Bobosaurus enters the LRT

Figure 1. Bobosaurus in situ with colors added. See figure 2 for a reconstruction. Colors help segregate the elements.

Bobosaurus forojuliensis (Dalla Vecchia 2006; Fabbri, Dalla Vecchia and Cau 2014; Dalla Vecchia 2016/2017; Late Triassic, Early Carnian; MFSN 27285; Figs. 1, 2) is a large eusauropterygian originally considered close to Pistosaurus and among pistosaurians, closer to plesiosaurians. It was originally assumed to have large flippers despite lacking large flipper elements.

Here
in the large reptile tree (LRT, 1430 taxa), high–spined Bobosaurus nests as a 3x larger sister to Corosaurus with small hands and feet, not flippers. The pectoral elements were overlooked or considered ribs. Corosaurus was among the taxa tested in Fabri, Dalla Vecchia and Cau 2014. Not sure yet how the topologies differed, but they nested turtles between Claudiosaurus and Lepidosauriformes (like Icarosaurus) + Ichthyopterygia, a hypothesis of relationships not confirmed by the LRT.

Figure 2. Bobosaurus reconstructed to scale alongside the 3x smaller Corosaurus. Both share tall spines, small hands, a tiny ilium and other traits not found in sister taxa or pistosaurids. Not all ribs are shown.

In a similar story,
earlier we looked at another large eusauropterygian, Sachicasaurus, that was originally considered a small-handed pliosaur, but nested in the LRT with the more primitive Nothosaurus. 

Figure 4. Subset of the LRT focusing on Eusauropterygians (pachypleurosaurs, nothosaurs, plesiosaurs and kin).

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References
Dalla Vecchia FM 2006. A new sauropterygian reptile with plesiosaurian affinity from the Late Triassic of Italy. Rivista Italiano Paleontaleontologia, Stratigraphia 112 (2): 207-25.
Dalla Vecchia FM 2016. Comments on the skeletal anatomy of the Triassic reptile Bobosaurus forojuliensis (Sauropterygia, Pistosauroidea). Gortania Geologia, Paleontolgia, Paletnologia 38:39–75.
Fabbri M, Dalla Vecchia FM and Cau A 2014. New information on Bobosaurus forojuliensis (Reptilia: Sauropterygia): implications for plesiosaurian evolution. Historical Biology 26 (5): 661-9.

wiki/Bobosaurus

Want to read a GREAT story about paleontology?

Click this link:
The Day the Dinosaurs Died | The New Yorker

This article appears in the print edition of the April 8, 2019, issue,
with the headline “The Day the Earth Died.”

And some blowback:
https://thewire.in/the-sciences/new-yorkers-dino-fossils-profile-features-pitfalls-of-science-as-performance

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References

https://www.newyorker.com

What is Galulatherium?

O’Connor and 6 co-authors 2019
report on “a new mammaliaform genus and species,Galulatherium jenkinsi (Mammalia, Fig. 1), from the Upper Cretaceous Galula Formation in the Rukwa Rift Basin of southwestern Tanzania. This represents the first named taxon of a mammaliaform from the entire Late Cretaceous of continental Afro-Arabia, an interval of 34 million years. Preliminary study of the holotypic and only known specimen (a partial dentary) resulted in tentative assignation to the Gondwanatheria, a poorly known, enigmatic clade of Late Cretaceous–Paleogene Gondwanan mammals (Krause et al. 2003).” Dr. Krause is also a co-author of the 2019 paper.

Figure 1. Galutatherium compared to the near-phalanger marsupial, Paedotherium, a taxon basal to Groeberia and Vintana in the LRT. Here the skull of Paedotherium is distorted and ghosted to fit the mandible of Galutatherium.

Too few traits are preserved
in the Galutatherium partial dentary (Fig. 1) to add it to the large reptile tree (LRT, 1428 taxa), but eyeballing this fossil suggests an affinity with the paedotheres, Paedotherium (Fig. 1) and two other enigma taxa, Vintana and Groeberia.

