Nesbitt and his Characters – part 7 – Lagerpetidae

Earlier in the last six blogs we looked at characters Nesbitt (2011) used to define Archosauriformes and nested clades up to Ornithodira with a special emphasis [his not mine] on the nesting of pterosaurs within all these clades. Nesbitt (2011) was satisfied that pterosaurs nested well here, but did not test competing candidates among the Fenestrasauria and Tritosauria, the only clades that actually provide a gradual accumulation of pterosaurian traits.

In Today’s Installment
Nesbitt (2009, 2011) nested Lagerpeton and Dromomeron at the base of the Dinorsauromorpha (Silesauridae + Dinosauria), robustly supported by five unambiguous synapomorphies, and these from a clade known only by the pelvis, anterior caudals and hind limb.

Closer to Pterosaurs?
Nesbitt (2011) found Lagerpeton closer to pterosaurs than to dinosaurs, chiefly due to ankle traits. Nesbitt (2011) lists these as 1) calcaneum and astragalus coosified; 2) ventral surface of calcaneum rounded like the astragalus; 3) no posterior groove of the astragalus; 4) calcaneum lacks any sort of tuber. All these traits are absent in basal dinosauriforms, including Marasuchus and Asilisaurus.

Unfortunately, Nesbitt (2011) considered the proximal tarsals (astragalus + calcaneum) fused to the tibia/fibula as homologous to the astragalus + calcaneum fused to each other distinct from the tibia/fibula. They are not. In fact, in Dimorphodon, they are separated from one another as two distinct condyles. A similar morphology is present in Sharovipteryx, which shares the three remaining traits (see above).

By the Way…
The large reptile tree found Lagerpeton to nest with Tropidosuchus, far from the Dinosauria/Dinosauromorpha. Nesbitt did not mention the ascending process of the astragalus, which is present on Lagerpeton and dinosaurs. However, in Lagerpeton this appears on the posterior surface, while in dinos the ascending process is an anterior structure. Also, Huehuecuetzpalli has a pterosaurian mesotarsal ankle composed of two larger proximal elements unfused to one another, which is distinct from traditional lizards.

Traits Shared with Dinosaurs…
Nesbitt (2011) reported the following traits lagerpetids share with other basal dinosauromorphs, and these are apparently absent in pterosaurs: 1) posterolateral portion of the femoral head ventrally descended; 2) a straight cnemial crest; 3) the longest metatarsal longer than 50% of tibial length; 4) metatarsal V ‘‘hooked’’ proximal end absent; 5) articular face for distal tarsal 4 subparallel to shaft axis; and 6) metatarsal V without phalanges and tapers to a point.

Well, 1) can’t comment on this because I’m not sure what this is; 2) shared with Tropidosuchus and Chanaresuchus; 3) ditto; 4) ditto; ditto; ditto.

I hate to keep pointing out the same Achilles heel in all these recent studies, but workers should not be afraid to add a few fenestrasaurs to their studies to get a fix on this situation instead of digging themselves deeper and deeper into the SOS.

Tomorrow: Silesaurids.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Nesbitt and his Characters – part 8 – Silesauridae

Earlier in the last seven blogs we looked at characters Nesbitt (2011) used to define Archosauriformes and nested clades up to Ornithodira with a special emphasis [his not mine] on the nesting of pterosaurs within all these clades. Nesbitt (2011) was satisfied that pterosaurs nested well here, but did not test competing candidates among the Fenestrasauria and Tritosauria, the only clades that actually provide a gradual accumulation of pterosaurian traits.

In Today’s Installment
Nesbitt (2009, 2011) nested Silesaurus, SacisaurusEucoelophysis, Asilisaurus, Pseudolagosuchus and Lewisuchus within the Silesauridae, the sister clade to the Dinosauria and together these form the Dinosauriformes. The large reptile tree did not test all these taxa, but Lewisuchus nested with the basal bipedal croc, Pseudhesperosuchus). At this point the Nesbitt (2011) study includes more pertinent taxa than the large reptile tree, which was focused on broader patterns throughout the Reptilia. I apologize ahead of time for the several characters I cannot comment on around the braincase and pelvis.

