proceedings of the American Philosophical Society (20): 447–461.
Marsh OC 1878. Introduction and succession of vertebrate life in America: Popular Science Monthly, v. 12, p. 513-527, 672-697.
Modesto SP 1994. The Lower Permian Synapsid Glaucosaurus from Texas. Palaeontology 37:51-60
Reisz RR and Berman DS 1986. Ianthasaurus hardestii n. sp., a primitive edaphosaur (Reptilia, Pelycosauria) from the Upper Pennsylvanian Rock Lake Shale near Garnett, Kansas. Canadian Journal of Earth Sciences 23(1): 77–91.
Reisz R R, Berman DS and Scott D 1992. The cranial anatomy and relationships of Secodontosaurus, an unusual mammal-like reptile (Pelycosauria: Sphenacodontidae) from the early Permian of Texas. Zoological Journal of the Linnean Society 104: 127–184.
Romer, AS 1936. Studies on American Permo-Carboniferous tetrapods. Problems of Paleontology, USSR 1: 85–93.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.

wiki/Ianthasaurus
wiki/Edaphosaurus
wiki/Secodontosaurus
wiki/Dimetrodon
wiki/Sphenacodon

Ophiacodon and the origin of mammals: bone studies are supportive

A recent paper
by Shelton and Sander 2015 provides confirmation to the heretical hypothesis that Ophiacodon is a Therapsid/Mammal precursor, discussed here several years ago.

Figure 1. Varanosaurus, Ophiacodon, Cutleria, Biarmosuchus and Nikkasaurus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology.

Figure 1. Varanosaurus, Ophiacodon, Cutleria, Biarmosuchus and Nikkasaurus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology.

From the abstract: “The origin of mammalian endothermy has long been held to reside within the early therapsid groups. However, shared histological characteristics have been observed in the bone matrix and vascularity between Ophiacodontidae and the later therapsids (Synapsida). Historically, this coincidence has been explained as simply a reflection of the presumed aquatic lifestyle of Ophiacodon or even a sign of immaturity. Here we show, by histologically sampling an ontogenetic series of Ophiacodon humeri, as well as additional material, the existence of true fibrolamellar bone in the postcranial bones of a member of ‘Pelycosauria’. Our findings have reaffirmed what previous studies first described as fast growing tissue, and by proxy, have disproven that the highly vascularized cortex is simply a reflection of young age. This tissue demonstrates the classic histological characteristics of true fibrolamellar bone (FLB). The cortex consists of primary osteons in a woven bone matrix and remains highly vascularized throughout ontogeny providing evidence to fast skeletal growth. Overall, the FLB tissue we have described in Ophiacodon is more derived or “mammal-like” in terms of the osteonal development, bone matrix, and skeletal growth then what has been described thus far for any other pelycosaur taxa. Ophiacodon bone histology does not show well-developed Haversian tissue. With regards to the histological record, our results remain inconclusive as to the preferred ecology of Ophiacodon, but support the growing evidence for an aquatic lifestyle. Our findings have set the evolutionary origins of modern mammalian endothermy and high skeletal growth rates back approximately 20 M.Y. to the Early Permian, and by phylogenetic extension perhaps the Late Carboniferous.”

References
Shelton C and Sander PM 2015. Ophiacodon long bone histology: the earliest occurrence of FLB in the mammalian stem lineage. PeerJ PrePrints 3:e1262
doi: https://dx.doi.org/10.7287/peerj.preprints.1027v1 preprints

Kenyasaurus not a tangasaur… not a diapsid… It’s a very basal dromasaur!

Earlier we looked at marine younginiformes. Perhaps conspicuous by its absence was Kenyasaurus, which was originally considered related to tangasaurid younginiformes. Last night I found data, plugged it into the large reptile tree and was surprised at where Kenyasaurus nested.

Kenyasaurus mariakaniensis
(Harris and Carroll 1977; Early Triassic; KNM-MA1, National Museum of Kenya) is represented by a headless skeleton with only a partial forelimb and pectoral girdle (Fig. 1).

Figure 1. Kenyasaurus in situ. Click to enlarge. This rather plain specimen nests not with tangasaurids, but with dromasaurids according to the large reptile tree. Boxed area: the primitive dromasaur, Galechirus and its foot to scale for comparison. Haptodus foot for comparison, not to scale. Pink and green tarsals are absent in Kenyasaurus and dromasaurs.

Figure 1. Kenyasaurus in situ. Click to enlarge. This rather plain specimen nests not with tangasaurids, but with dromasaurids according to the large reptile tree. Boxed area: the primitive dromasaur, Galechirus and its foot to scale for comparison. Haptodus foot for comparison, not to scale. Pink and green tarsals are absent in Kenyasaurus and dromasaurs. Note the similarity of the pes of Kenyasaurus and Haptodus, sharing the same number and proportion of pedal elements, less the two tarsals.