O’Connor et al. 2019 considered Vintana a comparable taxon,
along with two other ‘teeth only’ taxa I have not seen nor tested, but list below. The teeth are often without cusps and roots, which mean, like rodent incisors, they grew continually throughout the life of the animal.

The keywords,
‘Paedotherium’ and ‘Groeberia’ are not mentioned in the O’Connor et al. paper, so taxon exclusion may have been a problem in their systematics.

From the O’Connor et al. text:
“Galulatherium jerkinsi RRBP 02067, partial left dentary with single incisor and four cheek teeth (Fig. 1). Formerly referred to as NMT 02067 in Krause et al. 2003, permanently deposited at the National Museum of Tanzania (Dar es Salaam) under agreement with the Tanzania Antiquities Unit.”

Figure 1. Subset of the LRT focusing on Metatheria after the addition of Diprotodon and Palorchestes. Some new clades are proposed here.

Paedotherium typicum (Burmeister 1888, Cerdeño E and Bond M 1998; Miocene-Pleistocene) was originally considered a rabbit-like typothere notoungulate, but here nests between the primitive marsupial kowari (Dasyuroides) and the phalanger (Phalanger) at the base of the phytometatherian clade that also includes Groeberia and Vintana among the marsupials, not placentals.

According to Wikipedia
“Bharattherium – known from eight isolated fossil teeth, including one incisor and seven molariforms (molar-like teeth, either premolars or true molars).”

“Lavanify – known from two isolated teeth, one of which is damaged.”

“Gondwanatheria is an extinct group of cynodonts that lived in the Southern Hemisphere, including Antarctica, during the Upper Cretaceous through the Miocene (and possibly much earlier, if Allostaffia is a member of this group), making them by far the latest surviving stem mammals. They are known only from isolated teeth, a few lower jaws, two partial skulls and one complete cranium. Because of this fragmentary knowledge their placement is not clear. Gondwanatheres known from cranial remains almost universally have deep, robust snouts, as befitting their specialised herbivorous lifestyle.”

‘Gondwanatheria’ is not a clade in the LRT.

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References
O’Connor PM, Krause DW, Stevens NJ, Groenke JR, MacPhee RDE, Kalthoff DC, and Roberts EM. 2019. A new mammal from the Turonian–Campanian (Upper Cretaceous) Galula Formation, southwestern Tanzania. Acta Palaeontologica Polonica 64(1):65-84.
Burmeister CV 1888. Relaciónde un viaje a la Gobernaciónde Chubut. Anales del Museo Nacional de Buenos Aires 3:175–252,
Cerdeño E and Bond M 1998. Taxonomic Revision and Phylogeny of Paedotherium and Tremacyllus (Pachyrukhinae, Hegetotheriidae, Notoungulata) from the Late Miocene to the Pleistocene of Argentina. Journal of Vertebrate Paleontology 18(4): 799-811.
Chimento NR, Agnolin FL and Novas FE 2015. The bizarre ‘metatherians’ Groeberia and Patagonia, late surviving members of gondwanatherian mammals. Historical Biology: An International Journal of Paleobiology. 27 (5): 603–623.
doi:10.1080/08912963.2014.903945
Krause DW, Hoffmann S, Wible JR, Kirk EC, and several other authors 2014. First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism. Nature. online. doi:10.1038/nature13922. ISSN 1476-4687.
Krause DW et al. 2014. Vintana sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology Memoir 14. 222pp.
McKenna MC 1980. Early history and biogeography of South America’s extinct land mammals.
Patterson B 1952. Un nuevo y extraordinario marsupial deseadiano. Rev Mus Mun Cienc NatMar del Plata. 1:39–44.
Simpson GG 1970. Addition to the knowledge of Groeberia (Mammalia, Marsupialia) from the mid-Cenozoic of Argentina. Breviora 362:1-17

wiki/Vintana
wiki/Groeberia
wiki/Bharattherium
wiki/Lavanify
wiki/Gondwanatheria

Splitting the frontals in pliosaurs

Pliosaurs are like derived pterosaurs in very few ways,
but this one stands out: The premaxilla extends all the way back to the parietal (Fig. 1) in both clades.