13 Synapomorphies
Here the Nesbitt study and the large reptile tree are coming to closer accord because pterosaurs are finally out of the inclusion list. However, the large reptile tree found several of the above taxa to nest within the Dinosauria (along with the poposaurs and Lotosaurus), not just outside it. According to Nesbitt (2011)m “This clade (Silesauridae + Dinosauria) is supported by the following 13 unambiguous synapomorphies:

1) Anterior tympanic recess on the lateral side of the braincase present (101-1); Can’t comment on this not readily visible character.

2) Auricular recess extends onto internal surface of epiotic/ supraoccipital (133-1); Can’t comment on this not readily visible character.

3) Atlantal articulation facet in axial intercentrum, shape concave with upturned lateral borders (178-1); Can’t comment on this not readily visible character.
4) Crest dorsal to the supraacetabular crest/rim confluent with anterior extent of the anterior (= preacetabular) process of the ilium (265-2); Can’t comment on this not readily visible character.
5) pubis more than 70% or more of femoral length (278-1); Trait shared with putative outgroup, Poposauridae.
6) extensive medial contact between the ischia, but the dorsal margins are separated (291-1); Trait shared with putative outgroup, Poposauridae.

7) sharp ridge (= dorsolateral trochanter of some) on the dorsolateral margin of the proximal portion of the femur (307-1); Can’t comment on this not readily visible character, but Nesbitt (2007) notes a sharp proximal ridge (not sure if this represents the same ridge), on Effigia and Shuvosaurus, two putatitve outgroup taxa.
8) Straight transverse groove on the proximal surface of the femur (314-1); Can’t comment on this not readily visible character.

Ankle of Effigia.

Figure 1. Ankle of Effigia. Arrow points to tibial distal flange oriented toward fibula.

9) Posterolateral flange of the distal portion of the tibia nearly contacts or contacts fibula (334-1); Perhaps shared with Effigia, but apparently present on Lotosaurus among putatitive outgroups.
10) Anterior edge of the proximal portion of the fibula tapers to a point and arched anteromedially (342-1); Apparently present in Effigia among putative outgroups.
11) Midshaft diameters of metatarsals I and V less than II–IV (384-1); Present in Poposaurus and Effigia among putative outgroups. 
12) Distal articulation surface of metatarsal IV deeper than broad (391-1); Can’t really comment on this not very visible character.
13) and metatarsal IV length subequal to or shorter than metatarsal II (395-1). Present in Lotosaurus, Shuvosaurus, Poposaurus and Effigia among putative outgroups. 
These 13 character states represent a significant increase in the knowledge of character-state transformations immediately outside Dinosauria.”
Many of these traits are found in poposaurids, considered closer to rauisuchians than dinosaurs in the Nesbitt (2011) study. Poposaurs are considered to be dinosaurs in which the calcaneal tuber redeveloped in the large reptile study. This was shown to be convergent with the redevelopment in crocs.
Within the Silesauridae, the base of the clade (a composite taxon consisting of the hips and hind limbs of Pseudolagosuchus together with the front of Lewisuchus) is “well resolved and is supported by four unambiguous character states including: foramina of the hypoglossal nerve (XII) nearly aligned in a near anteroposteriorly plane (113-1); rugose ridge on the anterolateral edges of the supraoccipital (127-1); cervical centra 3–5 longer than middorsal (181-1); notch ventral to the proximal head of the femur (304-1).” I can’t comment on such minutia. However, I earlier made the case that Lewisuchus is closer to the basal pre-croc, Pseudhesperosuchus and Pseudolagosuchus nested with Silesaurus.
Within the Silesauridae, taxa closer to Silesaurus share the following “seven” (I only counted six) unambiguous synapomorphies: 1) anterior extent of the dentary tapers to a sharp point (155-1); 2) dentary teeth absent in the anterior portion (166-1); 3) maxillary and dentary crowns apicobasally short and subtriangular (173-1); 4) sacral ribs shared between two sacral vertebrae (208-1); 5) straight medial articular facet of the proximal portion of the femur (309-1); 6) distal condyles of the femur divided posteriorly between a quarter and a third the length of the shaft (324-1).
That sharp and toothless anterior dentary may be the predentary or something like it. I wonder if the sacral rib and distal femur characters are shared with Lotosaurus?