Originally considered
a relative of Tangasaurus and Hovasaurus, the large reptile tree nested Kenyasaurus with the arboreal herbivorous dromasaurid synapsids. If so, the purported ‘well-developed sternum’ (Fig. 1, lavender) must instead be the posterior coracoid because synapsids do not have a sternum*. Harris and Carroll (1977) noted the long tail was unlike those of the Tangasaurus and Hovasaurus and that the tarsus lacked a fifth distal tarsal, as in dromasaurs. The caudal transverse processes gradually diminished over 30 vertebrae creating a cylindrical, muscular tail similar to those found in dromasaurs, only longer.

Figure 2. Two other dromasaurs, Suminia and Galechirus.

Figure 2. Two other dromasaurs, Suminia and Galechirus. Note the similar ilium shapes.

Currie 1982
also examined Kenyasaurus. At that time Currie did not have a computer or software to test traditional nestings. And he had just been studying Tangasaurus and Hovasaurus. So he considered Kenyasaurus a tangasaurid.

Currie (1982) diagnosed Kenyasaurus on the basis of five autapomorphies:

  1. low but anteroposteriorly elongate neural spines in the dorsal region
  2. 56 caudal vertebrae and
  3. 28 pairs of caudal ribs and transverse processes.
  4. Astragalus almost triangular rather than primitive L-shape
  5. Pronounced process on fifth metatarsal for insertion of peroneus brevis

How do these compare to dromasaurs?

  1. Neural spines in known dromasaurs and outgroups are taller than long
  2. About 45 caudal vertebrae are present in Galechirus, but they get very tiny at the tip, 52 are present in Suminia
  3. 22 pairs of caudal ribs and transverse processes are present in Suminia
  4. Astragalus triangular present in Suminia, square present in Galechirus
  5. No pronounced process on fifth metatarsal

So, no wonder Kenyasaurus was not considered a dromasaur.

Bickelmann, Müller and Reisz 2009
did have a computer and software to test traditional nestings. They found support for two distinct families within “Younginiformes”: the aquatic Tangasauridae, and the terrestrial Younginidae. However, they found no support for the inclusion of Kenyasaurus within any of those families. Unfortunately that study also included the unrelated Lanthanolania, Palaegama, Saurosternon and Coelurosauravus (all basal lepidosaurifomes related to Triassic rib gliders) within the same clade that also included Claudiosaurus and the Younginiformes. Very odd.

Shift Kenyasaurus closer to Tangasaurus
and you’ll add 10 steps to the most parsimonious tree. Whether a sternum was present or not makes little difference.

Delete the two dromasaurs
from the large reptile tree and Kenyasaurus creates a large polytomy (loss of resolution) among basal synapsids.

Post-pectoral characters shared by Kenyasaurus and dromasaurs
to the exclusion of basal synapsids include:

  1. Gastralia present and rodlike (otherwise last seen in Ophiacodon)
  2. Ventral pelvis: separate plates, small medial opening
  3. Pubis orientation: medial
  4. Overall size: < 30 cm tall, 60 cm long

So, not a lot to work with.

Other dromasaurids
are known from the Late Permian, so Kenyasaurus would have been a late-survivor in the Early Triassic, despite its more basal nesting. That’s another black mark against Kenyasaurus being a dromasaur. Nevertheless, among the 542 taxa in the inclusion set, Kenyasaurus is most attracted to the dromasaurs with the present data set and scores.

Some final thoughts
Kenyasaurus displays no reduction of the middle phalanges of digits 3 and 4 of the manus and pes, so it resembles more primitive pelycosaur-grade synapsids in this regard. Based on this fact, the reduction of the three middle pedal phalanges may have occurred by convergence  within Therapsida, once in anomodonts and again in the main line beginning with biarmosuchids.

Basal anomodonts likely split from basal therapsids, like Stenocybus and Cutleria in the Early Permian. So Kenyasaurus was a very late (Early Triassic) remnant of that earlier radiation. So the autapomorphies that Currie listed (above) could have evolved during those tens of millions of years. And yes, I am making excuses for this taxon because it does not exactly match the ideal we might imagine. But those excuses could be true.

References
Bickelmann C, Müller J and Reisz RR 2009. The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of “younginiform” reptiles. Canadian Journal of Earth Sciences 46:651-661.
Currie P 1982. 
The osteology and relationships of Tangasaurus mennelli Haughton. Annals of The South African Museum 86:247-265. http://biostor.org/reference/111508
Harris JM and Carroll RL 1977. Kenyasaurus, a New Eosuchian Reptile from the Early Triassic of Kenya. Journal of Paleontology 51:139–149.

* We’ll look at the mammals sternum/manubrium issue later….