Figure 1. Kronosaurus dorsal skull colorized from originals in McHenry 2009. Note the premaxilla/parietal contact. Here the nasals appear to fuse to the maxillae, something added here in pink that was not intended by McHenry 2009. n = naris. o = orbit.

Other sauropterygian taxa split the nasals
as the premaxilla extends to the frontals. Pliosaurs also split the frontals with the invading premaxilla (Fig. 1).

By contrast,
in pterosaurs the premaxillae more or less sits on top of the nasals and frontals whenever the premaxillae extend posteriorly.

If you’ve ever had an interest in the giant pliosaur, Kronosaurus
you will be thrilled to read McHenry 2009. It’s chock full of details like this (Fig. 1).

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References
McHenry CR 2009. ‘Devourer of Gods’ The palaecology of the Cretaceous pliosaur Kronosaurus queenslandicus. PhD dissertation, U of Newcastle.

Another Crassigyrinus paper unsure of its nesting

Figure 1. Crassigyrinus compared to Whaatcheeria.

Herbst and Hutchinson 2018
summed up current knowledge on the fully aquatic basal tetrapod with vestigial limbs, Crassigyrinus (Fig. 1): “The Carboniferous tetrapod Crassigyrinus scoticus is an enigmatic animal in terms of its morphology and its phylogenetic position. Crassigyrinus had extremely reduced forelimbs, and was aquatic, perhaps secondarily. Recent phylogenetic analyses tentatively place Crassigyrinus close to the whatcheeriids. Many Carboniferous tetrapods exhibit several characteristics associated with terrestrial locomotion, and much research has focused on how this novel locomotor mode evolved.”

“We used computed tomographic scanning to search for more data about the skeletal morphology of Crassigyrinus and discovered several elements previously hidden by the matrix. These new discoveries give us a better understanding of the anatomy of this aberrant animal and the morphological variation present in early tetrapods.”

Not an enigma in the LRT
In the large reptile tree (LRT, 1428 taxa) Crassigyrinus (Fig. 2) nests with Ventastega, and slightly more distantly with Whatcheeria and Pederpes. Herbst and Hutchinson do not mention Ventastega. Reversals in the pectoral girdle are at play here. This is yet another temnostylian taxon returning to a more aquatic lifestyle, adopting a more tadpole-like morphology into adulthood.

Figure 2. Ventastega nests with Crassigyrinus in the LRT.

Crassigyrinus scoticus (Watson 1926, Clack 1998; 2m in length; Early Carboniferous, Viséan, 340 mya) has been described as taxonomically enigmatic (see below). Wikipedia has more on its history of discovery and taxonomy.

This aquatic tetrapod had tiny limbs and likely a long deep tail. The palate has been described as ‘very fish-like’. The vertebrae were not well ossified with no sign of posterior facets to unite them. The postfrontals contacted each other medially, separating the frontals from the parietals. The skull was relatively tall on this active predator with large teeth. The basioccipital is not developed into a formed occipital condyle, but then the neck is so short that the pectoral girdle starts beneath the lateral skull bones.

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References
Ahlberg PE and Milner AR 1994. The origin and early diversification of tetrapods. Nature 368, 507-514.
Clack JA 1998. The Scottish Carboniferous tetrapod Crassigyrinus scoticus (Lydekker) – cranial anatomy and relationships. Transactions of the Royal Society of Edinburgh: Earth Sciences 88, 127-142.
Clack JA 2002. Gaining Ground: The origin and evolution of tetrapods. Indiana University Press.
Herbst EC and Hutchinson JR 2018. New insights into the morphology of the Carboniferous tetrapod Crassigyrinus scoticus from computed tomography. Earth and Environmentsal Science Transactions of the Royal Society of Edinburgh 1–19.
Panchen AL 1985. On the amphibian Crassigyrinus scoticus Watson from the Carboniferous of Scotland. Philosophical Transactions of the Royal Society of London B 309: 505-568.
Panchen AL 1990. The pelvic girdle and hind limb of Crassigyrinus scoticus (Lydekker) from the Scottish Carboniferous and the origin of the tetrapod pelvic skeleton. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 81(1):
Panchen AL 1991. The early tetrapods: classification and the shapes of cladograms in: Origins of the Higher Groups of Tetrapods: Controversy and Consensus. Eds. Schultze HP and Trueb L. Comstock Publishing Associates, Cornell University Press, Ithaca and London.
Watson DMS 1926. Croonian Lecture – The evolution and origin of the Amphibia. Philosophical Transactions of the Royal Society B 214:189–257.