Tomorrow: Dinosauria

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Nesbitt (2011) and His Characters – Part 6 Ornithodira

Following remarks from fellow paleontologists asking for my study to include more Nesbitt (2011) characters in the large reptile study, I thought we should dive right into them, taking a few days to digest them all — a bite at a time. Earlier we considered more basal clades in parts 1, 2, 3, 4 and 5.

Nesbitt Characters for Ornithodira
Sterling Nesbitt (SN) reported, (1) Distal end of neural spines of the cervical vertebrae unexpanded (191-0). The neural spines of the cervical vertebrae are unexpanded in pterosaurs.
Note: This trait is also found in lepidosaurs, including fenestrasaurs.

(2) Distal expansion of neural spines of the dorsal vertebrae absent (197-0). The expansion of neural spines of the dorsal vertebrae is absent in pterosaurs.
Note: This trait is also found in lepidosaurs, including fenestrasaurs.

(3) Second phalanx of manual digit II (= 2.2) longer than first phalanx (255-1). This character is present in basal pterosaurs and in dinosaurs.
Note: While present in basal pterosaurs, this trait is not present in the dinosaurs, Herrerasaurus, Thecodontosaurus or Scelidosaurus.

(4) Trenchant unguals on manual digits I–III (257-1). Present in basal pterosaurs.
Note: Also present on the fenestrasaur, Longisquama. Not present in quadrupedal dinosaurs.

(5) Tibia longer than the femur (299-1). Present in basal pterosaurs.
Note: Also present in the fenestrasaurs, Sharovipteryx and Longisquama. Not present in quadrupedal dinosaurs. 

(6) Distal tarsal 4 transverse width subequal to that of distal tarsal 3 (347-1). Present in basal pterosaurs.
Note: In pterosaurs distal tarsal 3 is very tiny, never as large as distal tarsal 4. However the centrale, which is typically confused with distal tarsal 3 in character matrices, is usually just as large. 

(7) Size of articular facet for metatarsal V less than half the width of lateral surface of distal tarsal 4 (348-1). Present in basal pterosaurs.
Note: Also present on basal fenestrasaurs. However, the articular facet for mt5 is usually the majority of the lateral surface of distal tarsal 4 in pterosaurs. 

(8) Anterior hollow of the astragalus reduced to a foramen or absent (357-1). Present in basal pterosaurs.
Note: This trait is also present in all tritosaurs, including fenestrasaurs. 

(9) Anteromedial corner of the astragalus acute (361-1). Clearly present in Dimorphodon (fig. 46).
Note: Nesbitt confused the distal tarsals with the proximal tarsals, which are fused to the tibia. 

(10) Compact metatarsus, metatarsals II–IV tightly bunched (at least half of the length) (382-1). Present in pterosaurs.
Note: Also present in most tritosaurs, including the fenestrasaurs (sans Sharovipteryx). 

(11) Osteoderms absent (401-0). Pterosaurs lack osteoderms.
Note: Also absent in tritosaurs, including fenestrasaurs. Osteoderms are present in Scelidosaurus.

(12) Gastralia well separated (412-1). The gastralia of the holotype of Eudimorphodon are well separated as they are in dinosaurs. In contrast, the gastralia of most non-ornithodiran archosauriforms form an extensive, interlocking basket.
Note: The gastralia are also well separated in lepidosaurs, including fenestrasaurs.

Note: The large reptile tree does not recover a monophyletic Ornithodira, but finds pterosaurs and traditional archosaurs separated, evolving several traits by convergence.

Nesbitt (2011) reported, “The character states supporting pterosaurs as members of Archosauria and Ornithodira are not restricted to character states related to locomotion as suggested by Bennett (1996). As demonstrated in the list above, the character states cover features present all over the body, not just in the hind limb.” Unfortunately, Nesbitt (2011), like so many paleontologists before and since, did not even look at the competing candidates within the fenestrasauria, but force fit those square pegs into those round holes. Not sure why everyone is so afraid to let a large tree recover more parsimonious nestings.

Perhaps now you see why there is something terribly wrong with our current pterosaur studies. It’s an Alice-in-Wonderland world out there.

