Early Jurassic Hadrocodium now nests with extant Dromiciops

These two taxa,
Early Jurassic Hadrocodium and extant Dromiciops (Fig 1), now nest together in the large reptile tree (LRT, 2338 taxa). Despite their separation in size and time, they share more traits with each other than with any other tested taxon.

‘What took so long?’ you might be wondering…

Figure 1. Juvenile Early Jurassic Hadrocodium compared to adult extant Dromiciops to scale. ” data-image-caption=”

Figure 1. Juvenile Early Jurassic Hadrocodium compared to adult extant Dromiciops to scale.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/dromiciops-gliroides-large588.jpg?w=109″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/dromiciops-gliroides-large588.jpg?w=371″ class=”size-full wp-image-92261″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/dromiciops-gliroides-large588.jpg” alt=”Figure 1. Juvenile Early Jurassic Hadrocodium compared to adult extant Dromiciops to scale. ” width=”584″ height=”1613″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/dromiciops-gliroides-large588.jpg?w=584&h=1613 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/dromiciops-gliroides-large588.jpg?w=54&h=150 54w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/dromiciops-gliroides-large588.jpg?w=109&h=300 109w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/dromiciops-gliroides-large588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1. Juvenile Early Jurassic Hadrocodium compared to adult extant Dromiciops to scale. Premolar 1 takes the place of the lost canine. Molar 4 is a vestige in Dromiciops, lost in Glironia and Hadrocodium.

The identity of the canine-like premolar1
was one issue. Note the double root. Canines are simple cylinders. We looked at a homologous replacement earlier here with Glironia (Fig 2). Academics call these double-rooted canines. A broader view with more taxa refutes that identity.

Also note the small number of molars in Hadrocodium (Fig 1). Molar 4 is a vestige in Dromiciops (Fig 1), lost in Glironia (Fig 2) and Hadrocodium (Fig 1). So the smallest skull had the fewest molars, whether juvenile or not.

Otherwise
both taxa share a long list of traits, from 5 incisors (a primitive trait reduced in derived mammals) and a similar post-parietal skull architecture.

Figure 2. Above: Dromiciops skull and tooth row (right rotated). Below: Glironia skull and tooth row (right, rotated). ” data-image-caption=”

Figure 2. Above: Dromiciops skull and tooth row (right rotated). Below: Glironia skull and tooth row (right, rotated).

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=163″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=558″ class=”size-full wp-image-88629″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg” alt=”Figure 2. Above: Dromiciops skull and tooth row (right rotated). Below: Glironia skull and tooth row (right, rotated).” width=”584″ height=”1073″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=584&h=1073 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=82&h=150 82w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=163&h=300 163w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 2. Above: Dromiciops skull and tooth row (right rotated). Below: Glironia skull and tooth row (right, rotated).

Another Early Jurassic relative,
burrowing Docofossor (Fig 3), also lacks a canine, has a canine-like premolar1 and it has 4 molars.

That high molar count makes the extant taxon more primitive than the Early Jurassic relative. This demonstrates it did not take long for marsupials to diversify after their Late Triassic origin.

Recent work by U of Bristol researchers led by Janis 2025
indicated, “many mammals were already shifting toward a more ground-based lifestyle leading up to the end-Cretaceous mass extinction.”

Early Jurassic terrestrial marsupials indicate that shift occurred much, much earlier.

“We also knew that tree dwelling mammals struggled after the asteroid impact. What had not been documented, was whether mammals were becoming more terrestrial, in line with the habitat changes.”

= flowering plants. The Bristol U authors focused on limb bone joint surfaces.
The LRT uses complete skeletons and phylogeny.

Figure 3. The teeth of Early Jurassic Docofossor compared to extant Glironia. Both lack a canine, but replace it with a large, double-rooted premolar 1. ” data-image-caption=”

Figure 3. The teeth of Early Jurassic Docofossor compared to extant Glironia. Both lack a canine, but replace it with a large, double-rooted premolar 1.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/glironia.docofossor588.jpg?w=224″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/glironia.docofossor588.jpg?w=584″ class=”size-full wp-image-92274″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/glironia.docofossor588.jpg” alt=”Figure 3. The teeth of Early Jurassic Docofossor compared to extant Glironia. Both lack a canine, but replace it with a large, double-rooted premolar 1.” width=”584″ height=”783″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/glironia.docofossor588.jpg?w=584&h=783 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/glironia.docofossor588.jpg?w=112&h=150 112w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/glironia.docofossor588.jpg?w=224&h=300 224w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/glironia.docofossor588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 3. The teeth of Early Jurassic Docofossor compared to extant Glironia. Both lack a canine, but replace it with a large, double-rooted premolar 1.