wiki/Crassigyrinus

The origin of fingers and toes in basal tetrapods

If you ever wondered
how five fingers and toes came to be the ‘standard’ for reptiles (including mammals), we can turn to the large reptile tree (LRT, 1426 taxa; subset Fig. 1) to sort out this question.

With so many taxa
among basal tetrapods known only from skulls, the following is an exercise in phylogenetic bracketing.

Figure 1. Graphing the presence of fingers and toes in basal tetrapods, updated today with the addition of 4 digits in Panderichthys.

We start with lobefins
These are fish that have no fingers or toes. The most primitive bony fish, like Cheirolpis, had lobe fins and rays. Sarcopterygians emphasized the lobe part. Bony fish reduced the lobe part and emphasized the ray part. Within the lobe the humerus, radius, ulna and smaller parts appeared (one bone, two bones, many bones). Originally the radius was much longer than the ulna.

Dvinosauria
are the most primitive taxa in the LRT to have a sub equal radius and ulna (preserved in Laidleria) and a sub equal tibia and fibula (preserved in Gerrothorax). Gerrothorax is the most primitive taxa to preserve metacarpals. They were poorly ossified, but there were five in number.

Colosteus 
(Fig. 2) preserves four fingers (1-4) on a tiny forelimb. Only the front half of this taxon is known.

Pholidogaster
(Fig. 2) more or less preserves five toes. The manus was not preserved, but the radius and ulna were slender beneath a robust humerus.

Figure 6. Colosteus relatives according to the LRT scaled to a common skull length. Their sizes actually vary quite a bit, as noted by the different scale bars. Only Pholidogaster and Colosteus are taxa in common with traditional colosteid lists.

The vast majority of basal tetrapods
retained this digit pattern: four on the forelimbs, five on the hind limbs.

Exceptions include
Acanthostega (Fig. 3) with 8 fingers and 8 toes. Ichthyostega has 7 toes (manus unknown).

Acanthostega demonstrates a reversal:
The radius is twice as long as the ulna, as in lobefin fish. Apparently neotony produces this reversal as Acanthostega became sexually mature as a more fully aquatic ‘tadpole’, much smaller than its ancestor, Ossinodus (Fig. 2), for which only a few toe parts are known.

We looked at the convergently more aquatic Ichthyostega
earlier here. Both are Late Devonian taxa, appearing tens of millions of years later than the Middle Devonian trackmaker.

Figure 3. Ossinodus is the more primitive taxon in the LRT compared to the smaller Acanthostega, the tadpole of the two.

Proterogyrinus had five fingers and five toes,
but it appears to have developed the extra digits all alone and convergent with amniotes (= reptiles) and their kin (see below).

Cacops and kin (Dissorophidae, Lepospondyli)
also developed five fingers and five toes by convergence with reptiles. Other lepospondyls, like the frog, Rana, did not have more than four fingers.

The first taxon in our lineage with five fingers and five toes
is Utegenia, which gave rise to the clade Seymouriamorpha and the clade Reptilomorpha (by definition, taxa closer to reptiles than to salamanders: Eusauropleura, Gephyrostegus, their last common ancestor and all their descendants).

As early as the Late Devonian
the basal reptilomorph, Tulerpeton (Fig. 4), developed an exceptional and tiny sixth finger. Since no more taxa in this lineage had a sixth finger, this is not a reversal, but a novel digit. Originally this taxon was thought to have six toes (digit 5 had to be fully restored), but new reconstructions do not confirm this hypothesis.