Tomorrow: Lagerpetidae

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Nesbitt (2011) and His Characters – Part 5 – Archosauria

Following remarks from fellow paleontologists asking for my study to include more Nesbitt (2011) characters in the large reptile study, I thought we should dive right into them, taking a few days to digest them all — a bite at a time. Earlier we considered more basal clades in parts 1, 2, 3 and 4.

Nesbitt Characters for Archosauria
Sterling Nesbitt (SN) reported, (1) Palatal processes of the maxilla meet at the midline (32-1). Not known in basal pterosaurs.
The palate is known in basal pterosaurs and the maxillae do not meet at the palatal midline in dimorphodontids, but do meet at the midline in eudimorphodontids.

(2) Lagenar/cochlear recess present and elongated and tubular (118-1). Not known in basal pterosaurs.
Such a process is absent in lepidosaurs including fenestrasaurs and pterosaurs. 

(3) External foramen for abducens nerve within prootic only (122-1). Not known in basal pterosaurs.
Such a foramen is absent in lepidosaurs including fenestrasaurs and pterosaurs. 

(4) Antorbital fossa present on the lacrimal, dorsal process of the maxilla, and the dorsolateral margin of the posterior process of the maxilla (the ventral border of the antorbital fenestra) (137-2). This character is difficult to score for any pterosaur as also observed by Bennett (1996). However, a slight fossa in Dimorphodon (BMNH 41212) suggests that an antorbital fossa surrounded much of the border of the antorbital fenestra.
Note: There is no antorbital fossa on any pterosaurs antorbital fenestra. Dimorphodon was unusual, even among pterosaurs for its extremely large fenestra and extremely narrow skull bones. What Nesbitt (2011) considered an antorbital fossa is a paper-thin structural flange supporting a very weak bone intersection. 

(5) Posteroventral portion of the coracoid possesses a ‘‘swollen’’ tuber (225-1). Difficult to score in the highly modified coracoids of pterosaurs.
This is bogus. The coracoid of pterosaurs is no more “highly modified” than a bird coracoid and is shared with basal fenestrasaurs. The coracoid of fenestrasaurs may be expanded ventrally or not. In basal forms it is not.

(6) Lateral tuber (5 radial tuber) on the proximal portion of the ulna present (237-1). Not known in basal pterosaurs.
Note: This tuber is absent on all lepidsaurs, including pterosaurs.

(7) Ratio of longest metacarpal to longest metatarsal, less than 0.5 (245-1). The apomorphically elongated metacarpal IV in pterosaurs nullifies the scoring of this character.
This is bogus. Metacarpal IV is not elongated in basal pterosaurs (relative to the ulna), it is simply more robust and axially rotated. Taxa from Huehuecuetzpalli to Sharovipteryx match this trait. Longisquama and the basal pterosaur MPUM6009 do not. 

(8) Anteromedial tuber of the proximal portion of the femur present (300-1). Clearly present in Dimorphodon (fig. 39). 
Note: This trait has not been identified on other basal fenestrasaurs and pterosaurs, most of which have a crushed femur. A similar structure can be seen in the basal tritosaur, Huehuecuetzpalli. 

(9) Tibial facet of the astragalus divided into posteromedial and anterolateral basins (366-1). Not known in basal pterosaurs.
This is bogus. The astragalus tibial facet is not divided in pterosaurs. 

(10) Calcaneal tuber orientation, relative to the transverse plane, between 50 degrees and 90 degrees posteriorly (377-2). Pterosaurs lack a tuber; therefore, this character could not be scored.
Note: Not only do pterosaurs lack such a tuber, so do all tritosaurs, including fenestrasaurs. 

Note: The large reptile tree does not recover a monophyletic Archosauria, but finds pterosaurs and traditional archosaurs separated, evolving several traits by convergence.

Tomorrow: Ornithodira

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Nesbitt (2011) and His Characters – Part 4 – Crurotarsi

Following remarks from fellow paleontologists asking for my study to include more Nesbitt (2011) characters in the large reptile study, I thought we should dive right into them, taking a few days to digest them all — a bite at a time. Earlier we considered more basal clades in parts 1, 2 and 3. Today we take on the Crurotarsi.

Nesbitt Characters for Crurotarsi (Phytosauria + Crocodylomorpha)
Sterling Nesbitt (SN) reported, (1) Parabasisphenoid plate absent (96-2). Not known in basal pterosaurs.
Note: Also absent in lepidosaurs and fenestrasaurs.