Dromiciops gliroides
(Thomas 1894, Marshall 1978) is the extant, monito del monte or colocolo opossum, a tiny living basal marsupial from South America. The middle ear (endotympanic + alisphenoid (epiterygoid) is greatly inflated. Note the canine-like premolar 1. The canine is absent in this clade.

Hadrocodium wui
(Luo, Crompton and Sun 2001; Early Jurassic; skull length: 1.2cm), known only from a skull. Hadrocodium was originally considered a juvenile basal mammal, due to the emerging teeth and low molar number. The first premolar replaces the lost canine. A close relative is the living Dromiciops.

Wikipedia Hadrocodium reported,
“It is considered as the closest relative of the class Mammalia.”

Luo et al 2022 reported,
“Hadrocodium is similar to other mammaliaforms in retaining a mandibular middle ear, contrary to our earlier interpretation.”

In the LRT Hadrocodium is a mammal. During ontogeny the posterior jaw bones take time to assemble themselves into middle ear bones. In Early Jurassic taxa those changes may have retained marks. This may be another immature trait.

The authors’ observation of the posterior jaw has to be tested with extant and extinct relatives identified in this blogpost (Figs 1–3) to see how widespread and at what ontogenetic stage such marks disappear.

Luo et al reported,
“The mandible exhibits a large postcanine diastema from shedding of anterior premolars without replacement, an age-dependent feature better developed in older adults. Another adult feature is the alignment of the ultimate molar to the coronoid process. The two-rooted upper canines are more derived than other Early Jurassic mammaliaforms from the same fauna, although similar to docodontans.”

Double-rooted canines are re-identified here as premolar1.

Luo et al reported,
“The CT examination corroborates that there are no unerupted teeth in the upper or lower jaws, and the holotype of H. wui is dentally and osteologically mature and capable of independent feeding.”

The largest teeth in the jaws of Hadrocodium (Fig 1) have a length of 1mm. That’s tiny. Luo et al presented P3 with roots, but no crown and that unit aligns below the gum line of the erupted teeth. So the µCT scans may not be showing everything that was there.

The authors’ presentation (their figure 6) graphically whited out all data except for the teeth they wanted to show. In a drawing in the same figure P2 and p2 (labeled P1 and p1) are both missing – but an alveolus is indicated for both. So that requires more data and consideration.

Glironia may help provide answers.
Extant Glironia (Figs 2, 3), similar in size and morphology to Dromiciops, also has five incisors, lacks a canine, has three premolars and only three molars.That means Hadrocodium has another relative among extant marsupials with a similar tooth number and morphology.

Glironia lacks a pouch. Dromiciops has a fur-lined pouch.
Just imagine how tiny those Hadrocodium babies would have been.

Luo et al 2022 did not mention
Dromiciops or Glironia, so comparisons were not made.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.

References
Janis CM et al 2025. Down to earth: therian mammals became more terrestrial towards the end of the Cretaceous. Palaeontology 68 (2): e70004; doi: 10.1111/pala.70004
Gray JE 1821. On the natural arrangement of vertebrose animals. London Medical Repository 15(1):296–310.
Luo Z-X, Crompton AW and Sun A-L 2001. New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics. Science 292 (5521): 1535–1540. Bibcode:2001Sci…292.1535L. doi:10.1126/science.1058476. PMID 11375489.
Luo Z et al (4 co-authors) 2022. Reexamination of the Mandibular and Dental Morphology of the Early Jurassic Mammaliaform Hadrocodium wui. Acta Palaeontologica Polonica. 67. doi:10.4202/app.00949.2021
Mao F et al. (8 co-authors) 2024b. Fossils document evolutiionary changes of jaw joint to mammalian middle ear. Nature 1759(266): onlline
Marshall LG 1978. Mammalian species, Dromiciops australis, American Society of Mammalogists, pp. 1-5. pdf

wiki/Hadrocodium
wiki/Dromiciops