Figure 4. Tulerpeton peps in situ and several reconstruction options using published images of the in situ fossil. The one at upper left most closely resembles sister taxa and has more complete PILs (parallel interphalangeal lines).

The above LRT fish-to-tetrapod transition
only partially replicates and confirms the traditional one provided by Clack 2009 (Fig. 5) with far fewer taxa.

Figure 5. The classic paradigm illustrating the fish-to-tetrapod transition from Clack 2009.

If anyone knows of taxa pertinent to this subject
please let me know and I will add them. At present very few taxa represent many more taxa (phylogenetic bracketing) since so many taxa in the above subset do not preserve extremities or they were overlooked and not collected or published.

References
Clack JA 2009. The fish-tetrapod transition: new fossils and interpretations. Evo Edu Outreach (2009) 2:213–223. DOI 10.1007/s12052-009-0119-2

Enigmatic Laidleria joins the LRT

Short one today
on a small taxon with a nearly equilateral skull in dorsal view, Laidleria gracilis (Fig.1).

Figure 1. Originally considered a trematosaur, and then an enigma, Laidleria gracilis nests at the base of the Dvinosauria/Plagiosauria in the LRT.

Laidleria gracilis (Kitching 1957; Warren 1998; Early Triassc) Originally considered a trematosaur and long considered an enigma taxon, Lairleria was a small, flat head, wide-body, late-surviving, basal dvinosaur with a trinagular skull in dorsal view. Apparently the intertemporals, prefrontals and an interfrontal were overlooked by Warren 1998. If not their fusion to neighboring bones is an autapomorphy.

A close relative, Uruyiella,
(Piiñeiro, Marsicano and Lorenzo 2007), nests these two taxa in a new clade, Laidleriidae. They report, “The Plagiosauridae and the Laidleriidae form a clade at the base of Dvinosauria, which is the sister group of a clade that includes Stereospondyli and Archegosauroidea.” The large reptile tree (LRT, 1426 taxa) agrees with this nesting.

The interesting thing about this specimen
and its sisters is the lack of fossil fingers and toes. We’ll look at this issue using phylogenetic bracketing soon.

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References
Kitching JW 1978. A new small stereospondylous labyrinthodont from the Triassic beds of South Africa. Palaenotologia Africana 5:67–82.
Piñeiro G, Marscano C and Lorenzo N 2007. A new temnospondyl from the Permo-Triassic Buena Vista Formation of Uruguay. Palaeontology 50(3):627–640.
Warren A 1998. Laidleria uncovered: a redescription of Laidleria gracilis Kitching (1975), a temnospondyl from the Cynognathus Zone of South Africa. Zoological Journal of the Linnean Society. 122 (1–2): 167–185.

wiki/Laidleria
Archegosauroidea
wiki/Stereospondyli
wiki/Plagiosauridae
wiki/Dvinosauria

Avimaia and her enormous egg

Bailleul et al. 2019 reported
on the posterior half of an Early Cretaceous enantiornithine bird from China, Avimaia schweitzerae (IVPP V25371, Figs. 1,2), including an enormous eggshell within her torso. The authors commented on the eggshell, which had not one, but several several layers, an abnormal condition, probably leading to the demise of the mother.

Phylogenetic analysis
The Bailleul et al. 2019 phylogenetic analysis nested Avimaia with eight most closely related taxa, of which only one, Cathayornis (Fig. 1), was also tested in the large reptile tree (LRT, 1425 taxa, subset Fig. 3) and likewise nested with Avimaia. Significantly, Cathayornis also has a very deep ventral pelvis capable of developing and expelling very large eggs.

Figure 1. Avimaia compared to Cathayornis to scale. Cathayornis is the only other tested enantiornithine bird to have such a deep ventral pelvis.

A long, thin, straight, displaced bone was found
beneath the rib cage and identified as a rib by Bailleul et al. 2019. I wonder if it is instead a radius (Fig. 1) because it is not curved like a rib and it does not have an expanded medial process. The radius is vestigial. Regardless of the identify of this slender bone, Avimaia, appears to be ill-suited for flying based on her robust tibiae, short dorsal ribs  and giant egg. Cathayornis (Fig. 1) appears to be better-suited for flying, based on its chicken-like proportions.