(2) Semilunar depression on the lateral surface of the basal tubera of the parabasisphenoidabsent (98-1). Not known in basal pterosaurs.
Note: Also absent in lepidosaurs and fenestrasaurs.

(3) Absence of teeth on palatal process of the pterygoid (175-1). Eudimorphodon is the only pterosaur reported with pterygoid teeth (Wild, 1978). The pterygoid teeth are present on the palatal process of the pterygoid.
Note: I have not been able to confirm Wild’s (1978) observation. This absence is also found in tritosaurs and fenestrasaurs. 

(4) Cervical ribs short and stout (196-1). The cervical ribs are short in basal pterosaurs.
Note: This trait is also present in the basal fenestrasaur, Longisquama. By short and stout apparently Nesbitt means these ribs extend the length of two cervicals. If so, that also includes all fenestrasaurs. 

(5) Ventral articular surface of the astragalus calcaneum concavoconvex, with concavity on calcaneum (368-1). Not known in basal pterosaurs.
Note: Actually the ventral surface of the proximal tarsals are straight to convex in tritosaurs, including fenestrasaurs and pterosaurs. 

(6) Ventral articular surface for distal tarsal 4 and the distal end of the tuber of the calcaneum separated by a clear gap (371-1). Not known in basal pterosaurs.
Note: There is no calcaneal tuber on tritosaurs including fenestrasaurs and pterosaurs.

(7) Articular surfaces for fibula and distal tarsal IV on the calcaneum continuous (380-1). The articular surfaces for fibula and distal tarsal IV on the calcaneum are continuous in Dimorphodon (Padian, 1983; Sereno, 1991a).
Note: This trait is shared by tritosaurs including fenestrasaurs and pterosaurs.

Note: Nesbitt (2011) reported: ORIGINAL DEFINITION: Ornithosuchidae, Parasuchia, Aetosauria, Rauisuchia, Crocodylomorpha, and all extinct descendants that are most closely related to these taxa (Sereno and Arcucci, 1990).

REVISED DEFINITION: Node: The least inclusive clade containing Rutiodon carolinensis Emmons, 1856, and Crocodylus niloticus Laurenti, 1768 (new).

The large reptile tree does not recover a monophyletic Crurotarsi, but finds phtyosaurs and proterochampsids separate from the other traditional archosauriforms, evolving several traits by convergence.

Tomorrow: Archosauria

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Nesbitt (2011) and his Characters – part 3 – Vancleavea + Archosauria

Following remarks from fellow paleontologists asking for my study to include more Nesbitt (2011) characters in the large reptile study, I thought we should dive right into them, taking a few days to digest them all — a bite at a time. Earlier we considered more basal clades in part 1 and part 2.

Nesbitt Characters for Vancleavea + Archosauria
Sterling Nesbitt (SN) reported, (1) Postparietal(s) absent (146-1). Postparietals are absent in pterosaurs (Bennett, 1996).
Note: As in certain lepidosaurs, including all fenestrasaurs.

(2) Postaxial intercentra absent (177-1). Postaxial intercentra are absent in pterosaurs (Bennett, 1996).
Note: As in certain lepidosaurs, including all fenestrasaurs.

(3) Ectepicondylar flange of the humerus absent (234-1). An ectepicondylar flange is absent in pterosaurs (Bennett, 1996).
Note: As in certain lepidosaurs, including all fenestrasaurs.

(4) Distal condyles of the femur not projecting markedly beyond shaft (318-1). Distal condyles of the femur not projecting markedly beyond shaft in basal pterosaurs.
Note: As in certain lepidosaurs, including all fenestrasaurs.

Long time readers will remember that Vancleavea nests as a thalattosaur close to Helveticosaurus, not an archosauriform, as recovered by the large reptile tree.

Tomorrow: Crurotarsi (Phytosauria + Crocodylomorpha)

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Nesbitt (2011) and his Characters – part 2

Following remarks from fellow paleontologists asking for my study to include more Nesbitt (2011) characters in the large reptile study, I thought we should dive right into them, taking a few days to digest them all — a bite at a time. Yesterday was bite one.