Figure 2. Avimaia in situ. Some bones were originally mislabeled. Here the egg is reconstructed with a more traditional egg shape.

Mislabeled bones
The right ‘pubis’ (Fig. 2) is the right ischium. The reidentified pubis has a pubic boot and the ischium does, not as in sister taxa. The authors failed to identify vestigial pedal digit 5.

The egg was originally reconstructed as a sphere (drawn as a circle) inside the abdomen. Here (Figs. 1, 2) the egg is reconstructed in a more traditional egg shape more likely to pass through the ischia and cloaca.

Figure 3. Subset of the LRT focusing on the clade Enantiornithes and the nesting of Avimaia as a derived taxon within that clade.

Most birds
lay more than one egg in a clutch. Another exceptional bird that develops a very large egg is the flightless kiwi (Apterypterx, Fig. 4).

Figure 2. Jurapteryx, Pseudocrypturus, Apteryx and Proapteryx to scale.

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References
Bailleul AM, et al. 2019. An Early Cretaceous enantiornithine (Aves) preserving an unlaid egg and probable medullary bone. Nature Communications. 10 (1275). doi:10.1038/s41467-019-09259-x
Pickrell, J 2019. “Unlaid egg discovered in ancient bird fossil”. Science. doi:10.1126/science.aax3954

wiki/Avimaia

‘Armored’ Peltobatrachus enters the LRT

Panchen 1959
reported on a 70cm armored basal tetrapod from the Late Permian of Tanzania. Traditionally considered a temnospondyl, Peltobatrachus (Fig. 1) inspired the addition of several basal tetrapod taxa, including taxa that defined the Temnospondyli (see below).

Figure 1. Armored Peltobatrachus.

No teeth or distal limb elements are preserved.
Peltobatrachus is atypical in having wider ribs along their entire length. Related taxa, like Eryops and Sclerocephalus, have only the distal portion of their ribs (intercostal plates) expanded to overlap succeeding ribs.

Intercostal plates disappear in most tetrapods,
but make a reappearance (reversal) in the derived cynodont, Thrinaxodon.

Schoch (2013)  defined the clade Temnospondyli 
as the least inclusive clade of Edops and Mastodonsaurus. In the large reptile tree (LRT, 1423 taxa), Edops and Mastodonsaurus nest in two adjoining sister clades, the latter in the larger specimen clade of the Lepospondyli, distinct from the  smaller specimen clade that includes extant frogs and salamanders.

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References
Panchen AL 1959. A new armoured amphibian from the Upper Permian of East Africa. Philosophical Transactions of the Royal Society of London B 242:207-281.

wiki/Peltobatrachus

Tiny flightless Early Cretaceous bird from Spain

Kaye et al. 2019
illuminated feathers with laser-stimulated fluorescence (Fig. 1) in a tiny, unnamed, enantiornithine bird specimen, MPCM-LH-26189 from the Las Hoyas locality (Barreminian, Early Cretaceous) of Spain. Based on the presence of those illuminated feathers and the size of the specimen (Fig. 2) the authors judged it to be a precocial hatchling, capable of walking shortly after hatching. This is the same specimen first described and not named by Knoll et al. 2018.

Figure 1. Specimen MPCM-LH-26189 a tiny enantiornithine bird in situ under white light (above, plate and counter plate) and under laser stimulated fluorescene (below). DGS colors added and used in the reconstruction in figure 2. Not sure what those red highlighted items are at lower right. See figure 2b for skull details.

A reconstruction of the new extra-tiny bird
is shown (Fig. 2) alongside that of another tiny coeval and closely related enantiornithine bird, Iberomesornis, to scale. Note the tiny fingers in the tiny MPCM specimen indicating flightlessness. The lower crus, distal tail and feet extend off the matrix block, so they remain unknown. Contra Kaye et al. 2019, the tiny MPCM specimen does not appear to have juvenile proportions, despite its reduced size.