Nesbitt Characters for Erythrosuchus + Archosauria
Sterling Nesbitt (SN) reported, (1) Absence of a large anteriorly opening foramen on the anterolateral surface of the maxilla (31-0). Also absent in basal pterosaurs.
Note: Absent also in basal fenestrasaurs.

(2) Basipterygoid processes directed anteriorly or ventrally at their distal tips (93-1). Not known in basal pterosaurs.
Note: The same holds true for lepidosaurs, fenestrasaurs and all pterosaurs.

(3) Absence of a ridge on lateral surface of inferior anterior process of the prootic ventral to the trigeminal foramen (94-1). Not known in basal pterosaurs. 
Crushing makes this ephemeral detail difficult to observe.

(4) Verticalized parabasisphenoid (97-1). Not known in basal pterosaurs.
Note: This conjoined bone is never vertical in lepidosaurs, fenestrasaurs or pterosaurs.

(5) Absence of supratemporals (145-1). Supratemporals are absent in pterosaurs (Bennett, 1996).
Note: Also absent in fenestrasaurs.

(6) Posteroventral portion of the dentary laterally overlaps the anteroventral portion of the angular (164-1). Present in Dimorphodon and a specimen referred to Eudimorphodon (BPS 1994 I 51).
Note: The purported angular in Dimorphodon is actually a displaced pterygoid. Even so, the lepidosaur Tanystropheus unambiguously shares this trait. 

(7) Thecodont tooth implantation (174-1). Present in basal pterosaurs.
Note: also present in tritosaurs including fenestrasaurs.

(8) Second primordial sacral rib is not bifurcated (203-1). Not known in basal pterosaurs, but the second primordial sacral rib is not bifurcated in Campylognathoides (BSP 1985 I 87).
Note: also present in lepidosaurs including fenestrasaurs.

(9) Entire anterior margin of the scapula is concave (217-1). Difficult to score with confidence in the highly modified scapulae of pterosaurs.
This is a bogus excuse. The entire anterior margin of the scapula is convex in fenestrasaurs including basal pterosaurs.

(10) Acromion process of the scapula distinctly raised above the ventral edge of the scapula (220-1). Difficult to score with confidence in the highly modified scapulae of pterosaurs.
This is a bogus excuse. There is no acromion process on the strap-like scapula of fenestrasaurs and pterosaurs.

(11) Distinct notch between the scapula and coracoid on the anterior margin (221-0).
Note: Also present in lepidosaurs, fenestrasaurs and pterosaurs.

Tomorrow: Vancleavea + Archosauria

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Nesbitt (2011) and his Characters – part 1

Following remarks from fellow paleontologists asking for my study to include more Nesbitt (2011) characters in the large reptile study, I thought we should dive right into them, taking a few days to digest them all — a bite at a time — then use the ones that make sense to use (will be readily visible across a long list of taxa).

Nesbitt Characters for Archosauriforms
Sterling Nesbitt (SN) reported, “My analysis finds pterosaurs well nested within archosauriforms as the sister taxon to Dinosauromorpha; this is a result obtained by all recent numerical analyses.”
Note: none of these included lepidosaurs including fenestrasaurs.

SN: “Therefore, I present a list of characters that unambiguously support archosauriform clades and indicate whether the character is present in (basal) pterosaurs:”

1) Absence of a parietal foramen (63-1). Basal pterosaurs lack a parietal foramen.
Note: As in certain lepidosaurs, including all fenestrasaurs.

(2) Jugal-quadratojugal contact present (70-1). Present in basal pterosaurs.
Note: As in certain lepidosaurs, including all fenestrasaurs.

(3) Ectopterygoid forms all of the lateral edge of the lateral pterygoid flange (88-1). Not known in basal pterosaurs.
Note: Actually this trait is known. This condition does not occur in any pterosaur. The ectopterygoid is fused to the palatine to form the ectopalatine and the pterygoid does not have a lateral flange.

(4) Ossified laterosphenoid present (92-1). Present in pterosaurs (Bennett, 1996).
Note: Bennett (1991, 2001) identified an indistinct portion of the entirely fused braincase of Pteranodon as the laterosphenoid. Cosesaurus has that same structure. Due to lack of sutures, this trait in pterosaurs has not been definitively determined. And if so, then it has to be applied also to Cosesaurus. 

(5) Antorbital fenestra present (136-1). Present in basal pterosaurs.
Note: As in all fenestrasaurs.

(6) Lateral mandibular fenestra present (138-1). Present in Dimorphodon and a specimen refered to Eudimorphodon (BPS 1994 I 51).
Note: These are bogus identifications both due to a slipped surangular. No other pterosaurs have this trait.

(7) Presence of tooth serrations (168-1). Present in Dimorphodon.
Note: This was claimed for only the third premaxillary tooth in one specimen, but was not visible in the evidential photograph. No other pterosaur but the two specimens of Austriadatylus have serrations, but several have multiple cusps, like those found in higher fenestrasaurs. Here is why we should score characters at the genus level. Choosing Austriadactylus as our “pterosaur” would yield one score. Any other pterosaur would yield the other. 

So far, not a strong case, and a case that makes sense to Nesbitt (2011) only by keeping a blind eye to competing candidates among the fenestrasaurs.

Tomorrow: Erythrosuchus + Archosauria

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

News at the Base of the Therapsida

They’re largely ignored, typically crushed and often incomplete. They’re taxa at the base of the Therapsida and they’re worth another look.

Earlier we found anomodonts (here restricted to dicynodonts and dromasaurs) to be a basal offshoot clade of the Therapsida, derived from a sister to tiny Nikkasaurus, one of the rare therapsids without canine teeth.

The traditional reconstruction of Stenocybus.

Figure 1. The traditional reconstruction of Stenocybus.

The position of Stenocybus (Cheng  and Li 1997, Fig. 1) seemed odd. There aren’t many therapsids with such a round (in lateral view) skull. A convergence with Haptodus was noted earlier. Traditional reconstructions of Stenocybus (Fig. 1) separated the lacrimal from the naris, but a new look (Fig. 2) appears to demonstrate the opposite. Stenocybus is a 3D fossil that I have not personally examined, so the final word on this specimen is still in limbo.

 Stenocybus

Figure 2. Stenocybus and a tracing of the same. Previously identified as a juvenile Sinophoneus (Kammerer 2011), this taxon does not nest with Sinophoneus. The elongated anterior dentary teeth are distinguishing traits that ally this specimen to Anomocephalus, a similar basal therapsid.  This specimen does not appear to have the typical exposed quadrate/quadratojugal of other therapsids.

Anomolocephalus.

Figure 3. Anomocephalus. Are those flat teeth, or broken. If flat they are autapomorphic, unlike sister taxa. If elongated, they are synapomorphies, like sister taxa.

 

Figure 2. Anomocephalus skull with teeth replaced, no longer loose. It is apparent that the teeth ground against one another.

Figure 2. Anomocephalus skull with teeth replaced, no longer loose. It is apparent that the teeth ground against one another.

Anomocephalus
This anomodont (Figs. 3-4, Modesto et al. 2009) has been traditionally reconstructed with flat premaxillary teeth. And that may be true. However, I wonder if the those flat premaxillary teeth are broken? There are a few loose “tips” in this in situ tracing (Fig. 3) and several of the unbroken teeth appear to be elongated and fang-like. What if all the teeth were elongated and fang-like (Fig. 4)? This sort of reconstruction is quite similar to what we see in Stenocybus, with its elongated anterior dentary teeth and short round rostrum.

Tiarajudens
Traditionally nested with dicynodonts, Tiarajudens (Cisneros 2011) had a fang/tusk that would put any dicynodont to shame. The large crack in the skull (Fig. 5) has been traditionally considered like the Atlantic Ocean. Putting the edges back together creates a very short-faced dicynodont-like morphology. However, Tiarajudens nests with Anomocephalus, a taxon without tusks and with a longer rostrum.  Filling in the missing maxilla creates a more parsimonious reconstruction than subtracting the space.

Tiarajudens

Figure 5. Tiarajudens in situ with bones colorized. The top of the maxilla is colored green, like the rest of the maxilla, but this could be more lacrimal material. Posterior to the tusk we find pterygoid material with substantial flat teeth. As in Anomocephalus, the transverse processes of the pterygoid are rotated to the parasagittal plane. This is odd and re-identification of this material as posterior maxillary teeth would change things, but at present, the tusk is the posterior-most maxillary tooth, not the canine. Note the premaxillary teeth are not flat.