Figure 2. Tiny Iberomesornis compared to scale with even tinier MPCM specimen. Note the tiny fingers. Two tibial lengths are presented since this data remains unknown. The tiny MPCM specimen does not appear to have juvenile proportions.

Figure 2b. Skull of MPCM specimen traced using DGS methods and reconstructed using the resulting color parts.

It is always a good idea
to create a reconstruction (Fig. 2) from ‘road-kill’ taxa (Fig. 1). Such a reconstruction would have indicated the MPCM specimen did not have juvenile proportions, despite its small size… and it did not have traditional bird wings.

It is also a good idea to compare taxa
in a phylogenetic analysis to see how what you have relates to others of its kind. Here in the large reptile tree (LRT, 1423 taxa) the MPCM specimen nests close to Iberomesornis within the clade Enantiornithes.

Reversals
The MPCM specimen is the first enantiornithine to have short un-birdlike fingers (a reversal due to neotony) and such short forelimbs (another reversal).

If the tail lacked a pygostyle, as it currently appears, that would also be a reversal shared with long-tailed descendants, Pengornis and Protopteryx.

The small size of this possible adult specimen is also due to the same forces that led to tiny Iberomesornis in Early Cretaceous Spain. If the MPCM specimen had nested with much larger specimens, rather than tiny Protopteryx and Iberomesornis, then the MPCM specimen would more likely have been considered a juvenile.

Knoll et al. 2018 first studied the MPCM specimen
or its osteological correlates with other juvenile birds, not considering the possibility that phylogenetic miniaturization might make a tiny adult bird appear to be a juvenile. Perhaps that is why they concluded, “the hatchlings of these phylogenetically basal birds varied greatly in size and tempo of skeletal maturation.” Knoll et al. did not create a reconstruction nor put this specimen under phylogenetic analysis, probably on the basis of its presumed juvenile character. As your mother told you, if you assume something, you might miss out on its most intriguing aspects.

Phylogenetic analysis is so important
because it reveals so much more than just ‘eyeballing’ specimens.

Earlier we looked at other birds
that experienced a similar reversal from wings to hands. Among these are Mei long, Jinianhualong and Liaoningvenator.

In the Late Jurassic
tiny pterosaurs experienced a similar size squeeze. Traditionally considered juveniles, tiny hummingbird-sized taxa like B St 1967 I 276 (Fig. 3) and BMNH 42736 with fly-sized hatchlings, were among the few pterosaur lineages to survive the Jurassic and produce Cretaceous taxa.

From NatGeo.com
Paleo bird expert, Jingmai O’Connor reports, “All enantiornithines were super-precocial, born fully-fledged and ready to fly.”

A closer examination
indicates the MPCM specimen was never going to be ‘ready to fly.’ 

Figure 3. Smallest known bird, the bee hummingbird, compared to smallest known adult pterosaur, No. 6 (Wellnhofer 1970). Traditional workers consider this a hatchling or juvenile, but in phylogenetic analysis it does not nest with any 8x larger adults.

Is the MPCM specimen the smallest dinosaur?
If it is an adult, the MPCM specimen appears to be slightly larger than the smallest known dinosaur, the bee hummingbird (Fig. 3).

Since no one else wants to name the MPCM specimen,
probably because others considered this a hatchling rather than a phylogenetically miniaturized adult, let’s call him Microcursor sanspedes (‘tiny runner without feet”) in the meantime.

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References
Kaye TG, Pittman M, Marugán-Lobón J, Martín-Abad H, Sanz JL and Buscalioni AD 2019. Fully fledged enantiornithine hatchling revealed by Laser-Stimulated Fluorescence supports precocial nesting behavior. Nature.com/scientific reports (2019) 9:5006 https://doi.org/10.1038/s41598-019-41423-7

Knoll, F. et al. (16 co-authors) 2018. A diminutive perinate European Enantiornithes reveals an asynchronous ossification pattern in early birds. Nature Communications 9, 937 (2018).

Publicity

https://www.nationalgeographic.com/science/2019/03/dinosaur-era-birds-born-ready-to-run-fossil-feathers-show/