Tiarajudens reconstructed

Figure 6. Tiarajudens reconstructed with a hypothetical mandible. Here the tusk represents a posterior maxillary tooth, not the canine. No sister taxa have a large canine.

A tusk, not a canine
Unlike other therapsids, the tusk of Tiarajudens does not appear to be a canine. No sister taxa have an elongated canine. The tusk is unique to Tiarajudens. In other therapsids the canine is below the apex of the maxilla are far anterior to it. Only in Tiarajudens is the apex of the maxilla anterior to the largest tooth.

The new family tree
With the separation of the dromaeosaurs and dicynodonts at the base of the Therapsida, the traditional “basal” taxon, Biarmosuchus, moves further up the tree (Fig. 7). It should be noted that most of these nestings are supported weakly, based on skull-only traits. However, it’s interesting to note that Suminia retained a long tail and disc-like phalanges at p3.2, p.4.2 and p4.3. Suminia had been allied to dicynodonts, which have a short tail and those pedal phalanges are absent. The new tree clarifies relationships. Primitive Suminia retained the primitive long tail and primitive phalangeal formula.

A new tree for the Therapsida

Figure. 7. A new tree for the Therapsida based on the new reconstructions of taxa at its base. Here all sister taxa share more traits. Stenocybus appears to be the basal therapsid and, as predicted earlier, it shares traits with Ophiacodon and Haptodus, two pelycosaurs at the base of the Therapsida.

A tree of skulls
The new tree (Fig. 7) recovers a more gradual evolution of therapsids as shown by the tree of skulls shown here (Fig. 8). The flaring of the lateral temporal arch is convergent in at least two clades.

A new tree of basal therapsids illustrated by skulls.

Figure 8. Click to enlarge. A new tree of basal therapsids illustrated by skulls. More characters would refine relationships but only more taxa can provide more nesting opportunities. Shortening of the rostrum here does not indicate a juvenile trait. Neither does enlargement of the orbit.

This tree may change again as more data comes in and I learn more about therapsids. It’s an evolving project. For now it’s satisfying to see Stenocybus take a more parsimonious place, closer to more similar taxa, some with procumbent teeth, some with a large orbit, some with a short rostrum.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Cheng Z and Li J 1997. A new genus of primitive dinocephalian – the third report on Late Permian Dashankou lower tetrapod fauna. Vertebrata PalAsiatica 35 (1): 35-43. [in Chinese with English summary]

Cisneros JC, Abdala F, Rubidge, BS, Dentzien-Dias D, Bueno AO 2011. Occlusion in a 260-Million-Year-Old Therapsid with Saber Canines from the Permian of Brazil. Science 331: 1603–1605. DOI:10.1126/science.1200305PMID 21436452.

Modesto S, Rubidge B, and Welman J 1999. The most basal anomodont therapsid and the primacy of Gondwana in the evolution of the anomodonts. Proceedings of the Royal Society of London B 266: 331–337. PMC 1689688.

Are these the new faces of Tiarajudens and Anomocephalus?

Figure 1. Tiarajudens newly reconstructed.

Figure 1. Tiarajudens newly reconstructed. Here it appears that the tusk is not a canine, but the last maxillary tooth, posterior to the apex of the maxilla, distinct from all other therapsids and distinct from dicynodonts, with whom Tiarajudens has been nested in prior studies.

Anomolocephalus.

Figure 2. Anomolocephalus. Are those flat teeth, or broken. If flat they are autapomorphic, unlike sister taxa. If elongated, they are synapomorphies, like sister taxa.

Figure 2. Anomocephalus skull with teeth replaced, no longer loose. It is apparent that the teeth ground against one another.

Figure 2. Anomocephalus skull with teeth replaced, no longer loose. It is apparent that the teeth ground against one another.

Sorry for the lack of posts in the last two days.
This is what I’m working on. The therapsid tree is going to change a little. More tomorrow.

 

Change made Nov. 16, 2013

The skull of Anomocephalus is more accurately reconstructed below (Fig. 3) based on Modesto and Rubidge 2000.

References
Modesto S and Rubidge B 2000. A basal anomodont therapsid from the lower Beaufort Group, Upper Permian of South Africa. Journal of Vertebrate Paleontology 20(3):515-521.